i have a 2001 cummins i bought it at 80,000 mile.when i first got it it needed a lift pump.i bought a replacement from cummins west at 85 k i put edge w attitude 4 inch exhaust,afe intake. noticed on monitor that lift pump pressure was 0 to 3 psi. immediatly bought new lift pump. school bus model it gave me 15 -17 psi .now i have 118 k it has been constant for all this time. until i recently took a trip towing 5th wheel at sonora pass ca 9,000 ft plus.
(i nave had this truck two plus years, and half the time its towing 5th wheel on all high sierra pass's grand canyon utah arizona,washington oregon.) i fueled up and we camped at the pass it froze that night, the next morning i unhooked and went cruising and noticed right away i was only building 8-9 psi. i changed the fuel filter,didn;t change anything.i put some degel injector cleaner stuff from the napa and marvel mystery oil. came home running about 8-10 psi under exelliration as low as 6 psi for a couple seconds. after i was home driving around empty. at about 10 psi i bought some diesel kleen and marvel mystery oil
i was building up to 14 psi sometimes.its really hot here 100 plus. when i accelerate it drops to quick if i hit it real hard it will drop to 5 psi. just to note i don't hot rod at all i have the abillity to but i don't do it.now i bought another lift pump and it changed nothing.also i am wondering what hot fuel is all about i have a gauge but don't know what the normal temp should be

 

First welcome to the site

For the most part, the VP-44 injector pump is cooled and lubricated by the fuel...so fuel temperature does play a factor. As well, a lot of fuel is returned to the tank through the fuel return line (I cant remember how much at the moment), so when you dont have a full tank the fuel does get pretty hot which in turn, causes the VP-44 to increase in temperature. Also, Im thinking that the thinner ULSD factors in here as well. Running an additive is always a good idea but I would suggest switching from Marvel Myster Oil to TCW-3 2 stroke oil. I believe Marvel as some solvents in it.

As well, you need to quit buying the OEM lift pumps - they are complete JUNK. Spend a little cash and get yourself a good lift pump (I would suggest an Airdog), then you wont have to dink around with lift pump problems anymore. Just my 2 cents worth.

 

i'll check out the air dog. i have almost emptied my tank so i'm thinking of pulling it and cleaning and inspecting.i must havesome kind of restriction. are you saying the lift pump runs continuously? and do you know where i might be able to find out what the fuel temp should be.
Thanx for your help

 

With you having FP issues before I would want to believe that the vp might have taken a hit. Are you measuring FP at the vp? It is time for a good lift pump for your truck...You might want to consider the draw straw also... The pump will prime itself before cranking then run all the time after it cranks..

 

i am measuring psi at vp44. the vp 44 has been replaced before i bought the truck
i had it checked and it checked out.

 

Yes it runs continously. It supplies low pressure fuel to the injector pump. Which in turn, supplies high pressure fuel to the injectors. The injector pump will run with a damaged or non functioning lift pump but it will quickly overheat and die.

 

what tells it to return to tank. so it builds up to a pressure and then sort of blows off after a certain psi

 

The VP will simply draw the fuel it needs and then sends the rest back to the tank.

 

I drove my truck with 0 psi at the vp for over 2 months...The vp was still working when I pulled it for a FBD SO pump.. MPG and HP went up a good bit and I added a FASS150.

It only takes 1 time at a low psi to the vp to start it's painfull death... This is why a good LP is a must...

 

The internal pressure is controlled in the VP with a spring loaded regulator valve. The banjo bolt on the outgoing side of the VP is the valve, it is different then all the other Banjo's on the truck. If this regulator was not changed when you replaced the VP then that also may be part of your problem. They wear out and the spring get's weak, then they won't regulate the pressure right.