firehawk

I have recently started experiencing a kind of strange fuel pressure problem. When Im come to an idle my pressure will drop from 15 to around 6 or7, then when I accelerate, like leaving a stop light, it will jump back up to normal. As long as Im driving its around 12 and 10 at WOT. The when I stop again it will go to 15 momentarily then drop to 6 or 7 again. Fuel filter is clean and LP has 2000 mi. on it. Any ideas?

ds1rider

What's your ampmeter saying?

firehawk

I havent seen anything unuasual like diming light or anything, gauge is around 14v. Im probably just going to have to change the LP but I cant figure out why the FP is dropping like it is at idle. Like I said while driving its pretty good.

xmr

Mine did the same thing. I changed out the lift pump and every thing went back to normal. These are the symptoms of a dying LP. Better change it quick before it fries your vp44

Flashpoint

Sounds like the same problem i had with the isspro fuel Isolator. Hook your line up direct to gauge or use a different gauge off the test port. I wasted half a day to find out my gauge isolator was the problem.

firehawk

Thanks flashpoint, Ill try that tomorrow.

Shovelhead

My last two lift pumps did that when they were on their way out.
One @ 37,000 miles, the other @ 20,000 miles.

My mechanical gauge doesn't use an isolator.

firehawk

Problem solved, just ordered a FASS.

piperca

I have the same problem and discovered it's the tank's inability to vent properly. I have loosened the fuel cap and my fuel pressure has returned to normal. When the fuel pressure would drop, I could clearly hear the LP cavitate. I wouldn't be surprized if that is the reason for so many LP failures. I'm in the midst of designing a mod, so I'll post more when I get something working ... until then it's driving with the cap loose!

Ph4tty

Interesting. So you think there a venting issue on the tank?

I'm looking at the same type of problem. Newish lift pump, lines, banjos, clean ff- when lugging the truck around in low rpms the fp drops 4-5 then when I get back on it fp returns to normal. If it drops it will stay down all day until I punch it hard for a 1/2 second or restart the truck. The fp actual increses while I'm hitting it when its jumping back up to normal.

I'll try loosening my fuel cap. So whats the idea Mr Piper? Maybe an air valve in the cap?

Austin'sDodge

My truck is doing the same thing. I guess I'll have to vent the tank and see what happens.

Shovelhead

If it was tank venting, why would replacing the pump make the symptom totally disappear?

piperca

Since I've owned this truck, I've had issues with a vacuum in the tank. I recently replaced my VP and did the LP at the same time. The pump that was on there was good, so I kept it as a spare.

Shortly after replacing the pumps, I began to notice the fuel pressure gauge dropping in pressure while idling. If I accelerated the pressure would return to normal. At times the pressure was dropping from 13 to 7, I blamed the pump. I changed it out for the old one and it started doing the same. Each time the pressure dropped, I could hear the pump getting louder and it appeared to be cavitating. I started thinking it may be my fuel pick-up and started researching a cure. While doing so, I got a tip from someone on another site ... "try leaving the fuel cap loose." I did and immediately noticed the fuel pressure returned to normal.

All of a sudden things started to make sense. If there's negative fuel pressure (vacuum) in the tank, that would make the LP work 10 times as hard, eventually causing the LP to fail and would most likely affect the VP too. The truck is running much smoother now.

I'm trying to source a fuel tank vent that can be fitted on the tank, something similar to what they are doing on the 3rd gens, up on the "hump." It would need to be a type of check valve that would not allow the fuel to leak if the truck was overturned, but something bigger than the puny rollover valve the module is equipped with. By the way; I was at the dealership this morning looking for a replacement rollover valve and they told me they were backordered (there was one at a parts counter a couple of cities away). The parts guy told me they sell an average of 53 valves a week, nationally ... that's a lot! Sounds like there's a problem with the valve!

Anyway, I'm trying to come up with a cure and when I do I'll let everyone know ... and please don't hesitate to chime in with ideas ... a group effort is always much better! This may be the cause of many an LP/VP setup going south ... our troubles may be over soon!

firehawk

I went out this morning and started my truck which Im not driving until I get my FASS. Idle pressure was 7psi, I took my fuel cap off and nothing changed. As soon as I touch the throttle the pressure comes up to 15psi then drops at idle. Doesnt make a difference if the cap is on or off. Although the tank vent theory makes complete sense, I dont think its my issue.

chadwick

i was thinking that if the tank had vacuem wouldent you hear it depressurieze when you took the cap off. Or would it lose it vacume though the system when it is not running. Also i thought gas caps where made to only hold a certain amount of pressure