Wacky Gas Guage

The most likely cause is a bad connection on the float arm for the sending unit. I am actually in the middle of writing a page for my web site that includes this repair, but it applies to older vehicles. What I would do for yours is to connect a scanner to view live data. That will show the reading coming from the sending unit. If it drops out intermittently, the float arm can be detached, then the movable contact can be bent very slightly so it makes better contact.

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You need to use a scanner to see what the Body Computer is seeing.

The question then is was the gauge reading correctly while he was looking at the sending unit input? If it was, any testing has to be done while the problem is occurring. That is where having your own scanner is better than having to pay someone by the hour to to try to make the problem occur. If the Body Computer showed it was reading a correct fuel level, then it becomes an issue of looking at what the instrument cluster is seeing for fuel level information that is sent from the Body Computer. Welcome to our world of frustration thanks to all this unnecessary technology.

One thing you might consider before getting too involved is starting with the 1996 models when the instrument cluster became an unreliable computer module, there have been a lot of problems caused by loose connector pins on the back of the cluster. Those can be soldered to the copper circuits to improve reliability.

If that happens when you shift to reverse with the engine not running, I would look at those instrument cluster connector pins. The shifter may be causing causing just enough movement on the wiring to have an affect. You should be able to cause the problem to show up by pushing on the cluster too.

If the engine has to be running, look for things that flex when the engine rocks. Wiggle on wiring harnesses that run through the engine compartment.