I installed a fuel pressure gauge 6 months ago. Mechanical
with no isolator. Worked great , 16 psi @ idle , 14 psi@ cruise ,
and 11 psi@ WOT. Last week went on a three day business trip
and left the truck in long term parking for the three days. Noticed
right away at idle gauge was jumping around , down to 9 psi, up to
16 psi , back down etc. Been this way for the whole week. Only does it
at idle. As soon as I put it in gear and put a load on the truck the gauge
acts normal - 14 psi@ cruising and 11 psi@ WOT , steady. Only acts funny
when I'm stopped and @ idle. Is it the pump going bad or a bad electrical
connection ? Lift pump and filter were replaced 4500 miles ago.

 

I'd check the wiring first, but that's exactly what my fuel pressures started doing a month ago.
(Flakey pressures only at idle, once over 1500 rpm, saw good pressure)
One new lift pump later, and all's well again.

 

Whats the warreny situation on these lift pumps. I bought this one
new from Cummins West about 4 months ago.

 

Not too sure, might be worth a phone call to find out though.

My two pumps were replaced about 40,000 miles apart.

 

I had a simiilar problem. New LP solved it.

dIma

 

The part I don't understand is if the lift pump was going bad,
wouldn't it cause low pressures under WOT. This pump is only
4 months old , I changed out my old pump even though I was
still getting 11 psi@ cruising and 7 psi @ WOT. New pump bumped
everything up to 16 psi and now it only lasts 4 months

 

I had the same problem, fuel pressure fluxuating. Liftpump and Vp replaced about 3 months prior. Dealership was going to replace liftpump again. Decided not too keep havin to worry about repeated L/p problems. Installed FASS II, problem solved.

 

It baffled the heck outta me when mine did the same thing.
All I know is with one new pump installed, I again have a rock solid 16 psi @ idle.

 

Mine did the same thing. One new LP back to 15psi for 120 miles then 0psi. On the plus side the next LP last 140 miles.
Stock LP =

 

6 month warranty on Cummins lift pumps.

 

Do you think the motor/pump bearings can get worn enough to set up a "wobble" when the pressure is likely to drop until the wobble corrects and everything runs good again...kinda like a bad heater blower motor?

 

It could also be the pump's relief (spill) valve hanging up at idle. When the pressure drops a bit (load on the engine), the valve finally closes and pressures stabilize.

Rusty

 

Im having fluctuation problems for last 2 months now at all conditions with stock and pusher set up. my holley works fine alone but the stock pump skips. i start at 20 psi and some times goes to 15 or 11 when ever it wants. I got my pump from heath diesel and bill said he would waranty my pump if if would go bad. so i told him bout it and he said might be something else. i ran each pump seperately and the holley is doin fine stock one is fluctuating even by it self. I have the holley wired off the stock pump harness and thought it might be drawing to much power but the stock pump just wont run to its fullest either way. I told bill that stuff to and he still thinks my pump is fine and now he isnt helping me after saying he will waranty my pump for its life.

 

All day today, mine was dropping to about 6 psi. I could pop the go pedal and it would go back up...? I changed the two week old fuel filter, and it cruised the 40 miles home at a steady 12 psi. No crude in the filter and it was still pretty clean looking (not charcoal gray like a 10 k filter).

Hopefully it will last until Sunday when I install my new FASS II.