94 Lift Pump

[CedarGrove]

A neighbor picked up a 94 NA and it has some intermittent skipping problems. I drove it today and things seemed fine. I suspect the lift pump but I need some pointers for troubleshooting. On my 98, I hear the lift pump run when I turn the key on before I start it. On this 94, I don't hear the lift pump run until I shut the engine off. I hear it run for 5 to 10 seconds after shut down.

I'll put a gauge on the bleeder and see what the pressure is while running. I definitely had some flow there with it running.

Should I hear the lift pump on this 94 run when I turn the switch on? Any pointers?

Lewis

[94blue]

Hey bud,
All I read was "turn the key to start while in drive and you'll activate the lift pump w/o startiing the engine". My 94 will not let you turn the key that far when it is in drive so that method would'nt work. I will usually get the lift pump to run if I crank it over a few times which lets the Oil pressure switch activate it. I also just disconnected the pump connector and put 12volts to it to see if it actually would work at all.

BTW, I just replaced the OPS last weekend and sprung for a new lift pump too ,just because.
The truck has 263K on it and what the heck. Mine was running really rough and would shut off. The OPS was the culprit but did the lift pump anyway.

Just my novice info. I learned all I needed from this excellent site.
94Blue

[JohnC]

On 94s and 95s the lift pump runs whenever there is oil pressure and whenever the starter is energized. It will not run if you just turn the key on.

95 is the only year the pump will run if you put the shifter in D and turn the key to start.

[DmaxMaverick]

If this truck has a N/A engine from the factory, check to see if it has a mechanical lift pump. If it does, they don't make noise unless something goes horribly wrong. It may also have MFI (no PMD), depending on the original application. In 1994, military and government fleet GM Diesels were all MFI with mechanical lift pumps. The same applied to box trucks (bread, UPS, etc.) and chassis builds. If it has a mechanical pump, it will be at the lower/front/right area of the engine, in the exact location you would find one on the old Chevy gassers. When mechanical pumps fail, they will usually leak fuel from the weep hole in the pod.

[CedarGrove]

This thing clearly has an electric pump. It was a state hwy truck in VA. Has a PMD on the pump which is not plugged in. Someone recently put a remote mount PMD under the bumper. Also looks like the crank position sensor has recently been replaced. I feel like the previous owner had been chasing this problem. Pretty much the same stuff I did to my 98 chasing a similar problem. No filter on the optical sensor.

I feel like it's time for a DS4, but I'll check the fuel pressure first. If that's steady I feel like I can rule out the OPS. I suppose an ignition switch could be a suspect as well.