Rob, I have a 1998 GMC Sierra Z71 4x4 with a 5.7-litre V8. I fuelled up last Wednesday, and when I turned on the ignition without starting the engine, the fuel gauge jumped back and forth a couple of times finally registering the fuel level as per normal.
When I drove away after my fill up I noticed the gauge needle quivering back and forth so that it was almost a blur. The movement of the needle is so small but is so noticeable because of this rapid movement.
Have you a diagnosis so I can avoid a hefty bill at the shop?
Please advise. Allen
I'm sorry to say that your symptoms are very real and unfortunately very common.
It gets worse - not only is the fix difficult to nail down, the bits and pieces required to complete the repair are also expensive. As a matter of fact, a buddy of mine had the same thing happen to him and when he discovered the extent of the repair, he just lived with the problem until he sold the truck.
Here's what's going on.
The GMT 400 platform, which your truck is based on, was a major breakthrough for General Motors. I happened to be an Instructor in the General Motors system when these trucks first hit the market.
Your situation is connected GM's electronic instrument cluster.This came with small electric motors that rotated the indicator needles of each gauge, but that's not all - these motors were controlled by a microprocessor.
Consider that GM, like most auto manufacturers, was just getting into the electronic era, and was facing the reliability challenges that came with these new systems.
Very small electric voltages were being used for the first time to do the job that 12 volt systems did just one year earlier.
Now, let's throw into this mix new formulations of gasoline, and vehicles expected to last two or three times longer than was typical only a few years earlier.
To keep this as simple as I can, I'll number the events:
1. The fuel gauge system that the GMT 400 platform used was good in its day, just not very user-friendly with Ethanol-blended fuels. Other fuel additives also created problems in the form of corrosion at the fuel level sending unit connections.
2. No problem - simply replace the sending unit - but the GMT 400 utilized a fuel pump module. This meant replacing the pump, filter and sending unit at significant cost.
3. Discovery by warranty auditors that repeatedly connecting the delicate electronic connectors created bent, folded over, misaligned terminals inside the newly designed plastic connectors. It was later determined that the body connect needed to be replaced each time a fuel pump module was replaced. This didn't happen enough.
4. Any circuitry faults caused by electrical connections at, or inside, the fuel tank caused erroneous voltage signals to be sent to the control module that operated the gauge motors. This did two things:
a. It created erratic gauge needle readings. In bad cases they would vibrate, stick to one side or swing back and forth from Empty to Full, or,
b. Destroy the control module completely. This would obviously affect the other gauges.
Either way, the instrument cluster had to be replaced.
5. As these trucks aged, normal corrosion began to occur and the fuel tank electrical grounds began to lose connectivity. This also contributed to the erratic gauge readings due to the poor electrical connection at the frame.
So Allen, your journey has just begun and the best road map for you will be a proper electrical diagram and service manual.
Start with the simple things such as tracing the fuel tank and gauge circuit grounds. If you have never had the fuel pump module replaced, this might be the time. Failing that, you could be looking at a new instrument cluster.
You can see why my buddy left his alone and lived with the inconvenience. Allen, I did a little research and not only did GM issue service bulletins on the issue, there are countless web forums that have "discussed" this issue.
As there is no direct solution, this is one of those cases (because of the electronics and the era) that cause customers and shop personnel to lose sleep.