Engine/Transmission (1998.5 - 2002) Interesting Fuel Pressure Readings

[Lube Man]

With 76K on my truck the LP pump appears to function normally when the engine has heated up for a few minutes. 13PSI at idle without wavering.



However, on a cold morning (40F) it will idle at about 9-10 psi. Give it a little rpm in the garage and it instantly goes to 0 psi. I also noticed at idle it may go to almost 0 when the air intake heater kicks in and then goes back up when it knicks off.



But after driving about 5 minutes everything works normally.

 

[BDMiller]

I'll be the voice of optimism here. I think it is just that the intake grids use a lot of battery power. Notice how the headlights dim? Mine did essentially the same as yours but not quite as extreme a fluctuation. However mine just stopped doing it recently. I don't know if these things just get used to the cold weather or not or maybe these sending units aren't so accurate. I use a westach by the way & it is not that accurate.

 

[Lube Man]

Yes, I tend to agree with you. It appears to definitely be an electrical thing and with that heater grid cycling on and off there are significant voltage fluctuations in the system.

 

[Leroy D]

Erratic fuel pressure

A while back I noticed that my fuel pressure began to have erratic readings and I thought that my lift pump was going crazy. I got all worried that it was lift pump time until I found that the connections at the sending unit had vibrated loose. After I tightened them up the fuel pressure readings settled down to where they were supposed to be. It was funny after I found the problem, but it did have me a little worried for a little while. :-laf

 

[KYLE4]

doesnt sound like bad lift pump definitely a guage issue get yourself a diagnostic guage just a liquid filled guage and a barbed tee to tap into fuel lines and a long enough hose to hold guage in cab so you can drive around to test underload I made one for the trucks without schraeder valve on vp44, works good

 

[cumminsday]

I understand that the ECM supplies power to the lift pump. I have heard that the early ISB's like your 99 have had problems with supplying power to the lift pump. I think that the heater grid operating in cold temeratures is taking juice from the ECM, in turn it is not sending the juice to the lift pump. I am not sure if they use a fuel pump relay or not. If not, one could be installed easily to control power to the lift pump.