Fuel pressure issue, very perplexed, need help

taverroi · #1 05-26-2011, 09:50 AM

I'm operating and IO-360 M1B in my RV-7A

Ok so heres the deal:

PROBLEM: Fuel pressure after flight drops below 15psi when taxing for a minute or two...EFIS warns me in red flashes..and if left without a boost recovery engine will cut off (sometimes, other times just spit and sputter and hover around 12-15psi) .. boost bring pressure back up to 27-28psi..when boost is then cut back off pressure drops into the red again. pressure can not be recovered under higher engine power only under boost. Keep in mind...all is well while flying without boost..this only happens when I land and am taxing back to the hanger.

Now on initial start up and run up there are no fuel prssure issues. Standard start procedure is...3sec of elec boost, then off...Engine start under mechanical pump...warm up and taxi...run up checks good...elec boost on for take off...1200agl boost off...mechanical pump pressures fine for all of flight (stays between..say...23-26psi.

One mechanic has said its vapor locking after landing because of the reduced airflow in the cowling and that in flight airflow is sufficiently cooling the fuel lines to prevent vapor lock...hmmm

Another has said vapor lock does not happen when the engine is running, but only after shut down and sitting and that the fuel pump in going bad and needed to be replaced.

Ok, so I replaced the pump with a brand new high pressure pump. Operation was flawless..Inial ground test showed 29 psi.. no boost with the new pump..test flight flawless...operating pressure..no boost bwt 23-26 in flight...even landed it this time with no boost pump on..everything perfect..right up until i was on the ground and taxing a minute or so and the the problem returned.

Soooooo. I know that the pump was not going bad..$650 bucks spent for nothing (parts and labor of course)...So what gives?...Has anyone had the same experience? is this normal for fuel injection engines?..Ive been told that the pressure after flight without any boost should remain at the same levels that I see in flight..Can I continue to fly and feel confindent that all is well?

Please help me if you can...what can I do?..suggestions?.. Please somebody...

Tango

Kahuna · #2 05-26-2011, 10:19 AM

A search on this forum of fuel pressure and hot taxi fuel vapor would have saved you a lot of money and time.
Take a search around and let of know if you have any unanswered questions.

DanH · #3 05-26-2011, 10:46 AM

Keep mechanic #1

Fire mechanic #2

Be sure your last check before any departure is "electric pump on".

You may be able to improve the situation by insulating a fuel line or adding a heat shield to an exhaust pipe.

Bavafa · #4 05-26-2011, 11:15 AM

I am not an A/C mechanic so FWIW

I have the exact same engine and have never experience this issue. My fuel pressure remains as normal when I land and engine operates also normal. The vapor lock issue may come to play if you shut a hot engine and let it sit for a few minutes or more.

What I suspect is the cooling issue on your fuel pump. It may help if you install a blast tub to your fuel pump or insulate it by adding a heat shield as it has been suggested.

As a note, I do not have the blasting tube nor the shield. But I do have two blasting tubes to my Mags which might help cool that area a bit too.

Good luck

Tandem46 · #5 05-26-2011, 11:56 AM

Quote:

Originally Posted by Kahuna

fuel pressure and hot taxi fuel vapor

Tango, I'm with Kahuna. We just had a big discussion on this subject a few weeks or months ago, but I can't seem to find the posts. Most of us with FI engines have had the exact same problems you are describing. Wish you would have searched here first, could have saved you time/money.

taverroi · #6 05-26-2011, 12:52 PM

Thanks guys!!

I hear voices of great wisdom.

I feel like such an idiot! My gut told me it had to be what all of you say.. all of the evidence pointed to it, but I am no mechanic and just did'nt want to take any chances..

With that being said I have good used fuel pump with 200hrs on it for sale! Any takers?

Tango

mv031161 · #7 05-26-2011, 01:00 PM

Next time if you want to troubleshoot further.... you can also get a calibrated manometer (gauge) and check your pump output pressure prior to your FI distribution spider...Vapor lock sucks!

on the ignition side of things....you may want to consider a Slick Start http://www.aircraftspruce.com/catalo...slickstart.php I have used this in several aircraft and buying one for my current sled! it works great in hot starts! Never ever a problems with hot starts!

erich weaver · #8 05-26-2011, 01:14 PM

Maybe Im being anal, but from my recollection, there is a difference between what was discussed on this forum recently and this case. Previous discussion was simply a rough idle after flight, with no loss of fuel pressure. This can be attributed to vaporization of fuel within the stainless steel injection lines coming off the fuel spider on an injected engine. People referred to it as vapor lock, but it really isnt, as there is almost no pressure in these lines. Other than experiencing rough idle, there is no real hazard from this condition, and it goes away by opening your throttle slightly and increasing your fuel flow.

The current situation involves loss of fuel pressure, and to me is a bit more serious, in that it may truly be associated with vapor lock, and is preventing the mechanical fuel pump from operating correctly. While higher fuel flows appear to be "fixing" the problem for now, I would still want to take some action in this case for fear of things getting worse and being a problem in the air.

comments welcome

erich

mv031161 · #9 05-26-2011, 01:27 PM

Quote:

Originally Posted by erich weaver

Maybe Im being anal, but from my recollection, there is a difference between what was discussed on this forum recently and this case. Previous discussion was simply a rough idle after flight, with no loss of fuel pressure. This can be attributed to vaporization of fuel within the stainless steel injection lines coming off the fuel spider on an injected engine. People referred to it as vapor lock, but it really isnt, as there is almost no pressure in these lines. Other than experiencing rough idle, there is no real hazard from this condition, and it goes away by opening your throttle slightly and increasing your fuel flow.

The current situation involves loss of fuel pressure, and to me is a bit more serious, in that it may truly be associated with vapor lock, and is preventing the mechanical fuel pump from operating correctly. While higher fuel flows appear to be "fixing" the problem for now, I would still want to take some action in this case for fear of things getting worse and being a problem in the air.

comments welcome

erich

therefore my reccomendation to confirm fuel pressure with a calibrated gauge. if you confirm good pressure prior to the FI Spider, the rest is just eaisier to trouble shoot! Spider, SS Lines, nozzles....thats it! by isolating the simple fuel distribution system in two parts, you'll get to the possible issue eventually.

Unfortunatelly now days A&P's dont take the time to T/S. Customers just wont pay the time to do it correctly and prefer to act like Airlines mechanics with an open source of parts by removing and replacing parts........very broad statement but unfortunatelly very true!

Bavafa · #10 05-26-2011, 01:56 PM

Quote:

Originally Posted by erich weaver

Maybe Im being anal, but from my recollection, there is a difference between what was discussed on this forum recently and this case. Previous discussion was simply a rough idle after flight, with no loss of fuel pressure. This can be attributed to vaporization of fuel within the stainless steel injection lines coming off the fuel spider on an injected engine. People referred to it as vapor lock, but it really isnt, as there is almost no pressure in these lines. Other than experiencing rough idle, there is no real hazard from this condition, and it goes away by opening your throttle slightly and increasing your fuel flow.

The current situation involves loss of fuel pressure, and to me is a bit more serious, in that it may truly be associated with vapor lock, and is preventing the mechanical fuel pump from operating correctly. While higher fuel flows appear to be "fixing" the problem for now, I would still want to take some action in this case for fear of things getting worse and being a problem in the air.

comments welcome

erich

I totally agree. I have two other friends with the same setup/engine and flying with them have not seen such behavior, nor mine has ever done it and my min fuel pressure is set to 19 PSI. The rough start/running typically happens when you shut down the engine and heat builds up but not right after landing and while taxing. My 2 cents as usually.