2000 GMC 6.5 intermittent Stalling/Loss of Power to Lift Pump

Hi guys browsing this forum has been so informative and helpful but I am hitting a wall on a truck that I am working on.

It's a 2000 GM C3500hd,6.5 with dual tanks 2wd and a dump body with around 89,000 miles on it. The complaint was intermittent stalling but always restarted. The PMD was replaced by someone else a few years back and has a relocate kit and a large heat sink that it's mounted to. The glow plugs and the controller were replaced a few months ago.

I swapped the PMD with a new one that i got for my truck and it fired and ran for a few hours with no stalling. Then it died on the owner.

I went back and did the following to the truck:

replaced with a brand new PMD (so I can have mine back)

checked alternator voltage and battery connections. output is 14.2 and clean connections.

replaced oil pressure sensor with a new ac delco sensor

checked fuses and verified they were ok

swapped fuel relay with a new one

cleaned the grounds on the top of the engine and the ground straps

tested the lift pumps with direct power and they function

jumped out the pins in the fuel relay to check lift pumps as well

unplugged the optical sensor and it does the same stalling

checked the function of the fuel shut off solenoid

checked the main plug and for chafing at the firewall for the ecm

it had a po231 code for low fuel pressure

I wiggled the ignition switch wiring and the key switch itself


I have lights and directionals and no other symptom that would be an indicator that the computer is bad.

I found that the Lift pump starts then cuts out after the glow plug controller cycles but I didn't think those circuits were related. Also, the fuel balance modulator needs to be replaced, but i'm not sure if it's related or not.

I also checked in for chafing or corrosion behind the glove box at the ecm.


The fuel Modulator is going to be replaced so they have a working gauge. I'm thinking my next step would be to check the PMD extension harness, and then i'm wondering about the crank position sensor as well. Is there a good way to determine if the ecm is bad and is there a schematic showing the wires for the lift pump circuit off the ecm?

I know it's a lengthy post but i wanted it to be known what was done and if I'm on the right path

Thanks in advance!

The lift pump ground wire on mine opened up the last time I had a problem with the pump but i couldnt find where the wire came out under the hood. So I just ran a nice hefty 12 gauge up to one of the batterys to give it a nice solid
ground.

Welcome to TDP :):):)

When was the fuel filter last changed ?????

The P0231 is an indicator of an issue it's not happy about.

Dirty filter
Intermittent clog of the fuel strainer in the tank/s

Start with a fresh filter...bleed the air out and see what shakes....

With engine idling.....open the water drain up front near the water cross over.

Fuel should come out and engine should not stall.

If engine stalls the lift pump is either shot or is losing power.

Filter first....

Get a pressure gauge in the feed line to the filter.

5 psi is good.
Be sure the lift pump is running any time the engine is running.....

Flow is the most important thing.....These units move a lot of fuel through and back to the tank.....

Keep us in the loop.

the lift pump is losing power while running- like it get unplugged. when it dies it fires right up with no problem. when the lift pump is working it has substantial flow and pressure from the T fitting on the top of the engine

You mentioned replacing the oil pressure sensor (OPS - Oil Pressure Switch). I would revisit that. They are weak on a good day, and every one should be relayed at the first hiccup. This takes most of the load off the switch, and provides positive power to the lift pump. You can also add a momentary switch to the relay for filter priming or test running.

The 1995 model is unique, in that the lift pump can be forced to run with the engine off and without jumpering. Simply (set E-brake or chock the wheels): engine off, key ON, vehicle NOT in P or N, turn key to Start (engine should NOT crank). Pump should be powered, but it bypasses the OPS, and uses the LP relay. The LP relay is only used during the start cycle while the key is in the start position.

Ok I get that but would I be seeing a drop or spike in pressure on the gauge in the dash when it stalls? Also is the 2000 different from the 1995 due to the OBD II and the lift pump running when cranking?

I'll definitely re-visit the OPS on the 2000. I did replace it with an ac delco but you're right even new parts can be faulty.

Based on how it was acting it seemed like it was the PMD or the OPS so i started with those. I also found that with the key on power you hear the lift running, then the relay under the hood clicks and it cuts out. It can also be verified with a meter or power probe.

coincidentally this happens as soon as the glow plug cycle ends. i'm under the impression that those functions are separate. Just a note, the truck had glow plugs and the glow plug controller replaced in December. This stalling issue has not been going on that long.

I searched the harness for chafed wires or pinches and found nothing but i did remove the grounds and cleaned them and re-installed

Apologies. I read your sig, and replied accordingly (I've slept since the original post, so...). The 2000 M/Y does not operate similarly. The LP is relayed and commanded by the PCM, but its operation is not monitored beyond startup. The OPS is redundant, in regards to LP operation.

No Worries! The 95 is mine and the 2000 is in my friends landscaping fleet that I maintain. I think realistically my concern is that it is the PCM. I checked the optical sensor on the IP and it still stalls. My other thought is does the crank position sensor come into play here like telling the pcm when to power the Lift pump. I know the ford 7.3's were notorious for this but once the crank sensor was shot it wouldn't fire up.

Is there any way to get a wiring diagram for the PCM to see which wire powers the LP

Most stalling issues will cause DTC's, including most of the IP electronics (except the PMD) and the crank position sensor. If it's stalling with no codes, it isn't likely a lift pump issue. Once the engine is running, it will continue to run w/o the lift pump, although with less power under load, and will stall if air is sucked in (such as opening the water drain). Unexplained stalls with restarts brings you back to the PMD, or something much more simple (and very often disregarded), like the ignition switch or its associated connections, or a weak chassis ground.

I Just installed a new chassis lift pump and cleaned di-electric greased the plug. I also pulled the sending unit and found that the tank was clean and the pick-up was as well. I ran a long length of clear tubing from the t-fitting to the tank and ran the truck about an hour with no evidence of air in the system.

this truck has dual tanks and I know the balance module in the frame is faulty. The fuel gauge is spinning and i know they haven't used the rear tank for at least a year because of the rear tank overflowing the front on its own. I don't think that would have anything to do with tis but I'm not sure. I do know that the front tank and sender are primary and the rear works off of the front.

This is a Massachusetts truck with 163,000 and one owner. This is one of the only trucks I've seen with no rust or corrosion to speak of. the owner completely shuts down operations in the winter and it has never plowed or seen salt

This intermittent stall is still occurring with the truck. Since my last post i have replaced the ignition and the key switch, cleaned and checked my grounds on the engine and the chassis, and replaced the relay that controls the dual tank Module. I was optimistic when replaced the ignition because the key switch was worn, however I am still have the same stalling issue. Would the faulty dual tank balance module be able to make the truck stall because it was trying to switch tanks?

The other question is would if i replace the ECM would the dealer be able to reflash or can i get one plug and play if i provide the vin number.