What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Stuttering 360

bjohnson1234

Well Known Member
I have a Mattituck XP-360 that has a stuttering issue. It's either not noticeable at cruise RPM or it doesn't happen, but I have definitely noticed it in flight and on the ground. Every 3 or 4 seconds it very briefly loses some power, maybe a drop of 50 RPM or so and then spins back up. It's both audible and visible in the prop. I have sent my engine data to savvyanalysis and everything came back looking good, so I'm pretty sure it's not ignition, intake leak or sticking valve. It also seems to happen more frequently when the engine is hot (maybe always?). I checked the timing recently and savvyanalysis confirmed that it's correct. I have a fixed pitch prop, so it's not a governor issue. All my fuel lines have firesleeve and I cleaned out the fuel filter in the fuel injection servo. I also checked to make sure there are no broken springs on the valves. Everyone I've spoken to is stumped. Has anyone else seen anything like this? Any ideas what it might be?
 
A little more info please

I am not sure I can help but I am sure someone will be able to point you in the right direction.
Can you tell us a little more about the history of your engine/airplane?
XP-360 I assume its a carbureted version right?
How many hours, did you build it, when did this start, has any maintenance been done prior to this RPM anomaly.
Ignition set up, mag EI or combination, what kind?
Engine instrumentation? and I see you have recording capability.
What kind of fixed pitch prop?
Anything else you can think of, even things that might seem unrelated.
We love to help you.
 
I bought it. It has about 180 hours on it, I've put about 100 of those on it and the problem has always been there. Been through annual and has good compressions.

It's fuel injected.

LASAR ignition system, but the problem is the same with the breaker pulled and running on either mag and fuel pump on/off makes no difference

AFS EFIS with engine monitor

Catto 3 blade prop

I have seen other people talk about a similar issue that turned out to be fuel vaporizing in the lines, but I have fire sleeve on all my fuel lines. The only unprotected lines are the ones leaving the distribution block.

Crossover exhaust, maybe too much heat on the gascolator?

I tried multiple times to replicate the problem with the cowl off and was unable. After putting the cowl back on, the problem quickly returned and then stayed after I took it back off again. Tomorrow morning I'll try starting it cold with the cowl on and see what happens.

I live in Orlando and it's really hot and humid.


I am not sure I can help but I am sure someone will be able to point you in the right direction.
Can you tell us a little more about the history of your engine/airplane?
XP-360 I assume its a carbureted version right?
How many hours, did you build it, when did this start, has any maintenance been done prior to this RPM anomaly.
Ignition set up, mag EI or combination, what kind?
Engine instrumentation? and I see you have recording capability.
What kind of fixed pitch prop?
Anything else you can think of, even things that might seem unrelated.
We love to help you.
 
I have never had this happen in the air but mine (IO-360-A1A) runs poorly on the ground hot, its fuel vaporizing in the injector lines, once you add throttle to take off the pressure in the injector lines increases to a level where the fuel remains liquid and the engine runs normally, it does not do it with the cowl off because you?re not trapping the heat. This behavior on the ground is totally normal, nothing to worry about.

Whatever is happening in the air is probably something else because the presser in the injector lines is up and the temp is lower, could not say what?s causing the inflight issue.
 
Fuel (Vapor) related

You have a Mattituck XP IO-360.
More questions:
My guess is like Russ pointed out, a fuel vapor related problem.
The fact that you cannot duplicate the problem with the cowl off, somewhat confirms my suspicion.
The problem is finding the location where your fuel lines are heated to the point
where vaporization occurs and your fuel delivery becomes sporadic and partially intermittent. As Russ pointed out this occurs most often during idling after long taxiing on hot days and with low octane fuel when fuel flow is nearly stagnant. Well built engine installations do not do this and the fact that it happens to you in flight, tells me that perhaps your exhaust pipes are too close to some part of your fuel system. (Gascolator?)
What is your fuel pressure in flight?
It should be between 20 and 25psi. (wrong fuel Pump maybe?)
Have you tried the electric boost pump on to see if the problem persists?
The boost pump will not help if the problem exists after the fuel controller.
There is only one hose going from the fuel controller to the spider and fire sleeved as you pointed out. Usually routed up between the cylinders and not a problem in other installations.
Do you have a Bendix or AFP fuel injection or other?
Are your injector lines installed with stand offs and adel clamps to keep them away from cylinder heat? If Mattituck built it, I am sure it was done right.
What is your oil temperature on average when this problem occurs?
EGTs and CHTs?
What is your engine's compression ratio and are you using 100LL for fuel?
Do you have any pictures to post of your engine installation?
Specifically fuel system installation.
I think we are getting somewhere.
 
