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Fuel Pump Issue

yeah, but Greg---your system isnt exactly stock:eek:, but we know it works.
Tom

Yes, and yes. It was a just a datapoint on the venting, if my system vents just fine at that altitude with all the plumbing pressure drops I have, then it's not likely that someone else with a stock setup would have an issue, unless they have damaged tube/pipe.
 
Thread resurrection time!

I posted this at the end of another thread today but decided after further research it would be more appropriate in this 11-page classic :p

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RV-10 in early phase 1 with IO-540 with AFP mechanical injection (no return lines) , Andair valve, EFII boost pump and red cube in the tunnel, 40 micron JEG's billet filters under the seat pans between each tank and the selector valve which were just inspected and cleaned after the first flight two weeks ago. Since the first flight on 100LL, running 93 octane no-ethanol mogas. No shroud/blast tubes on pump. One (right) heat muff and stainelss valve, left side muffler has a 60% radiant heat shield from Vetterman but no hoses or firewall flapper valve for heat. Y-connector in tunnel for front and rear heat from the ingle muff. Tunnel does not feel hot-- I did a fibrefrax and SS foil sandwich on the firewall.

All flights have been nominal until today, 3 hours into phase one, when in level cruise 80% power 16.5 gph indicated about 15 minutes into the lazy circles at 4000' I got the "Check fuel pressure!" command and saw 14 psi and falling. Boost pump brought it back to 25 psi immediately. This behavior repeated itself several times on three flights today and has seemed to increase in severity. It will drop very slowly to 5 or 6 psi. One time it caught me off guard and threw all 6 cylinders into the EGT red zone on the Advanced monitor and the engine lost power. (I hope it went quickly through the "red box" detonation zone because I was cruising at 80% power for break-in when it happened.)

I've had enough flight time today to narrow a few things down about this. It only happens on the left tank. The boost pump typically builds the pressure back quickly but the pressure has dropped to critically low values (although much more slowly) on the left tank even with the boost pump running. Switching to the right tank relieves the low pressure immediately and lastingly (or else I wouldn't be writing this until after SAR plucked me out of the Virginia mountains). Selecting Right tank fixes the issue every time, with or without the boost pump running.

The behavior certainly makes me think of a partly obstructed vent line on the left tank pulling vacuum on it gradually, as I can repeat the cycle over and over again. After running on the right tank awhile I can switch back to the left with no boost pump and it will take a minute or so before the pressure drop repeats itself. Back on the ground, I've pulled wing root panels and swiveled the vent lines around to where I can blow into them - and with the fill caps loose I feel like it takes the same effort to blow air through either side. As others have noted, there is considerable resistance to blowing through that length of 1/4" tubing!

Maybe it's a filter that needs cleaning and it takes a while for the engine driven pump to cavitate after switching over to the side with a partial filter blockage.

I plan to chase the vent lines with weed eater monofilament and be sure there's no plugging material. I'll do fuel flow tests again on both tanks tomorrow (initial values were 40 gph both sides before first engine runs). I might even fly with a temporary perforated tape covering over the filler neck on the left side to assure that tank is adequately vented.

May end up with my transducer getting relocated to the FWF area...

I did verify today that fuel flow with the boost off is a nominal (as-yet uncalibrated) 16.1 gph in full rich cruise and 16.3 gph (once it stabilizes) with the boost pump on.

Seems like I'm in good company, here. Lots of work ahead: testing, measurement, shrouds, new vent locations, maybe even abandoning mogas :(

I appreciate DanH's opinion that leaving the boost pump on continuously is merely emulating the norm in the automotive world, but it still makes me uneasy being dependent on that thing. And I've seen one pressure dip today that happened on the left tank while the boost was already on. That part concerns me greatly.
 
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Seems that this low indicated fuel pressure hasn’t/isn’t resulting in engine cut out. (and I’m certainly not advocating turning off the electric boost pump to see if that happens) Seeing that we understandably fixate on any abnormal pressure and/or temperature readings when flying (especially on first flights) usually the actual senders/probes are the first port of call. The makers of the red cube that we are mostly using to indicate fuel pressure and flow rates, recommend that the cube is fitted after the mechanical fuel pump in the fuel system, and following that, a lot of 10 builders have fitted the cube after the mechanical pump and before the fuel servo and also between the servo and the divider/spider. Vans on the other hand recommends, or at least they did recommend fitting the red cube in the tunnel after the electric fuel pump but before the mechanical pump. Be interesting to know the location of the Red Cube in the Aircraft that are experiencing the low indicated fuel pressures.
 
