Fuel pressuere drop - which fuel pump??
 

I have a fuel pressure problem in my '86 F150, 5.0L EFI engine - pressure was dropping like a ROCK when the throttle/load was applied - at idle pressure was good ~28 psi and vacuum off the regulator it was ~44 psi but as soon as the throttle was pressed the pressure dropped! I can't believe the thing ran as good as it did with such little pressure! Hopefully it'll help my gas mileage - I've been getting ~10.5 mpg lately!


The only nagging question is that the pump provided enough pressure at idle and under no/light load, but it just couldn't handle the volume required with the engine under a load - so would that be the in-tank pump or the high pressure pump??? Any opinions on this one? I really need to know since the in-tank pumps are $38 and the frame mounted ones are $80. and I don't want to just buy all 3 for a shotgun repair approach.

Thanks in advance for any info!

Do you have 2 tanks? Does the pressure at idle remain the same when running off of both tanks? Is one higher than the other? Un plug power to the high pressure pump and test. If it remains the same odds are it is the high pressure pump.

Regards

Jean Marc Chartier

2 tanks - pressure is the same off of both tanks, I Believe this has 3 pumps - one "lift" pump in the tank that deliver fuel to the one high pressure pump that delivers the required pressure - I'm guessing that the lift pumps provide only a couple of PSI to the HP pump.

how much did it drop. does the pressure come back if you say hold it a 3,000 r.p.m.. i believe a drop in pressure is normal.

Well long story short - the culprit - the filter in the selector valve - didn't even know that it was down there - I've replaced the frame mounted fuel filter and checked everything for obstructions, etc. Got the hint from the local dealer that said just to toss the filter as he's seen lots of them plug and cause problems - well, I purchased a replacement (if it's plugging with crap it's got to be doing it's job!) - no wonder why the in-line filter looked perfect - it's getting filtered fuel. Anyhow - now I can run foot to the floor all the way to 70 MPH without bucking or backfire and that's good enough for me!

I want to thank everyone for their help! Didn't have to waste too much $$ fixing it and in the end it was a volume probelm getting to the high pressure pump - just not where I thought it would be!