When the stuttering happens, are you running LOP???

If so, how well are your EGTs balanced?
 
I have an ECI XIO-360 and I used to stutter on the ground. Sometimes it would just quit altogether while taxiing. It was exacerbated by leaning for ground ops.

I diagnosed it as fuel vaporization. It occurred on a day that was 75, but not the next day when it was 65, or when I left the oil access door open. I noticed that the fuel pressure would decrease exponentially just before it stumbled and died. If prepared, a quick hit of boost pump would allow recovery.

Although infrequent, I did notice it one day while flying. It was intermittent.

I installed a blast tube on the fuel pump and so far the issue has nearly disappeared. If I lean during ground ops it will stumble more than when full rich, but it is not bad and has not died on me like before.

It is a simple one inch blast tube from the left baffle that points at the aft section of the fuel pump. You are correct, it just is too dang hot here in Florida.

You might try some tests to confirm this... Note temperature, open oil access door, perhaps while on the ground block open your rear baffling to allow some cooling air to circulate, watch fuel pressure closely and see if you can correlate the events.

Let us know what you find out.

Don
 
Wow, that's for all the feedback! Let me try to answer all the questions:

Fuel is 100LL and it's less than a week old

Crossover exhaust does come close to the gascolator, but turning the fuel pump on doesn't help

Fuel pressure is 22-24

The line going from the injector servo to the spider is routed behind the engine. I moved it yesterday when I cleaned the injector fuel filter and briefly I thought it was fixed, but the problem returned. There are no standoffs, but it doesn't go near the cylinders until it goes through the baffle. Could it be happening in the lines from the spider to the cylinders? They don't have any heat protection on them.

Precision fuel injector servo RSA-5AD1 part# 2576536-2

I have seen it happen both when the oil temp is high (in flight 180 or so) and at start on the ground (100ish)

#3 CHT runs 20-30 degrees hotter than the rest, otherwise EGT/CHT look normal as confirmed by savvyanalysis

Happens regardless of mixture settings

I have done an in-flight mag test, sent the results to savvyanalysis and it came back with no issues

I have some blast tubes going to the mags and LASAR controller, but nothing to the fuel pump.

The symptoms in the air are identical to the ones on the ground, could it still be the same issue? I have specifically noted it at 2000 RPM, but I will take it out today and see what point it stops at. I'll try to post some pictures as well. Thanks again!
 
Yesterday after the problem occurred with the cowl on, I took it off and the problem persisted. I only ran it for a few minutes after taking the cowl off, but any heat that was causing it at the point would have to be direct radiated heat or possibly some fuel system component gets too hot and causes the vaporization inside it instead of something in the lines.
 
Nobody has mentioned looking upstream of the boost pump for cracked fuel lines. You could be sucking air.
 
Stutter

Nobody has mentioned looking upstream of the boost pump for cracked fuel lines. You could be sucking air.

Wouldn't that show up as fluctuating fuel pressure?:confused: Maybe not if there were slugs of air being compressed by the pump. Something to check for sure.
 
I took it out this morning and I was able produce the problem and largely eliminate it pretty reliably. At Low power settings (1500 RPM or less) I could feel a noticeable "hiccup" in the RPM when the CHTs were around 300. I kept the throttle back and put the plane is a fairly steep dive to increase cooling to the engine and the problem was reduced substantially. At this point I'm fairly confident that it's a fuel vaporization issue, the only trouble is where.

Here are the things I'm thinking about trying:

1. There is a small gap on the bottom between the cowl and the baffle that is probably contributing to my high #3 CHT. I'm going to close that gap

2. Add a blast tube to the engine driven fuel pump and the gascolator

3. add a heat shield to the exhaust near the gascolator

4. add a larger heat shield to the exhaust to protect the fuel lines coming into and leaving the fuel injection servo

I can see the fuel lines from the fuselage to the electric fuel pump directly in front of the wing spar between the seats. I'm going to pull the cover and make sure there are no cracks anywhere inside.

