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Low fuel pressure

gafitts

Member
I am having problems with low fuel pressure with the engine driven fuel pump
It will run fine for a while maybe an hour or two at 28 psi but every now and then the pressure will drop to around five or six psi turning the electric fuel pump on brings the pressure right back to 28 psi
I sent the engine driven fuel pump back to the manufacture they ran it on the bench could not duplicate installed another engine driven fuel pump from them same problem Checked the fuel lines from the tank to the engine for leaks no leaks no leaks are around the engine driven fuel pump I am thinking when the fuel is low ram air from the fuel vent is letting high-pressure air in the tanks and then being sucked up by the fuel lines and cavitating the engine driven fuel pump this is just a guess
I say this because the engine driven fuel pump works fine on the ground
 
Similar observation

I have an IO 390 with Airflow Perf Fuel Inj with purge valve routed to port tank.

I too observed pressure drops on the mech pump that could be solved with electric pump.

Eventually I isolated the pressure drops to my port fuel run which leads me to believe that the purge valve for the fuel spider lets in air somewhere and my temporary fuel pressure drops were due to momentary slugs of air in the fuel lines.
 
Air intrusion

I meant to post my experience only to share how small slugs of air can manifest as momentary losses of pressure particularly with the mech pump. Sometimes they will even show a slight decrease in fuel flow which is most noticeable as a transient 'lean pop'.

How air gets into my system I have no idea. I have pressure checked all my lines short of the spider/diverter and have yet to find the source.

Perhaps the junction of the purge valve line to the port tank line simply demonstrates a high point where occasional air bubbles are accumulating and re-entering the system once atmospheric pressure decreases as a result of climbing? Just too much work to replumb that joint to fix a problem that isn't really a problem.
 
@gafitts, any updates? Have you learned anything? I have the same issue, except my dips in fuel pressure only occur with full tanks making me think I am getting fuel slugs in the vent lines.
 
I suggest looking at the more common causes of low fuel pressure. Vents tend to not be on that list assuming they are not blocked (check for mud dabbers and such). Some ideas:
- My experinece is air leaks into the fuel system is the most common, and hardest to find. Here disconnecting one end and drawing a vacuum on the line is a more definitive diagnosis.
- Fuel line construction. If the problem goes away when turning on the boost pump look for those things that are known to be issues, such as 90 degree fittings and such. I worked on RV with low fuel pressure problem that had eight 90 degree fittings between the tank and the engine. Cleaned that up and repaired the small air leak on the engine driven pump and all was good,
- I’ve never seen an AirFlow Performance purge valve leak, either internally or externally. You may be the first but I’d be surprised.

One question. Are you running mogas or just 100LL?

Carl
 
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