Paul Eastham

Paul Eastham

Well Known Member
This has happened to me three times now, wondering if other people have experienced this.

Carbureted engine w/ stock Facet boost pump.

On a hot day, after flying around a bit, shutting down, and then preparing to restart shortly after, my fuel pump does that really loud "I'm pumping air" banging noise prior to engine start. It doesn't go away on it's own, I usually just turn the pump off, start the engine normally, then verify that it's gone back to it's usual quiet hum after starting.

I guess fuel is boiling in the gascolator (just upstream from the pump), and the vapor is stuck at the pump since the engine is not running?

The concern, I guess, is vapor lock that bites me right after takeoff.
I have a blast tube on the gascolator but obviously that doesn't do much on the ground. All fuel lines firesleeved.

Thoughts? Speculation? :)
 
pumpin' air

Had that happen to my 9A this weekend after sitting in the hot hangar for a couple of days. Boost pump on and a gascolator sump, and pump was normal. As soon as I started the engine, all was well. No problems in flight, but I will monitor this issue.

Chris
 
Paul, I think you are right with your assumption. The pump is trying to compress vapor down stream and may be getting a strong back pressure pulse causing the noise? As the engine consumes the hot fuel on take idle / take off it cools down and the noise goes away. The heat soaked engine compartment (after shut down) is not something you can really get away from even with.

Do you have a mechanical fuel pump attached to the engine? What is your backup fuel pump?
 
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I had the exact same thing happen to me one time in my former-9A. Hot humid day and stopped for a 5 minute weather check and restroom relief break, got back in and same issue. It started fine anyways and as you did I verified the electric pump could increase pressure once the engine was running and then all was fine.

Scott
 
Living in TX where it's mucho hot all summer I experienced this same condition quite regularly so I elected to install a purge valve (primer valve) off the carb to purge the boiling fuel from the mechanical pump and lines. I had the foresight to install a purge line back to the tank when I was building for this reason and was glad I did. A flick of the purge switch and you could hear the pump come back to the normal pumping sound. I ran this system for 4 years and 800 hrs without a hitch.

Now that I've completed the switch over to the AFP injection, the same line is used for the purge valve it incorporates, a very smart decision on the part of AFP and part of the reason I chose their system. I might also add that I am extremely pleased with the perfomance and support from AirFlow Performance, great folks and products. I always try to run LOP and the new injection makes it all that much smoother. I did run LOP with the carb but it was not as smooth as the AFP is now. LOP was how I justified the cost to switch from carb to FI, I figured if I could save 1 gal/hr in a 1000 hrs the system will have paid for itself.

I am still in the process of tuning the nozzles and just switched to the .025 nozzles this weekend which seems to have improved both the hot idle issue and closed the gap on the EGT/fuel flow, probably less than .4 GPH spread now but I am looking for .2GPH before I'm done!

PS: If anybody is looking for a M4-5-4164 carb, fuel pump, Andair gascolator, primer/purge valve let me know.
 
I agree

The purge valve works really well..I have the AFP setup and hardly notice any difference between a cold start and a hot start after giving it a jolly purging..

If that doens't work giving a "jolly good talking to" does..:)

Frank
RV7a IO360 now featuring mogas +10% ETOH
 
Yep, purge value works great from Air Flow Performance. Both the throttle and the mixture control have to be full open for it to work though. It is nice to know the fuel system is cooled down before a "quick turn" hot take off.
 
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Purging with full tanks...

I have an AFP FI system with a purge valve in my -4. A typical flight for me is to an airport with cheaper gas than my home base. I shut down, fill-up and re-start. On a hot day, I would like to pump some fuel through the purge valve to help cool the system, but cannot as both tanks have just been filled. I never seem to remember to "short-fill" the return tank...
 
Todays cars never experiance vapor lock??!

I live in an area that normally does not get above 30-35?C. but I have been in a lot hotter places. Last year I visted Dubai, Abu Dhabi, Qatar, Bahrein and Saudi Arabia. Hottest temperature was 51?C (124?F). Even hot starts were no issue for the Honda's/Datsun's/Mazda's that I rented and that is with Mogas! (sealevel though!) :)

Why is it that cars do not have a vapor lock problem? :confused:

I plan on running mogas and therefore have a return line installed, with a return from the Precision Fuel Injection, back to the tank, to return vapor and/or hot fuel. Also the steady flow of fuel should keep it cool (no time to heat up) :cool:

Anyway, so far I have not planned any trips to Egypte or the Middle East, but because of global warming, I am not sure what our average temerature will be by the time I am finnished building this darn plane. :eek:

Regards, Tonny.
 