My fuel flow transducer is at this point in the standard location, following the electric pump in the tunnel. The fuel pressure transducer is the newest design supplied by Dynon/AFS, and I have no reason to suspect it isn't dead-on. The engine gets appropriately unhappy when the readout drops below 7-8 psi.

I would like to point out that I had a severe power reduction with all EGT's spiking and the engine just about cutting off in cruise yesterday while playing with the back-and forth tank switching to sort this problem out... I got "distracted" looking out the window for traffic and doing basic VFR navigation and wasn't fixated on the fuel pressure readout :rolleyes: - not sure why I didn't hear the "check fuel pressure!" in my ear that time - maybe I had tuned her out after the dozen other times...

This is a safety of flight issue and could send an engine into detonation as well as shut it down outright. I suspect it's more like a quick-pull of the mixture, but no one advocates doing that at 80% power as far as I know.
 
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I used to have fuel pressure drops on climb out and sometimes right after climb in cruise. My fuel system was stock with the exception of the FF transducer between servo and spider. Also I had added a 90 degree fitting where the fuel line leaves side of fuse. I decided it had to be turbulence and air vapor in the 90 degree fittings. I fabricated two new aluminum lines without any 90 degree fittings. One runs from the tank outlet to under the seat, and the other from under the seat to the Andair fuel selector. They are connected with a straight coupler under the seat.
This totally eliminated my problem. It is my belief that anything you insert, including 90 degree fittings, on the suction side of the fuel system is asking for trouble, the FF cube included.


It does sound like you may have blockage in your air vent
 
Bill,

If you are getting pressure drops one minute after switching back to the left tank, a venting issue seems improbable. Given that you can blow through the vent line, there is at least some air movement and it should take much longer than a minute to introduce enough vacuum to create a problem. Several people have actually drawn enough vacuum to dent their tanks and the engine was still running fine. If the tanks were full and had fuel in the lines, it would be a bit more probable, as you need to pull a slight vacuum to suck the fuel through the vent line. However, you pump is routinely pulling fuel against an 18" head pressure, so this type of work is well within the norm for a pump.

I would be looking for small leaks that are introducing air bubble or possible restrictions in the lines and filters on the left side upstream of the selector. Also check the outlet screen inside the left tank.

Larry
 
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Not sure how to help this thread, BUT in testing for our wing root filter option, we did a flow test on some 90* AN adapters--AN822-6D. Steve and I bought this unit. Now, full dislcaimer, it is NOT as accurate as Don's flow bench at Air Flow Performance, but you can definitely see the difference in the flow meter of 1 liter per hour. Not a real scientific test but served the perpose.

As you can see, we used a EFII standard injection pump, not the one for the duplex system, only because it maxed out at 30psi. So I did a free flow of just the pump at 30 psi and got a baseline of 150 liters/hour (39.625 gal/hr). I wasnt concerned whether the actual flow volume was correct, but establishing a baseline. The I tested the same set up with a 90* AN fitting on the suction side, and got virtually the same result, +- .5 liter/hr. I then moved the adapter to the discharge side, repeated the test and got the same thing.

Lastly, I took our prototype root filter plumbing, and put it on the suction side and retested. I got virtually the same result,+- .5 liter/hr. To verify the flow meter was actually reading, I increased and decreased the pressures to see the changes. Definite reading change.
So my conclusion, again not scientific but ok for our purposes, was that a true 90* AN fitting had negligible flow differences versus a straight one.

Maybe one of these days I'll make a trip to Don's and get some real cerified flow data from his bench.

IN the case of this thread---it appears that having 90*s in the system doesnt necessarily affect the flow. Seems some aircraft have an issue, and others of the same model and similar equipment dont. I think it comes down to installation, and not necessarily the design of the system.

Tom
 

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I have observed similar fuel pressure loss with mo-gas (in a dissimilar fuel system - carbureted RV-6). I believe my situation is caused by cavitation (vapor) on the suction side of the pump. Given your verified vent operation I would suspect this as a root cause in your case also. In my aircraft the symptoms are aggravated by:
1). Warm fuel in the tanks (sunshine heat soak, hot day)
2). High power settings (high fuel flow)
3). Nose high attitude (low speed high deck angle climb) resulting in higher static head for the pump to overcome.

