Engine/Transmission (1998.5 - 2002) Tell me about this fuel pressure symptom

[isb360]

I am smelling another failing LP, but before I jump to what I think might be an obvious conclusion, I am soliciting opinions/feedback on the following scenario.



I start the truck,,,,FP goes to 3-5psi, holds steady for 3-4sec, then climbs to "normal" 15psi over a 4 or 5 second duration, and is fine from then on. Each start lately has had the same exact scenario. Crank time this morning was a teeny bit longer than usual, but could be my imagination.



All my previous LP failure symptoms have basically been a dancing FP gauge followed by it flatlining at 5psi when it finally died. This is different and it's almost like the ECM signal telling the LP to run full-bore is being delayed by a few seconds. Pressure is rock steady (ha ha) while driving.



Let's hear 'em!



Tim

 

[GWedekind]

I have a friend that had a similar situation and he had a bad fuel filter. If your FF is old and you are pulling your pressure reading post FF I would change that first.

 

[isb360]

Fuel filter has about 5k miles on it. I change it every time I change the Amsoil engine oil and filter in 10k intervals. I would expect that a clogged/blocked fuel filter would provide additional restriction to the fuel system all the time and especially under WOT operation where I should see a drop in post-filter pressure greater than I am seeing now. With Smarty off (GASP! -- my buddy is borrowing it) and TST on #7, I can pull down to 11psi under WOT and I have not noticed any appreciable difference recently.



Tim

 

[Texas Diesel]

Thats exactly what mine was doing. Replace the LP, same thing. Replaced LP again, same thing. Ordered a LP from Cummins, problem gone and pressure steady.

 

[AdamFarnham]

I read pressure post-filter on mine.



Pressure goes to 10 psi during preheat, then drops off after pump stops priming. After starting, it usually gasps and wheezes 6 - 8 psi at idle. After letting it warm up for a few minutes, the pressure goes to 10 psi at idle.



I used to think it was the voltage drop from the air heater that caused the drop in pressure. Now, I've correlated the voltmeter and fuel pressure while driving, and don't think that is the problem. A start after a warm shutdown does not have this result. I don't know if the fuel viscosity is an issue, or perhaps the pump bearings are bad. I'm interested in tearing it apart after I get it out to see what's going on.



My lift pump is going bad. Saving $ for a FASS (and a steering box and A/C repair, etc. )

 

[CFAR]

What does the FP do if you just bump the starter?

 

[CRead]

Both of my last fuel pumps failed the same way. It seemed like it was worse when I had a full tank of fuel. The first time just replaced the pump and problem went away for about 3 months. The second time i want deeper in to this. First I put my snap-on gauge on prefilter to conferm pressure was the same as in-cab gauge. Changed filter just in case, no differance. Hooked my multimeter to the powerwires of pump to check pump voltage it stayed around 14. 5 volts, even during low pressure times. Pulled tank to check fuel pick up and change bad sending unit, everything was ok. So after all that i called cummins got a new pump and all is well now. I am going to out a holley blue pump on soon so i don't have to change it again which I know will happen soon. I'll bet thats its your pump good luck.

 

[isb360]

What does the FP do if you just bump the starter?

CFAR: good question. I will try it tonight or tomorrow and let you know.



Tim