I also noticed that if I run the boost pump too long before starting the engine, fuel leaks from between the spacer and the air intake. Maybe I should add some red RTV to seal that better?

Any other ideas?

Thanks!
 
Any Pictures?

Posting pictures is a bit of a hassle until you figure out how but if you could take some engine compartment pics that would be great.
There are literally hundreds of RVs with your exact engine set up and they don't have the problem you describe.
It should be fairly easy to spot the cause when comparing installation set ups.
A call to Precision would also be in order although they are probably busy in OSH this week.
We all have a mechanical fuel pump and in my circles we call that the "fuel boiler" and is generally regarded as the main suspect for fuel vapor problems.
A blast tube directed to the fuel pump or even into a fuel pump shroud can help a lot. However, we have not yet identified where the problem is, so I wouldn't start fixing yet.

Nobody has mentioned looking upstream of the boost pump for cracked fuel lines. You could be sucking air.
This is certainly suspect especially because at least initially you said the problem only occurred at high rpms. A tiny leak in the upstream fuel line would not leak fuel out but may suck air in when there is enough flow in the fuel line.(High RPM)
Once the air is introduced into the fuel system it would take very little heat to expand those air bubbles and cause erratic engine operation.
This would explain why this happens even if your system is set up like everyone else's. There are quite a number of fuel line connections leading up to the boost pump, check these but don't over tighten them.
I would expect to see a somewhat fluctuating fuel pressure if this was indeed the cause of your problem.

Most of us don't have a gascolator, depending on where it is located, it can be another heat sink in the fuel system especially when it is installed too close to the exhausts.

You have a number of people looking at this now so keep us posted
We want to see this fixed.
 
Actually, the problem only occurs at lower RPMs. I have never noticed it at cruise RPM, but I have noticed it as high as 2000 RPM, but that was holding on a hot day at about 3000 ft. I did call Precision but they said it didn't sound like anything in the injector servo and the savvyanalysis test showed that I have very good fuel distribution from the spider.

I've taken the cowl on and off so many times over the past couple days I was hesitant to take it off again, but if it will help spot the issue I'll try to get out there this afternoon.

Also, OAT this morning was 29C on the ground at 20C at 4500. I also don't think I've mentioned that it's a 6.


Posting pictures is a bit of a hassle until you figure out how but if you could take some engine compartment pics that would be great.
There are literally hundreds of RVs with your exact engine set up and they don't have the problem you describe.
It should be fairly easy to spot the cause when comparing installation set ups.
A call to Precision would also be in order although they are probably busy in OSH this week.
We all have a mechanical fuel pump and in my circles we call that the "fuel boiler" and is generally regarded as the main suspect for fuel vapor problems.
A blast tube directed to the fuel pump or even into a fuel pump shroud can help a lot. However, we have not yet identified where the problem is, so I wouldn't start fixing yet.


This is certainly suspect especially because at least initially you said the problem only occurred at high rpms. A tiny leak in the upstream fuel line would not leak fuel out but may suck air in when there is enough flow in the fuel line.(High RPM)
Once the air is introduced into the fuel system it would take very little heat to expand those air bubbles and cause erratic engine operation.
This would explain why this happens even if your system is set up like everyone else's. There are quite a number of fuel line connections leading up to the boost pump, check these but don't over tighten them.
I would expect to see a somewhat fluctuating fuel pressure if this was indeed the cause of your problem.

Most of us don't have a gascolator, depending on where it is located, it can be another heat sink in the fuel system especially when it is installed too close to the exhausts.

You have a number of people looking at this now so keep us posted
We want to see this fixed.
 
My bad

It's either not noticeable at cruise RPM or it doesn't happen, but I have definitely noticed it in flight and on the ground.

I misread your original post, skipped over that not word.

The good news is that the fix is much easier and the problem is much more common than what I thought you had described.
In this case I would definitely start with what you have outlined in your post and I am quite sure you will see positive results.

