Engine/Transmission (1998.5 - 2002) Lift pump question

[Oliver88]

Recently I've noticed my lift pump acting strange. In the past when I turned the swith to the "on" position the pump would build up to about 12 lbs. of pressure and hold it until the truck was started. Lately I've noticed that it will build to the 12 lbs. and drop back to 0. The truck starts fine and holds over 9 lbs of pressure while driving sometimes over 10 lbs. Should I be concerned? Is there some sort of check valve (back flow preventer) that is messed up and causing it not to hold the pressure it builds when the switch is turned to the "on" position? Just curious. Also about the time this started I was getting mail out of the box and I noticed the truck missed on a cylinder and it puffed black smoke. (Had just filled with diesel minutes before this happened) Hasn't done it since, and I've towed a 7,900 lb. camper about 500 miles. Thanks for the advise.

Greg Davis

 

[Bob Wagner]

the pressure drop off after key on is normal, the pump runs for up to 30 seconds then turns off.



your pressures are a little low though

 

[Oliver88]

Maybe I just hadn't noticed it before. My pump doesn't run for thirty seconds initially, maybe 10. I'll keep an eye on it.

Greg

 

[Rammin]

Lift pump only runs 2or3 seconds when the key is turned on on .

 

[Bob Wagner]

Lift pump only runs 2or3 seconds when the key is turned on on .

after the key is bumped?

 

[Amianthus]

The lift pump will run for two seconds when the truck is first turned to the 'on' posittion. When you hit the starter, the LP will pulse off and on to limit it's discharge pressure to something less than 4 psi. This helps prevent a hard start condition (but raises cain with the pump). Once the ECM has indication of a 'run' condition, the lift pump will turn on and stay on. As long as the truck is running anyway.



If you turn the truck 'on' and 'bump' the starter, the pump will run through a priming phase that will last one minute. That mode is there so that you can prime your fuel filter after you've changed it (too bad that doesn't work all the time).



If you turn the truck 'on' then 'off' then 'on' again, you will only get one quick priming cycle on the first 'on'. You will have to wait for 10 miutes before the ECM will again allow a 2 second pre-start pump cycle.



Does that answer your question?

 

[Oliver88]

Very helpful! Thanks,

Greg