Even in challenging conditions, I have never observed the problem with 100LL.

You might consider:
Fueling with 100LL in the left tank to confirm the auto-gas link.
Seeing (at altitude) if a nose high attitude induces similar problems on the right tank (w/ auto-gas).

Perhaps subtle differences in plumbing, filter condition, etc induce vaporization from your left tank more easily than the right.

If this is your problem, anything that creates a pressure drop upstream of the pump is an opportunity for improvement (vent flow, filters, pressure drop in lines and fittings, flow measurement, etc). Also any high temp locations on the suction side could contribute.

Hope this helps, please be careful in your further testing.

Peter

I posted this at the end of another thread today but decided after further research it would be more appropriate in this 11-page classic :p

**************
RV-10 in early phase 1 with IO-540 with AFP mechanical injection (no return lines) , Andair valve, EFII boost pump and red cube in the tunnel, 40 micron JEG's billet filters under the seat pans between each tank and the selector valve which were just inspected and cleaned after the first flight two weeks ago. Since the first flight on 100LL, running 93 octane no-ethanol mogas. No shroud/blast tubes on pump. One (right) heat muff and stainelss valve, left side muffler has a 60% radiant heat shield from Vetterman but no hoses or firewall flapper valve for heat. Y-connector in tunnel for front and rear heat from the ingle muff. Tunnel does not feel hot-- I did a fibrefrax and SS foil sandwich on the firewall.

All flights have been nominal until today, 3 hours into phase one, when in level cruise 80% power 16.5 gph indicated about 15 minutes into the lazy circles at 4000' I got the "Check fuel pressure!" command and saw 14 psi and falling. Boost pump brought it back to 25 psi immediately. This behavior repeated itself several times on three flights today and has seemed to increase in severity. It will drop very slowly to 5 or 6 psi. One time it caught me off guard and threw all 6 cylinders into the EGT red zone on the Advanced monitor and the engine lost power. (I hope it went quickly through the "red box" detonation zone because I was cruising at 80% power for break-in when it happened.)

I've had enough flight time today to narrow a few things down about this. It only happens on the left tank. The boost pump typically builds the pressure back quickly but the pressure has dropped to critically low values (although much more slowly) on the left tank even with the boost pump running. Switching to the right tank relieves the low pressure immediately and lastingly (or else I wouldn't be writing this until after SAR plucked me out of the Virginia mountains). Selecting Right tank fixes the issue every time, with or without the boost pump running.

The behavior certainly makes me think of a partly obstructed vent line on the left tank pulling vacuum on it gradually, as I can repeat the cycle over and over again. After running on the right tank awhile I can switch back to the left with no boost pump and it will take a minute or so before the pressure drop repeats itself. Back on the ground, I've pulled wing root panels and swiveled the vent lines around to where I can blow into them - and with the fill caps loose I feel like it takes the same effort to blow air through either side. As others have noted, there is considerable resistance to blowing through that length of 1/4" tubing!

Maybe it's a filter that needs cleaning and it takes a while for the engine driven pump to cavitate after switching over to the side with a partial filter blockage.

I plan to chase the vent lines with weed eater monofilament and be sure there's no plugging material. I'll do fuel flow tests again on both tanks tomorrow (initial values were 40 gph both sides before first engine runs). I might even fly with a temporary perforated tape covering over the filler neck on the left side to assure that tank is adequately vented.

May end up with my transducer getting relocated to the FWF area...

I did verify today that fuel flow with the boost off is a nominal (as-yet uncalibrated) 16.1 gph in full rich cruise and 16.3 gph (once it stabilizes) with the boost pump on.

Seems like I'm in good company, here. Lots of work ahead: testing, measurement, shrouds, new vent locations, maybe even abandoning mogas :(

I appreciate DanH's opinion that leaving the boost pump on continuously is merely emulating the norm in the automotive world, but it still makes me uneasy being dependent on that thing. And I've seen one pressure dip today that happened on the left tank while the boost was already on. That part concerns me greatly.
 
I just got back from snaking the fuel tank vent lines with .090" weed trimmer line and neither showed any evidence of an obstruction.

I had extensive conversation with Steve Tschurwald at Aircraft Specialty last week when my issue was repetitive clogging of the same injector restrictor with fine debris and we discussed my under-seat filters from Summit/JEG's at length - they are finer than spec for any injection system at 40 microns but don't have a very large filter surface. They are rated for plenty of flow, but that's at pressure, not on the suction side... At the time I was looking for ways to go finer, trying to block this crud from reaching the injectors. It seems to have all been flushed out now.