Here are the things I'm thinking about trying:

1. There is a small gap on the bottom between the cowl and the baffle that is probably contributing to my high #3 CHT. I'm going to close that gap

2. Add a blast tube to the engine driven fuel pump and the gascolator

3. add a heat shield to the exhaust near the gascolator

4. add a larger heat shield to the exhaust to protect the fuel lines coming into and leaving the fuel injection servo

There is a 100% sure fire fix for your problem but entails a bit of plumbing.
Try all the above mentioned things first.

I also noticed that if I run the boost pump too long before starting the engine, fuel leaks from between the spacer and the air intake. Maybe I should add some red RTV to seal that better?

Running the boost pump too long before starting will produce a fuel drip and accumulate fuel in the filter box. Most builders have either a small hole in the low spot of the filter box or a sniffle valve for the fuel to drain out.
I have a small hole #30 drill.
 
What brand plugs are you using? I have the same problem but at higher RPMs and when I lean more aggressively. I am changing to Tempest plugs from my old Champions as I believe that one of my Champions under stress is intermittent. The Tempest, from reading on their web site is a more advanced plug and less likely to fail. I will know soon!

Phil
 
I have the fine wire plugs, I forget which brand, but the savvyanalysis test confirmed that spark is good

What brand plugs are you using? I have the same problem but at higher RPMs and when I lean more aggressively. I am changing to Tempest plugs from my old Champions as I believe that one of my Champions under stress is intermittent. The Tempest, from reading on their web site is a more advanced plug and less likely to fail. I will know soon!

Phil
 
Fly on

This is simply a matter of fuel vaporizing in the injector lines just as I said in my first post. The injector lines are the ones that go from the spider to the injector nozzles, there sitting right on top of a big hot stove/your engine. Fuel under pressure is harder to vaporize, fuel under little or no pressure is much easier to vaporize, at low RPM your spider and injector lines are under low pressure and the fuel boils, this is all very normal on these injected engines, you even said the boost pump does not clear the problem and that your fuel pressure is 20+ PSI, this eliminates everything before the fuel servo. You're spending lots of time chasing something that is normal, you’re not too likely to relieve this, I would just fly it.

YMMV and if you think it’s not safe use your best judgment, none of us are actually there to verify our diagnosis.
 
Bad spark plug intermittently failing?

I had an almost brand new one fail in my old Cherokee. The problem presented itself when I throttled back in the pattern getting ready to land. It got my attention real quick. Discovered which one was bad using a benchtop spark plug tester.
 
I am pretty certain at this point that you are correct. My big concern was whether or not it was a safety issue and I don't believe that it is. I intend to keep flying it, but I also intend to make the cooling modifications to try to reduce the issue. I feel like the baffling will help lower the CHT (I'm also looking at the "washer" mod) which seems to have the most direct impact. With the blast tubes I'm thinking that keeping the fuel cooler before it reaches the spider might also help prevent it from getting vaporized. I think just some basic, inexpensive changes to reduce temperatures can only be a good thing in the long run anyway, right?

I do have one question though: if the injector lines are exposed to the most heat since they are on top of the cylinders, why are they completely unprotected? Why not use fire sleeve or have some kind of cooling fins on them? Even some kind of heat sink on the spider itself seems like it would help keep the fuel cooler.

Thanks for all the help!


This is simply a matter of fuel vaporizing in the injector lines just as I said in my first post. The injector lines are the ones that go from the spider to the injector nozzles, there sitting right on top of a big hot stove/your engine. Fuel under pressure is harder to vaporize, fuel under little or no pressure is much easier to vaporize, at low RPM your spider and injector lines are under low pressure and the fuel boils, this is all very normal on these injected engines, you even said the boost pump does not clear the problem and that your fuel pressure is 20+ PSI, this eliminates everything before the fuel servo. You're spending lots of time chasing something that is normal, you?re not too likely to relieve this, I would just fly it.

YMMV and if you think it?s not safe use your best judgment, none of us are actually there to verify our diagnosis.
 
You're spending lots of time chasing something that is normal, you?re not too likely to relieve this, I would just fly it.

