Engine/Transmission (1998.5 - 2002) fuel pressure

[KTobin]

Installed a mechanical fp guage last weekend and got 12psi at idle, 10 at 60 mph and around 6 at WOT. However, each morning when I fire it up, I get 8psi at idle, 5-6 under light throttle city driving and less than 4 at wot. After 10 to 20 min. of driving, the pressure climbs back to 12 at idle, 10 at cruise and 6 at wot. This only occurs in the morning and once the pressure picks up, it will be fine all day, even if left to sit for 8 hours at work. I think i'm looking at a new lift pump. I've seen a lot of great ideas here and believe I'll go with a pusher if need be. Thanks



Kevin

 

[KTobin]

Forgot to mentionthat when I shut off the truck, pressure drops to zero within 5 seconds. Does this seem right? I was of the impression that a check valve in the lift pump would hold a few pounds of pressure against the vp44.

 

[Camino-man]

I just had my lift pump replaced two weeks ago. The new one is doing this very same thing. Start up in the morning with 8-9psi and then slowly climbs to 10-11 and stays there. I shut down and come back 5 mins later and back to 8psi and it takes another 5 - 10 min to climb back up to 10psi. Anybody know what's going on here???

 

[KTobin]

Originally posted by Camino-man

I just had my lift pump replaced two weeks ago. The new one is doing this very same thing. Start up in the morning with 8-9psi and then slowly climbs to 10-11 and stays there. I shut down and come back 5 mins later and back to 8psi and it takes another 5 - 10 min to climb back up to 10psi. Anybody know what's going on here???

Camino-man, if these numbers are from an accurate guage, they are way too low. Check for obstructions, filter, kinks etc. if all is ok, I'd question your new Lp



Kevin

 

[jeepit]

For your reference, just installed a westach fp guage yesterday and I'm running 15 idle, 12 cruise, 8 wot. Seems normal for me.

 

[CORed]

My brand new lift pump, just installed 4 days ago, does the same thing. Starts out about 1. 5 lbs lower than normal operating pressure. So I get 13. 5 at idle, but after a few minutes, it hits 15 at idle and that is where it stays.



Figured it had something to do with cold fuel being a bit thicker than warmer fuel after the return line has fed the tank and made the fuel warmer overall but that is just my guess.



I wouldn't worry about that behavior.



BTW, 10 at normal cruise isn't that bad, but I'd keep an eye on it. Mine was around 9-10 at cruise and 5-6 WOT for awhile when it suddently just gave up... 3. 5 at idle and 0 at cruise.

 

[Camino-man]

Right after the new pump was installed the max pressure I saw was 13 at idle, 11-12 at cruise and 9 a WOT. It just stared this behavior last friday.



The machanical Isspro gauge would move around a bit but seems to have settled down. The pressures now after it warms up are 12-12. 5 idle, 11 at cruise and can't really pull it below 10 WOT. I don't know if the gauge was causing the fluctuation or what? I don't understand the low pressures at start-up.





BTW 1st lift pump lasted til 68K. Stock as a rock truck.

 

[KTobin]

Originally posted by Camino-man

I just had my lift pump replaced two weeks ago. The new one is doing this very same thing. Start up in the morning with 8-9psi and then slowly climbs to 10-11 and stays there. I shut down and come back 5 mins later and back to 8psi and it takes another 5 - 10 min to climb back up to 10psi. Anybody know what's going on here???

 

[KTobin]

Originally posted by Camino-man

Right after the new pump was installed the max pressure I saw was 13 at idle, 11-12 at cruise and 9 a WOT. It just stared this behavior last friday.



The machanical Isspro gauge would move around a bit but seems to have settled down. The pressures now after it warms up are 12-12. 5 idle, 11 at cruise and can't really pull it below 10 WOT. I don't know if the gauge was causing the fluctuation or what? I don't understand the low pressures at start-up.





Camino-man, these numbers are a bit different from your last post. I assumed that your new LP was in question. I have the same ISSPRO mechanical guage setup as you. My instructions make no mention of bleeding the 1/8 inch o/d poly line from your issolator to the guage. I know that it may not be such a big deal considering the tiny I/D of the line (meant to keep the pusations of the vp44 down to a dull roar) but I did it anyway. Pascals law reminds us that even within a confined enviroment, a fluid cannot be compressed. Try bleeding or re bleeding it.



Kevin

 

[Camino-man]

The pressures seem to fluctuate and build. Makes me think more and more that its the gauge. This morning on the way into work, the cruise pressure built up to 12. 5. Shut the truck off and then restart and the pressure is back to 8psi. I was told by the outfit that installed it that there is air in the line and it will eventually work its way out.



Ryan

 

[KTobin]

They're not a big deal to bleed. If you havethe transparent line you should see bubbles in it. Hit the starter to pressurise the line then crack off the big screw on your issolator untill all the air is gone. Keep a rag under the issolator to catch the diesel. If they loaded the guage side of the issolator properly with antifreeze, there should be enough reserve to fill the secondary line from issolator to guage. Put a rag under the guage and crack off the 3/8 inch compression fitting untill the line fills up with antifreeze and the air is expelled.



Kevin

 

[Diesel Gunner]

Hey Guys, just a reminder that if your grid heaters are on after a cold start up your lift pump pressure may be a bit lower due to them pulling alot of amps. Have the outside temps been low enough to cause the grid heaters to run?

 

[Camino-man]

Update for my truck. The isolator was bad. The shop replaced it under warrenty. Had it replaced yesterday and all is well. I'm a happy truck owner again! Lift pump pressures are within spec according to the technician. 13@idle, 10-11@cruise, WOT it will drop to 5 though.



Ryan

 

[RRFinch]

I think you are right. The bearings, I would guess, on your LP are getting tired. Rather than do an OEM replacement, i recommend an aftermarket LP and filter. You are, then, done with the problem. I have had 2 LP in my truck's life and next one will be aftermarket.