Questions and Answers

GMC 2500 Intermittent Fuel Pump Q. Dear Vince, I have an intermittent problem with my GMC truck which is driving me nuts. The truck is a 1997 GMC 2500, 4WD, 7.4 liter engine, manual transmission, with ABS, A/C, and power everything. The truck has about 87,000 miles. The problems started and were thought to be solved in November of 2002.

The truck failed to start one day and all symptoms pointed toward a bad fuel pump. The suspected bad pump usually buzzed quiet loudly and was now silent. A mechanic friend of mine installed a new NAPA fuel pump and the truck then started and ran fine, for a couple of days. The truck then failed to start and was towed back to the shop to have the fuel pump replaced again.

New fuel pump number two was installed and the truck ran fine, again for a couple of days. The truck then failed to start and left me stranded a couple of hours from home. When I retrieved the truck the next morning it started right up and I drove it on to the trailer to haul it home and I drove it off the trailer at the garage. I talked to another mechanic as well as a local GM dealer and they both gave me a couple of ideas.

I used the lift at my friend's garage and dropped the fuel tank again. I had been told to inspect the connectors between the sending unit and fuel pump for any signs of corrosion, carbon?, or anything to cause a bad connection. The connectors had some black areas on the prongs so I installed a new Delco sending unit and Delco fuel pump. The truck then started and ran fine until October of 2003.

In October the truck stalled one morning while driving up a hill. It would not restart immediately but did restart after about five minutes. About a week later I turned the truck off on incline at a boat ramp. The truck would not restart and I could not hear the fuel pump running. Again a few minutes later the truck started and ran fine.

On December 21, 2003 the truck again failed to start after being parked on an incline at a boat ramp. The truck started after about 5 minutes, was shut off for a couple of hours, and then restarted. A half hour later the truck stalled but restarted after about 15 minutes.

On December 26, 2003 I turned the truck off at a store and when I returned a few minutes later the truck started but then immediately stalled. When I tried to restart it the motor seemed like it was trying to fire but was not getting enough fuel to run. I finally remembered the location of a plug near the fuse block where I could hook a jumper up to energize the fuel pump without turning on the ignition.

When I hooked up the jumper wire to the power stud the pump energized as normal. I then disconnected the jumper and immediately started the truck. The truck has been starting and running normally ever since.

I hope you can understand my frustration. Any mechanics I talk to say they can't solve the problem if they can't duplicate it. I keep driving the truck almost hoping it will quit and not restart. The problem is where and when this will happen. Hopefully I'm not too far from home and not pulling my boat at the time.

Do you have any ideas? I really hate to have to pull the tank and replace the fuel pump and sending unit again unless I know for sure that is the problem. Since I was able to energize the fuel pump the last time with the jumper wire could the problem be electrical somewhere else?

I know that if you turn the key to "ON" without engaging the starter the fuel pump will energize for a short time and then stop when the system is pressurized. What tells the fuel pump to re-energize after the motor starts? Is there a valve or switch which could be causing the problem?

By the way, when the problem first occurred last year I did try substituting the fuel pump relays from another truck and they didn't make a difference.

I'm stumped. If you have any ideas I'd be glad to hear them.

Thanks in advance,
Ken

A. Well, at least you know the frustration all technicians feel when they get a job towed back for problems such as yours.

We obviously have a intermittent problem. I would suggest you get a factory wire schematic for this truck and trace EVERY wire involved from the ignition switch to the pump. Any loose terminals at any connector involving a fuel pump wire is suspect.

The only way to tell if a terminal is no good is to slide a new terminal of the opposite sex inside it to see how tight it is. My guess is you will find one, or more, that the new terminal will fall out of if held upside down. This means all terminals at the relay should be checked as well as the ones on the plug that goes to the tank. Sounds like one is getting hot and making a poor connection. Good Luck!