I disagree, it is not normal and most don't have this problem and I am certain you'll be able to completely eliminate that stuttering engine problem.
It is normal that we don't always agree on the solution to a problem
and it seems to me that John wants to fix a nuisance and a potential problem that could someday when it is really hot develop an engine stopping vapor lock problem. I agree it is not a reason to stop flying the airplane but minimizing the potential for vapor lock by ensuring the fuel system runs cool is a smart course of action. These are inexpensive tweaks like heat shields and blast tubes that may achieve the desired effect of keeping the fuel from boiling.
Your injector lines benefit from cooling air in the upper plenum chamber, insulating them would probably make it worse. Of course they are heat soaked after sitting on the ground and airspeed is needed to cool them down along with everything else.
I'm thinking that keeping the fuel cooler before it reaches the spider might also help prevent it from getting vaporized.
You have it exactly right!

Don at Airflow is THE expert in fuel delivery systems and owner of Airflow Performance Injection systems. He is also a gentleman and all around nice guy to give advise on a competitors product.
You may ignore all the advise you've gotten so far but you can take his advice to the bank, he knows what he is talking about.
 
I am pretty certain at this point that you are correct. My big concern was whether or not it was a safety issue and I don't believe that it is. I intend to keep flying it, but I also intend to make the cooling modifications to try to reduce the issue. I feel like the baffling will help lower the CHT (I'm also looking at the "washer" mod) which seems to have the most direct impact. With the blast tubes I'm thinking that keeping the fuel cooler before it reaches the spider might also help prevent it from getting vaporized. I think just some basic, inexpensive changes to reduce temperatures can only be a good thing in the long run anyway, right?

I do have one question though: if the injector lines are exposed to the most heat since they are on top of the cylinders, why are they completely unprotected? Why not use fire sleeve or have some kind of cooling fins on them? Even some kind of heat sink on the spider itself seems like it would help keep the fuel cooler.

Thanks for all the help!
I think your right to try to improve it so long as you understand what it is and that you?re not going to cure it. Don suggests deleting the gascolator, good advice, a blast tube on the fuel pump might send the fuel to the servo a little cooler, that might help, anything you can do to make the engine run cooler, baffling etc. can?t hurt either. Whatever you do, do not tinker with your injector lines, do not wrap them or modify them in any way! There is even an AD out with the recommended installation of these lines to keep them from breaking from fatigue, I?m sure Mattituck installed them accordingly so don?t mess that up.
 
Brian, I had a similar issue when I first started flying mine. It ran great WOT and also when cold out...but when I pulled it back in the pattern it popped, bucked , spit and died completely several times. Kinda scary as a new RV pilot doing phase 1 testing and the engine dies as you turn base leg....although I learned to fly a pretty tight high pattern just in case. Much troubleshooting came to the conclusion that fuel was boiling in the injector lines. I also had a hot engine issue(CHT >400 & oil temp 215-220) that required step climbing to keep things in check.

I finally was able to seal up my plenum and clean up the outlet area for better air flow. This dropped the temps and now never see above 395 CHT and oil runs 190-200. Engine operation when hot is MUCH improved. It'll still get rough running after landing and have a long slow taxi.
 
I disagree, it is not normal and most don't have this problem and I am certain you'll be able to completely eliminate that stuttering engine problem.
It is normal that we don't always agree on the solution to a problem
and it seems to me that John wants to fix a nuisance and a potential problem that could someday when it is really hot develop an engine stopping vapor lock problem. I agree it is not a reason to stop flying the airplane but minimizing the potential for vapor lock by ensuring the fuel system runs cool is a smart course of action. These are inexpensive tweaks like heat shields and blast tubes that may achieve the desired effect of keeping the fuel from boiling.
Your injector lines benefit from cooling air in the upper plenum chamber, insulating them would probably make it worse. Of course they are heat soaked after sitting on the ground and airspeed is needed to cool them down along with everything else.

You have it exactly right!

Don at Airflow is THE expert in fuel delivery systems and owner of Airflow Performance Injection systems. He is also a gentleman and all around nice guy to give advise on a competitors product.
You may ignore all the advise you've gotten so far but you can take his advice to the bank, he knows what he is talking about.
That?s fine, but just to clarify, he is not having vapor lock at all, vapor lock is when the fuel pump cannot pump fuel because it?s lifting the fuel causes the fuel to boil ahead of the pump, at that point it is vapor locked and the engine will not run, operating the boost pump however will correct that problem and is one reason for running it while taking off. Fuel vaporizing in the injector lines will not keep the engine from running and will clear with the application of throttle.
 
