Lift Pump Not Working?

[Steelkilt]

I replaced the fuel filter today in my new-to-me 98 suburban. Cleaned out the bowl before installing the new filter. Following the recommended procedures (this is my first diesel so I read a lot of the posts here first) I opened the bleed valve at the top of the fuel filter and triggered the ignition to run the lift pump, but no fuel comes up and the filter bowl remains empty. So now I'm wondering, am I doing something wrong here. My understanding is that switching on the ignition should operate the lift pump. If that is the case, then how can I find out whether the lift pump is working? could it have suddenly failed; truck seemed to have plenty of power before I changed the fuel filter. tank is nearly full btw. Is the lift pump in the tank itself? it has a pump on the frame rail under the driver's seat. Any advice would be appreciated. Thanks in advance.

[DmaxMaverick]

The only fuel pump (OEM) is the pump on the frame under the driver seat. It should have temporary power when the key is turned to RUN (before start). If it is getting power, check the pump for operation. You can jumper power to the pump, and it should run. If you do this and it doesn't run, the pump is bad. If you aren't getting power to the pump, the LP relay or the IGN switch could be bad. Connectors and grounds could also be suspect.

Start your diagnosis by verifying power to the pump, and the condition of the pump. If the pump checks good, but isn't getting power, you can trace your problem upstream from there.

[Steelkilt]

Thanks! So does that mean the pump on the frame rail is the lift pump? And I just have to get it working.

[DmaxMaverick]

Correct...

[Steelkilt]

of the two wires going to the pump, am i safe to assume the brown is the positive (as opposed to the black)? i don't have the wiring diagrams.

[Steelkilt]

it was the fuel pump, i put a new one in today. truck is running again. information and comments on this site were a great help. i appreciate it. Thanks.

[rustyk]

The only fuel pump (OEM) is the pump on the frame under the driver seat. It should have temporary power when the key is turned to RUN (before start). If it is getting power, check the pump for operation. You can jumper power to the pump, and it should run. If you do this and it doesn't run, the pump is bad. If you aren't getting power to the pump, the LP relay or the IGN switch could be bad. Connectors and grounds could also be suspect.

Start your diagnosis by verifying power to the pump, and the condition of the pump. If the pump checks good, but isn't getting power, you can trace your problem upstream from there.

Mine is an AMG, but the wiring is c.1992. The shop manual says the lift pump is powered in the "Start" position, and the relay is replaced by power through the oil pressure sender when oil pressure hits 4-6 psi.

I bring this up only because a start issue (detailed in another thread) has continued to plague me, even though a dead lift pump was replaced, and the new one works properly when the engine runs. Since I have this in a diesel pusher, the engine is a long way from the ignition switch, and checking it in the "Start" position is a PITA, but in "Run" is easy.

It sounds like later models have a different sequence?

[DmaxMaverick]

Mine is an AMG, but the wiring is c.1992. The shop manual says the lift pump is powered in the "Start" position, and the relay is replaced by power through the oil pressure sender when oil pressure hits 4-6 psi.

I bring this up only because a start issue (detailed in another thread) has continued to plague me, even though a dead lift pump was replaced, and the new one works properly when the engine runs. Since I have this in a diesel pusher, the engine is a long way from the ignition switch, and checking it in the "Start" position is a PITA, but in "Run" is easy.

It sounds like later models have a different sequence?

They changed. In 94, 95, then again in 96 (OBD II compliance).