Given the new issue with loss of pressure, I think a course reversal in my thinking may be appropriate: go to a coarser filter between each tank and the selector valve to reduce flow resistance and likelihood of cavitation under suction - and fashion it with an eye toward easier access and maintenance than the billet filters currently provide.

I'll be reaching out to Steve today to bat some ideas around.
 
Some more real world experience:
7A. IO360 Precision Airmotive Injection. Original Lycoming fuel pump. EZ Flo Fuel filters in wing roots. Began with 70 micron. Have reduced to 40 micron 250 hours ago. I take off without boost pump on, for at least the last 300 hours. Pressure stays at 25-27 thru all phases, either tank. Climbs to 12.5 no problem. (Hi as I go) I use boost pump for cold start only. 100LL AvGas only.
 
boost pump

... I take off without boost pump on, for at least the last 300 hours. Pressure stays at 25-27 thru all phases, either tank. Climbs to 12.5 no problem. (Hi as I go) I use boost pump for cold start only. 100LL AvGas only.
Just curious, why not use the boost pump on takeoff? I also have mine on for landing - one less thing to worry about close to the ground.
 
they are finer than spec for any injection system at 40 microns but don't have a very large filter surface. They are rated for plenty of flow, but that's at pressure, not on the suction side... At the time I was looking for ways to go finer, trying to block this crud from reaching the injectors. It seems to have all been flushed out now.


It is hard to overstate the problems associated with insufficient suction pressure on a pump. In my career, I have lost count of the times that I have found the root cause of an apparent pump performance problem to be actually poor suction piping design. The net positive suction head required (NPSHR) is an extremely important data point for any pump in a system, but oddly, is often not provided for pumps other than those intended for industrial service.

If you have the ability to install temporarily a pressure gauge in the suction line ahead of the pump you will likely see that the suction pressure is too low, possibly due to installing restrictions (filters) in the pump suction lines.

Especially for volatile fluids, ensuring adequate suction pressure is key for guaranteeing that a pump will perform to its design (or indeed at all).
 
Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.

Definite improvement related to vapor pressure, I assume...

Steve and Tom are making hoses for me to plumb the left side with a flex line from the tank straight to a Holley paper element 10 micron filter under the seat and another flex line through the tunnel side to the Andair valve, eliminating two fittings, and with them the possibility of cracked flares.

In the meantime, I'll be flying with avgas or a high avgas blend - which I certainly hope is only a temporary necessity.
 
What is alittle puzzleing about Bills situation, although easier to run down, is that both left and right sides have the same type of plumbing, filters, tanks, and I assume vents. Everything forward of the Andair selector valve is common to both subsystems, and there is no apparent issues with the right side. So that eliminates the pump forward. He is going to send us the original plumbing so we can inspect it, but quite honestly I'd be surprised if we find anything there.
Obviously cant take apart the tank, and I dont know the history. But eventually we'll get the root cause.

Tom
 
Tank built by an other owner before me (his second RV-10) and done well enough that it doesn't leak - although the other side has a slow leak through what I must assume is a loose B-nut inside on the vent line bulkhead fitting and out the vent, when the fuel level is high enough. The misbehaving side does not have this issue.

I've opened the under-seat filter on the left side and cleaned a tiny amount of tank debris out of the stainless mesh element before this issue started making itself known but not since. Will be interesting to see if there is anything new in there when Tom receives it as-is.
 
Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.

Definite improvement related to vapor pressure, I assume...

Steve and Tom are making hoses for me to plumb the left side with a flex line from the tank straight to a Holley paper element 10 micron filter under the seat and another flex line through the tunnel side to the Andair valve, eliminating two fittings, and with them the possibility of cracked flares.

In the meantime, I'll be flying with avgas or a high avgas blend - which I certainly hope is only a temporary necessity.

After Phase 1 flight testing in my RV-10 I ran one tank of ethanol free mogas with the other tank 100 LL exclusively. I never experienced a difference in fuel pressure when I swapped tanks during flight. I did use a fuel pump cooling shroud and insulated the bottom of the tunnel to reduce tunnel heat. I am not a fan of paper media fuel filters at all in airplanes.
 
I did the mogas/avgas separate tanks thing for a number of years in my 6A while I developed confidence in mogas as a fuel - and never had issue one with it in the O-320. I can do so again.