Delete the gascolator.

Don

I'm a little nervous about removing the gascolator completely. I have found water in the tank drains from a leaky gas cap after washing the plane. I've since replaced the gaskets, but I like the idea of having an additional buffer for water in the fuel system. However, I do wonder how effective the gascolator would be. When fuel is flowing, I would think it would pull any water straight through along with the fuel. Are there any real disadvantages to removing the gascolator or only imagined ones?
 
To Gascolate or not?

The gascolator debate has gone on as long as the proverbial primer war and the forum is filled with opinions.
A gascolator in the wrong place (engine compartment) is simply a heat trap
as well as a notoriously common place for air leaks.
At least bypass it and see what happens, you'll want to remove it if it solves your problem.
Properly sealing fuel caps are the solution if water gets in your tanks, you already did that and draining the sumps is required pre take off procedure.
 
This morning I went out and sealed the gap between the cowling and the baffle and used some RTV to fill a bunch of small leaks around the baffle. I don't have time to do anything with the gascolator right now, so for now I will just add a blast tube and see what happens.

I was able to get a really good seal between the air inlet and the baffle by doubling up the rubber gasket material and putting the fold just a little past the edge of the cowling. It naturally expands out and pushes against the bottom creating what appears to be a really good seal, but doesn't interfere with removing the bottom cowling. Doubling it up also makes it stiffer so it should have more resistance to being pushed down by the incoming air.

http://www.flickr.com/photos/99537167@N02/9394930810/in/photostream/
 
Last edited:
Gascolator

This is a letter we wrote to a customer about gascolator use in fuel injected engines.

To fully understand the situation you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.

Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.

One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.

Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.

1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated.


Hope this helps

Don
 
That does help a lot, thanks! I guess the only question I have then is if I remove the gascolator, do I need to add fuel filter somewhere to catch sediment? I know there is one in the fuel injection servo, is that all I need?

This is a letter we wrote to a customer about gascolator use in fuel injected engines.

To fully understand the situation you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.

Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.

One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.

Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.

1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated.


Hope this helps

Don
 
For now I've added a blast tube to the gascolator. If I remove it entirely I will re-purpose that to the fuel pump.

This is a letter we wrote to a customer about gascolator use in fuel injected engines.

To fully understand the situation you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.

Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.

One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.

Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.

1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated.


Hope this helps

Don
 
Installation for fuel injected engines

We recommend that the boost pump be installed on the cold side of the firewall also. There should be some type of fuel filter before the boost pump to protect the boost pump.

Don
 
My boost pump is installed directly in front of the wing spar between the seats. I'm not the builder, but I looked at your website and I think I may have your boost pump and fuel filter. My plan is to fly with the modifications I've already made for now and remove the gascolator and add a cooling shroud and blast tube for the fuel pump at the next annual. Because of the way it's installed, I can't bypass it without removing it from the firewall and I don't have time to do that right now. Thanks for all the help!

We recommend that the boost pump be installed on the cold side of the firewall also. There should be some type of fuel filter before the boost pump to protect the boost pump.

Don
 
Slightly off topic, a question for the folks who see this behavior with their engines -- what sort of OAT's do you see it at? I have heard much discussion of this happening on "hot" days, but I have encountered it on ground and in flight at anything over ~55F, which is a stretch to call "hot." The stutter is much as described in the OP. Disappears with higher fuel flows above 1200RPM, but the engine feels like it may quit at any moment on the ground (and has, three times now...I tend to taxi with more throttle than I'd like as a result.)

This is really the only problem I've had with my 9A...

IO-320, ECI from Aerosport.
AFP Injection system.
Catto 3 blade prop.
Vetterman muffler exhaust.
Andair pump and filter, which has been serviced along with the servo inlet filter.
Boost pump on/off makes no difference.
Red cube moved to cabin between pump and FW.
FWF fuel lines shortened as much as possible.
Engine fuel pump has been replaced, and blast tube w/shroud installed.
Servo to spider line covered with heat rejection sleeve.
Never had cooling problems on engine...oil temps work hard to get much above 180, and typical cruise CHTs average 300 in current weather.

I've just accepted it as normal so far, but it's bad enough that I would hesitate to take it to OSH as the long taxi would be tough to manage wrt throttle and brakes. I imagined that "hot" would be OAT 85+...
 
