How do GM techs test lift pump operation

[Perry]

I'm curious how the GM trained dealer techs to check for lift pump operation? Do they just open the bleed valve and see if fuel comes out? Or are they trained to test for fuel line pressure?

My Suburban started to stall out again, but this time it left me stranded on top of the mountain after a evening of skiing, so I had it towed to my Chevy dealer, glad I just upgraded to AAA Plus.

It started up fine, ran for about 2 minutes, and then sputted and stalled. When I restarted, it ran for about 30 seconds, then died. On the next attempt, it ran about 10 seconds. Next attempt about 2 seconds, then it wouldn't start at all. After waiting a few minutes, it would start again, but stall after about 15 seconds, and run for shorter periods of time until letting it sit for a while.

When I called the service writer this morning, they said the truck started fine and they were able to drive it around their lot....they would let it idle an hour, then put it on their "test equipment" and let me know.

I could not convince them to do a fuel line pressure test. This was the same dealer that couldn't figure out my intermittent stalling problem two years ago, and when I took it to an indy diesel shop, they found out my lift pump was bad....so what magic phrase do I need to tell the service writer to properly test the operation of the lift pump? I could just go to another shop, but I don't want to be stranded en-route.

[Hubert]

I am not sure a LP would cause that unless filter is dirty and fuel was that cold and "thick". The LP is a flow through design. If broken, the IP can still pull fuel from the tank for idle.

No OBD codes for actual LP operation I don't think. The service manual says to at idle (warm) fill a pint container and time flow. Should fill in a few seconds if too long diagnose fuel system.

They could check fuel pressure and voltage at wiring to further test but most would probably just replace LP. Labor cost for further testing would maybe be a gamble in some peoples mind I guess as LP is most common cause.

[Black95TD]

Perry,

I used to do a seat of the pants test for my lift pump by putting the shift lever in D and turning the key to start. The lift pump would run but the engine did not turn over. On the 95 it was a very simple plunger type pump. My take was that if I could hear the pump running it probably was not the pump. Not sure about the 99's Maybe someone with a 99 can comment on this test. Good luck.

[Barry Nave]

Lift pump should run with key on. 95 do run lift pump,gear out of park,start mode.Starter will not turn.
As a ruel,open water drain vavle Eng. does not die than good.
I do wonder about the fuel, Gelling would be my guess or very close to it. A filter check at that time would of told the tail.
Even a filter change would of gotten you up and running.

[Perry]

I don't have much experience w/diesels in cold weather as it doesn't drop below 30 that much here in Seattle. Its never had a problem running on those colder mornings.

While I was at the mountain pass, the weather was 33, and it was parked for only 2.5 hours, so I didn't think I had a fuel gelling problem...just for my education, at what temps will fuel start to gel?

[JohnC]

I wouldn't expect 30* to be a problem, especially with a fuel heater, but your symptoms sound exactly like fuel gelling...

You sure it was 30? Gelling in the tank could have plugged the pickup tube.

[charliepeterson]

I would guess that fuel wouldn't gell untill below 0* F. It doesn't happen often that a lift pump would cause a stall and hard start, but it can. I had it happen to me. I thought for sure it was the FSD, it wasn't. The injection pump can pull from the tank on a non working lift pump. I guess it depends on how the small electric motor fails.

Any Tech. worth anything will check for power at the lift pump. This procedure isn't hard and it is fool proof.

[Perry]

Well, I picked up my Sub from the dealer yesterday, and they said that my Pump Driver was failing and needed to be replaced. The estimate was for $400+ for the pump driver and intake gaskets and $388 for 4 hours of labor. 4 hours is laughable as my PMD is located on the skidplate using Heath Diesel's kit, I could switch it out in 10 minutes in a parking lot. And if that PMD was really going bad, why didn't they just unplug the harness and run the original PMD that I left in the stock location?

I'm not sure how they were able to give me a PMD diagnosis. I figured at least they would have replaced the fuel filter or give me a report on fuel pressures, but none of that. They said it started and ran fine after I had it towed in, and it did drive home ok even though they didn't do anything. $100 wasted. Lesson learned, never go to my Chevy dealer for anything more than an oil change.

[Barry Nave]

never go to my Chevy dealer for anything more than an oil change-

Don't know about that :eek:
I'd do my own even then

[moondoggie]

Good Day!

"I used to do a seat of the pants test for my lift pump by putting the shift lever in D and turning the key to start. The lift pump would run but the engine did not turn over." This test, unfortunately, ONLY works on 95 trucks.

As stated by others, the IP will pull fuel from the tank w/o the LP's assistance. In my experience, a failed LP system shows up as stumbling on 1/2 - full load, which might very well show up when climbing a mountain road. (The stumble feels almost exactly like an ignition miss on a gasser.)

Blessings!

[rjwest]

I have a dash mounted fuel pressure gage.

I have noticed that with the Truck camper loaded ,and less than 1/2 tank of fuel : while driving
on a highway with the cyclic bumps,
the kind that get the truck to galloping,
The fuel pressure would drop to zero,
in sync with the bumps....

I would suspect that if lift pump was inop or weak
that this would cause some intermittent IP starvation for fuel....