I think the filter inbound from Steve is a stainless mesh 100 micron, but the Holley inserts are interchangeable I believe between that and 10u paper. I'd prefer to start with the looser mesh to minimize flow resistance.

My tunnel is barely warm to touch in flight.

Since the boost pump has been unable to prevent the pressure loss consistently when feeding from the left tank using mogas, I doubt the issue is amenable to a fix by cooling the engine driven pump, but I'm not ruling it out either.
 
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After Phase 1 flight testing in my RV-10 I ran one tank of ethanol free mogas with the other tank 100 LL exclusively. I never experienced a difference in fuel pressure when I swapped tanks during flight. I did use a fuel pump cooling shroud and insulated the bottom of the tunnel to reduce tunnel heat. I am not a fan of paper media fuel filters at all in airplanes.

As Bill mentioned below...the filters that we utilize are all high quality with a stainless mesh filter.
 
Have you noticed anything with regards to the rate of pressure loss versus fuel consumption? You'll always see increasing vacuum as measured at the pump suction with increasing flow. If you've got a moderate obstruction on one side, you should clearly see a difference in the vacuum between the two sides, but also should find that the severity your problem varies with fuel flow rate. Clearly not a solution to the problem, but measuring that vacuum and how it responds to different conditions should confirm whether this is a restriction issue or not. If on the other hand the vacuum is lower (closer to ambient pressure) on the problematic side, you have a leak and you're drawing air.

When it comes to measuring vacuum, you also may want to consider using a differential gauge/sensor (other side open to atmosphere) rather than a single port. With single port you'll need to consider your current density altitude when comparing values.

Best luck getting to the bottom of this.
 
Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.

Definite improvement related to vapor pressure, I assume...

One thing to consider, is that fuel tank fill height will change that amount of suction required to pull fuel. The fuel pump draws a good bit less suction when pulling fuel from a full tank than a near empty tank. Something to consider when reviewing symptoms. I would expect a small air leak to show more significant symptoms on an empty tank compared to a full one.

Larry
 
Have you noticed anything with regards to the rate of pressure loss versus fuel consumption? You'll always see increasing vacuum as measured at the pump suction with increasing flow. If you've got a moderate obstruction on one side, you should clearly see a difference in the vacuum between the two sides, but also should find that the severity your problem varies with fuel flow rate. Clearly not a solution to the problem, but measuring that vacuum and how it responds to different conditions should confirm whether this is a restriction issue or not. If on the other hand the vacuum is lower (closer to ambient pressure) on the problematic side, you have a leak and you're drawing air.

When it comes to measuring vacuum, you also may want to consider using a differential gauge/sensor (other side open to atmosphere) rather than a single port. With single port you'll need to consider your current density altitude when comparing values.

Best luck getting to the bottom of this.

I have not done reduced flow rate pressure testing - purposefully avoiding prolonged ground runs with CHT 300* upper limit and running >=75% power all the time for engine break-in, except in the pattern which is not the best place for inducing an expected loss of fuel pressure. As things stand I will probably have the left side plumbing swapped out before the break-in is completed.

I don't anticipate that ground-based fuel flow testing using only the boost pump would tell me what I need to know, but it might...

I am hatching a scheme in my head today at work - to use my garden sprayer brake bleeder rig to pressurize the fuel lines and leak test with soap solution before I dismantle the plumbing and send it to Tom.
 
One thing to consider, is that fuel tank fill height will change that amount of suction required to pull fuel. The fuel pump draws a good bit less suction when pulling fuel from a full tank than a near empty tank. Something to consider when reviewing symptoms. I would expect a small air leak to show more significant symptoms on an empty tank compared to a full one.

Larry

YES. The suction head available is related to the head of liquid in the tank. The problem should be worse when the tank level is low and it might appear to be non-existent with a full tank.

A combination of a filter introducing an excessive dynamic head loss (pressure drop when flowing) and a low tank level will result in too low a suction head at the pump.

This will be particularly evident for a positive displacement pump, like the engine driven fuel pump. If the suction head is too low, a volatile fluid such as gasoline will tend to vaporize on the pump's intake stroke.

A pump with an internal recirculation may not show this, as some fluid on the discharge side will return to the suction side and increase the average suction pressure. Think of a jet style well pump here. I'm not certain of the configurations of the various electric fuel pumps, but it's likely they have some sort of internal recirculation setup to allow flow past the pump if it fails/is turned off.
 
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