Last edited:
You don't give me much hope for fixing it on mine, but I am starting to wonder if we see this problem more often with wood props because a heavy metal prop might just spin right through it without much of a hiccup. I have the Catto 3 blade and one of the A&Ps across from my hangar mentioned that the Long EZ next door also does it a little bit and he has a Catto 3 blade too.

Slightly off topic, a question for the folks who see this behavior with their engines -- what sort of OAT's do you see it at? I have heard much discussion of this happening on "hot" days, but I have encountered it on ground and in flight at anything over ~55F, which is a stretch to call "hot." The stutter is much as described in the OP. Disappears with higher fuel flows above 1200RPM, but the engine feels like it may quit at any moment on the ground (and has, three times now...I tend to taxi with more throttle than I'd like as a result.)

This is really the only problem I've had with my 9A...

IO-320, ECI from Aerosport.
AFP Injection system.
Catto 3 blade prop.
Vetterman muffler exhaust.
Andair pump and filter, which has been serviced along with the servo inlet filter.
Boost pump on/off makes no difference.
Red cube moved to cabin between pump and FW.
FWF fuel lines shortened as much as possible.
Engine fuel pump has been replaced, and blast tube w/shroud installed.
Servo to spider line covered with heat rejection sleeve.
Never had cooling problems on engine...oil temps work hard to get much above 180, and typical cruise CHTs average 300 in current weather.

I've just accepted it as normal so far, but it's bad enough that I would hesitate to take it to OSH as the long taxi would be tough to manage wrt throttle and brakes. I imagined that "hot" would be OAT 85+...
 
no luck

Unfortunately improving the baffling and adding the blast tube really didn't help at all. My CHTs are maybe down 5 or 10 degrees on average, but it's hard to say for sure.
 
Hot idle problems.

Yes, of course no weight on the crank doesn't help the hot idle problem either. But you have some installation issues to clean up yet. Remove the gascolator from the firewall, remove any pressure drops from the suction side side of the pumps. The flow meter should be in the metered fuel line.

If all that does not help then the last resort is to install smaller nozzle restrictors to raise the nozzle back pressure. This will not completely cure the problem but usually helps. We need to know the take off fuel flow and inlet fuel pressure to calculate the nozzle size that can be installed.

Don
 
Spark Plugs.....Hmmm

Bj, Just couldnt help but notice this comment on the Champion fine wire plugs.
I never liked them. They have a screw in the center of the plug where the connection (electrical) is made. Below it is a spring and a formed resistor. The resistance from the electrode end and that screw should be 1K ohms to maybe
2K Ohms, that s it. I have found a number of plugs that have 20K and greater resistance which gives you a "miss", but probably not like what you are seeing. What if many of these plugs have this high resistance, Hmmmm... High temperature, greater resistance. I only use Tempest now on my 180Hp Lyc and REM40E only. These have a sealed resistor which reads 1K no matter how many hours I put on them. Take a few plugs out and check, can't hurt.
 
I'm not sure what you mean by the "metered" fuel line. Do you mean it should be installed between the injection servo and the spider? Mine is installed after the boost pump before the gascolator. I'm going to be in Atlanta at least through the weekend, maybe I can stop up there.

Yes, of course no weight on the crank doesn't help the hot idle problem either. But you have some installation issues to clean up yet. Remove the gascolator from the firewall, remove any pressure drops from the suction side side of the pumps. The flow meter should be in the metered fuel line.

If all that does not help then the last resort is to install smaller nozzle restrictors to raise the nozzle back pressure. This will not completely cure the problem but usually helps. We need to know the take off fuel flow and inlet fuel pressure to calculate the nozzle size that can be installed.

Don
 
I'm not sure if they are champion or tempest, but since it has the LASAR system I'm guessing they're Tempest.

Bj, Just couldnt help but notice this comment on the Champion fine wire plugs.
I never liked them. They have a screw in the center of the plug where the connection (electrical) is made. Below it is a spring and a formed resistor. The resistance from the electrode end and that screw should be 1K ohms to maybe
2K Ohms, that s it. I have found a number of plugs that have 20K and greater resistance which gives you a "miss", but probably not like what you are seeing. What if many of these plugs have this high resistance, Hmmmm... High temperature, greater resistance. I only use Tempest now on my 180Hp Lyc and REM40E only. These have a sealed resistor which reads 1K no matter how many hours I put on them. Take a few plugs out and check, can't hurt.
 
