My new theory

Limited

Limited

Slow N' Quiet
Joined
Apr 17, 2008
Messages
2,193
Well I have had problems with my fuel pressure guage, its an electric 30psi. It either sits around 12psi and drops upon acceleration, or sits around 2psi and zeros as soon as you touch the pedal. I thought that the guage was just bad, but I am beginning to think it was right all along. It hasn't gone over 2-3 psi in a long time. Is it possible that before the lift pump was going out and only worked occasionally,(when the psi was up) and now is completely dead? Would this thing run with 0 fuel pressure? I have recently noticed than with zero psi, when you get on it hard it has a bit of cackling or rapping out of the exhuast, kinda like a PSD or D-max. My only thought is that it is the VP starving? BTW the sender is tapped into the top of the fuel filter housing.

I came up with this theory today and I am going to go take the smarty off in the mean time. So I'm sure many of you have had this happen, should I just change the lift pump and see if that changes what the guage reads?(meaning the guage has been correct all along) Or what do I do?
 
Check it with a manual gauge, just get a T fitting and a section of hose. And yes, the truck can run with 0 fuel pressure, way back when I first got my truck and didn't know jack squat on VP's I drove to a shop day at Jeff Garmon's in Atlanta on a dead lift pump. Guys there helped me check out the truck and found that mine produced 0 PSI :bang
 
I did some reading on Blue Chip(thanks Chip) and did a little testing. Its official, the guage works perfectly, the lift pump is fried. I bumped the key on and off about a dozen times and only got the lift pump to come on once. I heard it humming and the guage sat right at 12psi for like 30 seconds until I heard the pump kick off, and then the guage dropped to 0. When I start the truck it usually jumps to about 12psi for a second before dropping back to 2psi. So thats my problem. Now I just have to fix it!
 
My buddy has a few stock lift pumps lying around, so a free oem sounds good to me!!!
 
My buddy has a few stock lift pumps lying around, so a free oem sounds good to me!!!
Click to expand...

Just the smarty alone on a bone stock truck can kill off an oem lift pump very easily. fuel pres is the life of your vp. It would save you alot of 30-45 minute headachs to just put a good fuel supply that runs a line STRAITE to the side of the vp like FASS or air dog. Also the price of a new vp out weighs the cost of one of these fuel pumps.
 
At the least relocate the pump to the frame rail in front of the tank. Many a person has done this and had very good results since the stock pump is actually a very good unit, just in a crappy location.
 
Well I got a different pump on it today. It kicks on and runs, but makes no pressure. Truck won't start. After cycling the lift pump several times and cranking it for a while, it started and ran for about 5 seconds. It reved up to around 2500 on its own, then died,. Won't start again. If you tap the starter to make the lift pump run, it runs but makes no pressure.
HOWEVER, if you crank on the starter for 12-15 seconds and let off, the pump continues to run AND THEN makes pressure(around 10psi). After 25 seconds it shuts off. Tap the starter to make it come on again, it runs but makes NO PRESSURE! Also when the guage reads pressure you can tell that pump is bogging down and making pressure.
SO....Is the pump only supposed to make pressure after you crank a bit, or after it starts? If you crank on it for less than 10 seconds or so, the pump continues to run but make NO PRESSURE! It will only create pressure after cranking for quite some time. Also, after you crank and get it to make pressure, you can crank some more and it still wont start.
ANY IDEAS??? My immediate fear was that I just finished off the VP, but it seems unlikey as it has thrown no new codes. So in conclusion the lift pump runs but makes no pressure(except after cranking) and cummins refuses to start or even try. I am assuming that if I can get pressure at the top of the filter housing where the sender is, then there cant be any air in the system anymore.
HELP!!!
 
Sounds like your LP is losing prime. When you crank, the VP helps draw fuel up to it so you have pressure when the LP runs after cranking. But as soon as it stops, you're losing the fuel in the line and it won't pick back up.

Sure you have no leaks in the lines between the tank and the LP? Also, you should hear a difference in the sound of the LP when it has picked up fuel and when it hasn't.
 
Yes, when I get pressure you can hear that the LP pulls down a bit, where as otherwise it just sounds like its free wheelin. Only way i have a leak is if its on the inlet of the LP, its the only thing I took off before the LP.Would fuel leak out there, or just air in? Cause its not leaking fuel anywhere.
 
Depending where the hole or leak is, it may not leak fuel it may just suck air or let enough air in to allow the fuel to drain back to the tank.
 
I took it back off, all seals were on and lines were tight, no way its sucking air. Before I took it off I tried to bleed air out in a few places and it didnt help.
 
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