Engine/Transmission (1998.5 - 2002) Is this true?

[RichardFerrell]

I just talked to a Cummins Rep at their plant. Trying to determine the problem with my lift pump. It goes from good pressure to low pressure, back up to good (above 8 psi). I can even vary it by adding and letting off throttle. He told me the LP VARIES IN SPEED according to the feed from the computer. Is this true? I've never heard that before. I asked him if the computer could be reprogramed so the LP would keep possitive pressure to the VP44 and he said he didn't know. He wasn't an engineer. Said to take it to a dealer. Yea, right!

 

[ntillm01]

The only time it varies speed is during crank. The PCM pulses the lift pump voltage at 50% on/50% off to "prevent over-fueling of the injection pump" at start-up. Once the engine is running, it should be 100% voltage constantly. I think you've got something else going on. Your pressure varies w/throttle because of the vary-ing fuel demand. Sounds like the pump is getting weak.

 

[Cliffman]

Rich, that sounds like a smoke screen to me buddy. Your lift pump is going south. I've been through 3 of'em.

 

[RichardFerrell]

variable lift pump

Originally posted by Cliffman

Rich, that sounds like a smoke screen to me buddy. Your lift pump is going south. I've been through 3 of'em.

First installed gauge. Pressure low. Installed new LP. Gage read about the same. About 700 miles on gage. 600 miles on new LP. Called Cummins in Phoenix and they varified variable flow as stated by factory rep. He asked if truck was 2001 and he stated the LP is a variable flow as data is fed from VP44. I think I am confused!

 

[Cobra]

lp pressure

Richard,

I am confused. If you have a gauge mounted in your truck, what pressures are you seeing and at what throttle settings.



i. e. In my truck I show 14. 5 psi at idle. At cruise my gauge shows mostly 12 psi, and at wot it drops to about 8 psi.

 

[RichardFerrell]

Cobra-- This is what is confusing me. Idle --16 to 17. Shifting through gears (6 spd) some times down to 10. Really get on it as in heavy foot in 5th gear, sometimes as low as 4 psi. cruising in 6th empty at about 70 mph 10 to 12 psi. Momentary WOT 70 mph psi will drop to zero come back to 4, sometimes back up to 8. Towing a light trailer up a hill in AZ I was in 5th at about 65 and the psi dropped to zero. Went WOT and the psi climbed up to 8 and stayed there as long as I was WOT. By the way I've call the factory, Phoenix Cummins and Salt lake City Cummins and all three give me the same story about what the SLC guy called "Pulse width Modulation" via the ECM.

 

[JSHAW]

Sounds a little fishy. I have a 2001 and so do 4 of my freinds. We all have fuel pressure guages, some electric some mechanical. None of us see any of the symptoms you have unless the pump is going bad.



Jim Shaw

 

[Gary - K7GLD]

Check to make sure the power plug to the bottom of the LP is clean and fully plugged in - they sometimes get dirty or otherwise make poor/no contact, obviously affecting PSI - varying engine torquing/movement might create just enough motion or change of position to affect how good contact the plug makes - simply UNplugging, then plugging the connector might do the job - those are really POOR connectors for this application. not enough surface area and engagement for the current flow, in my opinion...

 

[A Johnson]

Pulse width modulation

Is only in effect during engine cranking, to my knowledge.



You should get the same PSI after the engine is running if you were to run straight 12VDC from your engine battery. Of course varying load gives different pressures due to you are sending more burnt fuel out the tailpipe, and the lift pump is allready running at full duty cycle so it can't pump anymore fuel.



A few guys on here have had bad lift pumps right out of the box. Of course you could also have an air leak or restriction before the lift pump that is compounding the problem. Also be sure to follow Garys' advice and check the electrical connections.



A Johnson

 

[RichardFerrell]

Originally posted by A Johnson

Is only in effect during engine cranking, to my knowledge.



You should get the same PSI after the engine is running if you were to run straight 12VDC from your engine battery. Of course varying load gives different pressures due to you are sending more burnt fuel out the tailpipe, and the lift pump is allready running at full duty cycle so it can't pump anymore fuel.



A few guys on here have had bad lift pumps right out of the box. Of course you could also have an air leak or restriction before the lift pump that is compounding the problem. Also be sure to follow Garys' advice and check the electrical connections.



A Johnson

Thanks, many thanks! Going to try and get time tomorrow to do the connection thing. I've seen suggestions here to dialectric grease every connection on the engine. Don't know if that means where the ecm hooks up and all? By the way, had the truck out for about 20 or 30 miles today and it kept the readings about like they should be. I even tryed to get the pressure to go down and couldn't. I think this truck is trying to see if I will go crazy or not! Still having fun in it tho. I'v about run out of excusses to my wife to get it out on the road!

 

[Bob Wagner]

thanks again for hangin around AJ



you are a great resource

 

[TEaton]

Richard,

I had the same problem but it turned out to be the sending unit for the fuel pressure gauge. Took it to the dealer and they checked the fuel pressure mechanically and it was fine.

Installed a new sender and everything is back to normal.

Troy