Check

No time or $ for OSH. I'll be in the shop, just give me a call if your flying over.
The flow meter should be in the metered fuel line, that is, between the fuel control and the flow divider.

Don
 
This thread has caught my interest, plus I'm new to the forums.

I have a Mattituck XP-360 (IO-360B1A2), dual slick mags, C/S Hartzell 2-blade, Sam James plenum, Bendix RSA-5 fuel servo. I have a Mooney-style 90 degree butterfly valve that allows either filtered air or direct intake ram air. I've had the airplane 10 years and it has logged 400 hours since new; I was not the builder (2nd owner).

My engine performs great from takeoff to touchdown but experiences a similar RPM "hunting", 40-50 RPM on the ground, frequently on hot days. I've never worried that it would quit, but it sounds terrible and is embarrassing during a long, hot taxi in. Just like the previous poster, mixture & boost pump don't seem to affect the problem. My inflight CHTs stabilize between 330-380, oil temps 185-200 depending on season & OAT.

- I've had the fuel servo rebuilt - no change.
- I've sealed gaps around the plenum - no change.
- Tried fire sleeve on the injector lines - no change.
- Van's gascolator, which I am ready to remove.

Here is where my problem goes beyond erratic idle: My engine will not shut down with the mixture at I/CO on hot days once oil temps are above 210F. Increasing throttle doesn't seem to help much. At its worst, the engine may appear to stop and then spin several revolutions backwards before coming to a stop. I had just about decided to start over with a more traditional baffle system when I discovered this thread. The plenum seems to work great for X/C flying but is a nightmare for summertime ground ops.

I like the idea of a fuel pump blast tube, but my problems seem a bit more complex than the original poster. Suggestions welcome.

Ken
 
I'm headed to Spartanburg tomorrow to have Don take a look at mine. I'll let you know how it works out.

This thread has caught my interest, plus I'm new to the forums.

I have a Mattituck XP-360 (IO-360B1A2), dual slick mags, C/S Hartzell 2-blade, Sam James plenum, Bendix RSA-5 fuel servo. I have a Mooney-style 90 degree butterfly valve that allows either filtered air or direct intake ram air. I've had the airplane 10 years and it has logged 400 hours since new; I was not the builder (2nd owner).

My engine performs great from takeoff to touchdown but experiences a similar RPM "hunting", 40-50 RPM on the ground, frequently on hot days. I've never worried that it would quit, but it sounds terrible and is embarrassing during a long, hot taxi in. Just like the previous poster, mixture & boost pump don't seem to affect the problem. My inflight CHTs stabilize between 330-380, oil temps 185-200 depending on season & OAT.

- I've had the fuel servo rebuilt - no change.
- I've sealed gaps around the plenum - no change.
- Tried fire sleeve on the injector lines - no change.
- Van's gascolator, which I am ready to remove.

Here is where my problem goes beyond erratic idle: My engine will not shut down with the mixture at I/CO on hot days once oil temps are above 210F. Increasing throttle doesn't seem to help much. At its worst, the engine may appear to stop and then spin several revolutions backwards before coming to a stop. I had just about decided to start over with a more traditional baffle system when I discovered this thread. The plenum seems to work great for X/C flying but is a nightmare for summertime ground ops.

I like the idea of a fuel pump blast tube, but my problems seem a bit more complex than the original poster. Suggestions welcome.

Ken
 
Airflow Performance @ KSPA

After landing head west down the taxi way to the end. We're the last hanger. I'll have the gate open in the morning.

Don
 
I'm headed to Spartanburg tomorrow to have Don take a look at mine. I'll let you know how it works out.

Definitely keep us posted, I'm hoping to learn something vicariously! If I was based nearer to Don's shop, I likely would have hired him to take a look at it months ago.
 
If I find that it is not feasible to place a fuel filter in line prior to the electric boost pump, are there any specific recommendations on quality/durable fuel filters placed in the space about to be vacated by the gascolator?

Ken
RV-8
 
Back
Top