Ah??

Dean Pichon said:
I have an AFP FI system with a purge valve in my -4. A typical flight for me is to an airport with cheaper gas than my home base. I shut down, fill-up and re-start. On a hot day, I would like to pump some fuel through the purge valve to help cool the system, but cannot as both tanks have just been filled. I never seem to remember to "short-fill" the return tank...
Click to expand...

I guess thats another advantage of the electric pumps in the wingroots design.

I have my return piped to the right tank, so i simply turn on the right wingroot pump and circulate away...The tank cannot over fill because it solely recirculating through the right tank.

So I can fill both of mine to the brim.

Frank
 
here we go..:)

Pilottonny said:
I live in an area that normally does not get above 30-35?C. but I have been in a lot hotter places. Last year I visted Dubai, Abu Dhabi, Qatar, Bahrein and Saudi Arabia. Hottest temperature was 51?C (124?F). Even hot starts were no issue for the Honda's/Datsun's/Mazda's that I rented and that is with Mogas! (sealevel though!) :)

Why is it that cars do not have a vapor lock problem? :confused:

I plan on running mogas and therefore have a return line installed, with a return from the Precision Fuel Injection, back to the tank, to return vapor and/or hot fuel. Also the steady flow of fuel should keep it cool (no time to heat up) :cool:

.
Click to expand...

The short answer is bacause airplane fuel system are about the worse you can possibly design form a vapour lock (VL)perspective.

In any pumping system that involves a high vapour pressure (VP) liquid the worse thing you can do is..

1) heat the liquid
2) Suck on the liquid.

Look at where the fuel pump is...bolted to the back of a HOT engine inside a HOT cowl.

Mogas on average has a bit higher VP than 100LL does hence you have to be careful about swapping fuels with a standard system...especially at a high airport on a HOT day.

Basically if you heat and suck on fuel it will boil..it does this at the lowest pressure point in the system..I.e at the inlet to the mechanical fuel pump. It also does it during periods of max flow..like on takeoff!

The Vans systems overcome this by putting a boost pump in a cooler location.But this is a bit of a band aid at best.

The right thing to do from a VL perspective is to put the pumps at the source so no part of the system is being sucked upon and its at the coolest..This would be IN the fuel tanks.

Guess where the fuel pump is for the car?..Yup IN the fuel tank.

You can get very close to designing the ideal system by putting electric fuel pumps in the wingroots. And if you do that you might as well remove the mechanical pump altogether..In fact it is highly desirable to do so to keep it simple.

I have flown my own design of both a carburetted system (zenair Zodiac 400 hours)and a FI'd system on my current RV 330 hours so far.

Is this necessary?..Debatable....Does it work?..yes very well, does the system have shortcomings?..One or two.

If interested do a search and on my name and you will how the system is built.

So that answers VL..but I think you have two questions...the other question I believe is "why do we have hot start issues?"...This is a different phenomina.

Whats happening on a FI'd engine is the fuel is boiling in the injector lines..this is a minor irritation but it is not the same as VL.

Cheers

Frank
 
Pilottonny said:
Why is it that cars do not have a vapor lock problem? :confused:


Regards, Tonny.
Click to expand...

Modern autos have a fuel return line in the system, the engine uses what fuel is needed for operation, rest is returned to tank. One of the benefits of using electronic fuel injection.
 
Hmm

Mike S said:
Modern autos have a fuel return line in the system, the engine uses what fuel is needed for operation, rest is returned to tank. One of the benefits of using electronic fuel injection.
Click to expand...

true, but that is not the whole reason...Its just a convenient by product of putting the pumps in the correct place.

Actually what this gives you is an "auto purge" feature that greatly enables starting.

But to put this on a standard Lycoming system is still a bandaid to a bad (from a VL perspective) system.

Would be realtively easy to add to a standard system though using an Andair 6 port tank selector however.

Would be difficult to add this to the "pump in the wingroots" system.

Frank
 
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