Engine/Transmission (1998.5 - 2002) erratic fuel pressure

[sootgrinder]

I know this has been beat to death, and no one is too excited about looking at more lift pump questions, but I have a stumper. I just installed my fuel pressure gauge last weekend, and I have got some strange numbers coming from it. I purchased the billet cap, put the fuel pressure sender in it and wired it to a Westach gauge. I hope I just have a problem with the wiring. During idle, I have around 14 psi. At WOT, I can suck it down to around 7 psi. At cruising speed, the gauge will bounce down to about 3 psi, and then show negative pressure. All I have to do is take my foot off the throttle to get it back to 14 psi, or I can accelerate and the pressure comes back.



Is there any connection between the TPS and the lift pump that might be my problem, could it be there is a problem in the wiring, or do I have the dreaded LP problem?



I have 45,000 miles on it.



Thanks.

 

[BigMike]

I had a similar problem with mine after installing my guage. I kept thinking it was something wrong with the install, because sometimes it would work fine and other times it didn't. Turned out to be the LP. Luckily, I was still under warranty at 80,000 miles. Now I consistently get 15 psi at idle and 11-12 WOT. Thank god for the guage. Go see your dealer and a new LP. Better yet, if you can find a diesel shop that's authorized to do warranty work on, go see them. All they have to do is get authorization and won't give you the hassle a dealer can sometimes inflict.

 

[fmj]

soot, its more than likely the lp, but here's something else you might look at. When I installed mine I ran a hose from one of Rays (Geno's sells them) banjo fittings to the sending unit on a bracket that I made to attach to dipstick bolts. All well and good and the gage worked, but it did flakey things everyonce in awhile. This drove me nuts and I decided that I needed to find out why. I checked wires, they were ok. I pulled out my ohm meter to see if there was a bad ground. There was a weak ground at best. In looking at i figured out why. Everything the sending unit was attached to is alum. I made a ground wire from sending unit case to engine ground and problem has never returned. You might want to check this on your set up as the fuel filter cap is alum and you, I'm guessing only have one wire from sending unit, which is what I had. The instructions that I had were explicit on not using thread sealant on sending unit due to ground, they assumed that we attach directly to engine for ground. Just something you might try.

 

[MWilleford]

Sootgrinder,

My last lift pump lasted from 90k to 140k. I installed a pressure guage at 90 (home made from a bar-LED graph) and when my truck started showing similar symptoms-- I though it was a grounding issue.

Did not make sense that the fuel pressure came back at WOT! Hmm

Most problems of lost pressure came in at idle (where the least amount of fuel was being consumed and most fuel was bypassed back to the tank)



Home testing (engine not running, but bumped starter)

-Pressure would build to 14psi, then drop to ~2psi

-Restart pump cycle by starter bump-- it would jump to 14 again, hold for a few secs and drop to 0-- pump still running of course, or at least making a noise different from when at 14psi.

-I also checked and tested the relief in the return bango fitting-- no probelms. It was clear when I replace the LP that it was the problem



Sure enough, the lift pump was bad. New pump, all good all the time now.



Funny thing-- I've looked at alot of vein pump failures (hydrauilc pumps mostly) and the failure on this pump was not from worn veins or from the rotor.

For what it is worth, my 2 cents, IMO etc-- the lift pump failure was not mechnaical-- it was electrical. This pump failed similar to my last pump, but I cannot be 100% as i did not have the pressure gauge at that time. I would have like to look at RPM vs Current draw when the pump was failing. I kept the old pump, and would like to determine root cause when i get time by attaching the rotor to a friction disk (apply load) and monitor the current as the load increases.

Sorry for being so long winded.

 

[sootgrinder]

Thank you gentlemen for the responses. I am going to dig into it this weekend and check grounds etc. The eratic way it is acting, makes me think it is the wiring, or a bad sender.