Fuel Pressure Fluctuations

Fastwind67 · #1 03-16-2014, 12:01 PM

This is not a new topic here on the forum. Perhaps this thread will add to what's already out there.

My RV-4 experienced in flight fuel pressure fluctuations. From reading the forum and talking to Van's techs it seems I'm not alone. The typical occurrence was a drop in fuel pressure to about 1.5 - 2 psi when in a climb with the boost pump off. Activating the boost pump ALWAYS fixed the issue. Occasionally the pressure would drop in level flight. Van's described the occurrence as fairly common, not entirely understood, and maybe nothing to worry about. That is probably reasonable advice. Finally in level flight I experienced a fuel psi near zero, my EGT rose and I experienced engine roughness (from an excessively lean mixture I assume). I wasn't sure the engine would continue running without intervention. Pushing the mixture in probably would have helped, but I just turned on the electric boost pump and it immediately gave me 5 psi and all was fine, except it made me think maybe I needed a SPARE electric boost pump since it seemed an electric boost pump failure could at times lead to a forced landing....

So, I no longer was of the "probably not really a problem" mindset and began to consider the problem again. I have numerous pieces of construction equipment that use the same facet electric fuel pump as on my RV-4. Anytime this pump fails (usually due to electrical problems, circuit breaker, etc. - I've never actually had the pump physically fail) my equipment will not run very well or for very long, even with an engine mounted mechanical fuel pump. I attributed this to internal resistance in the pump, since it has no internal bypass. Incidentally, from blowing into a couple of new and used pumps I found the newer the pump, the higher the internal resistance. I did not "quantify" this resistance with any gauge, but just what it seemed from blowing through them.

I installed a check valve (Andair valve from Vans) parallel to my electric pump to provide a fuel bypass so fuel did not have to flow through the pump when the pump was turned off. Schematically this is very simple. In actuality it was a bit tricky due to the limited space (especially the proximity of the pump to the fuel valve) in my RV-4. I used a couple "uncommon" aeroquip aluminum fittings that had to be special ordered and moved the electric pump about 8" aft of its original location, but it all worked out well in the end.

I've only flown about 25 hours since this change, but the common occurrence of fuel pressure falling to 2 psi is now a very rare occurrence of fuel pressure falling from 5 psi down to maybe 4 or 4.5 psi, never lower. At idle with the electric pump off (and the engine driven pump running slowly) I occasionally see a fuel pressure down to 2 psi but never in flight.

I believe the internal resistance of the electric pump is a significant cause of the in flight fuel pressure fluctuations. I am not a fluid engineer, or engineer of any type, so this info is "for what its worth". Perhaps it will be of use to someone...

I would be happy to provide more detail or pics of my assembly.

Pics here: https://picasaweb.google.com/1176086...3490/FuelPSI02

flightlogic · #2 03-16-2014, 06:14 PM

Glad you brought the topic up. I have never quite been satisfied by the data presented so far. I watch the pressure like a hawk, because the excursions seem random. I would rather gaze out the window more. Hoping some fluid engineers and mechanically curious will come forward again.

GLPalinkas · #3 03-17-2014, 06:39 AM

Anxious to see the new set up. If I did the same I could alleviate the wife tapping the fuel gauge every so often

See you soon....

Fastwind67 · #4 03-17-2014, 10:30 AM

OK, here's a try at posting pics of the setup:

https://picasaweb.google.com/1176086...3490/FuelPSI02

RFazio · #5 03-18-2014, 12:56 PM

I too had low fuel pressure problems at times. I noticed it most when I pulled up steeply, my pressure would drop to zero or near zero. I have a Dynon Skyview installed and my girl would announce "fuel Pressure". No big deal I would flip on the electric and continue playing. The other day it went to zero while flying straight and level. "fuel Pressure" was announced and I flipped on the electric. I didn't think too much of it. I shut off the electric and it was fine for a while, then "fuel Pressure" again. Hmmpf! what is going on. I landed and pulled the cowl. I check the gascolator, clean as a whistle. I was looking for something and my friend Skip comes over. He is building a 7 and has the same motor, O-360. He looks at my fuel pump and say's "what's that plug doing in there?" I say what plug! He's pointing to a 1/8" pipe plug I've got in the fuel pump. He tells me it is the vent side of the pump and it is supposed to be open to the atmosphere. Duh, I had a plug in there and you are supposed to put a fiting with a 90 degree down and a short piece of tubing facing the bottom of the cowl. So I took the plug out and took off. Right away I saw a higher fuel pressure than I've ever seen with the mechanical pump. I pulled up sharply and the pressure drops, but nowhere near what it used to. Then.... "fuel pressure" again in level flight. So a new pump is in my hand and it will go in this weekend. I think I screwed up the pump having that plug in there. We'll see this weekend.

Fastwind67 · #6 03-18-2014, 04:32 PM

I experienced just what you described, usually in a steep pull up, but sometimes in level flight. I don't have the pretty lady saying "fuel pressure" but all the rest sounds the same.

My fuel pump does have the 90 degree vent fitting in it, so that was not my problem. I agree it should not be a plug, so putting in the vent fitting is surely the right way to go. I'm curious if you still have the same occurrences with your new pump. Waiting anxiously, please advise us.........

My first thought was to change the fuel pump, even though it had 50 hrs TTSN - sometimes they leave the factory broken. When I realized I had to pull the engine forward (or push the airplane back) to change the pump (RV4) I thought maybe I'd focus elsewhere. I turned my search to the forum, where I read it was somewhat common. There can't be that many broken fuel pumps out there, could there?

RFazio · #7 03-23-2014, 06:01 PM

I posted above about my fuel pressure problem. I installed a new pump and flew it today. The problem of the fuel pressure going to zero is gone. It still does drop when pulling up steep, but no where as bad as it was. I have higher pressure all the time. at all different attitudes. I think I messed it up having the plug in there.