Take your main ground wire off. Wire brush the spot clean put some dielectric grease on the bolt and tighten it up to spec.

Not to long ago they had a batch of bad fuel that would ruin the fuel sensor. Just a thought.

u know silverados have the same problem but its 2000's to 03 i believe i have an 01 and the fuel gauge keeps going from empty and back to a correct reading and stops when driving though my dealer says its the instrument panel could be the same for u.

Check resistance to chassis with the car ignition on. Clean or find a clean metal spot on the car and check and make sure it has 0 ohms or close to it from the neg. side of the battery to the chassis.

Or you could do this. Check continuity between the surface of the PCM and chassis ground and remove the ground wires from the engine until you find which one no longer allows continuity from the surface of the PCM.

It's in the Wiring Harness Dude. I'll put money on it! Has to be!

Justin...I've read some of this and nothing makes sense that they are saying.

I've been pondering this since I came over and looked at this. You don't have a short to power b/c you would be blowing fuses left and right. You have an open ground that the voltage cannot find it's path to ground through. That is why you got surface voltage on the PCM. Try taking a test light and connecting it to ground and start probing the ground wires in the wiring harness and the engine grounds. If you get voltage, it should light up the light. I think you got a ground and hot wire touching in the harness. I would start with the ground wire in the J1 harness with the pcm disconnected. Or because we had continuity through the J1-20 power wire connector and chassis ground, that could be it right there.

just the gauges go crazy or like ur power windows or something else?. there could be a melted wire in the harness somewere that might be grounding. might also be bad plug connection on the junctionbox as well

just try it. that is a sympton of a iac no cold idle when it warms up it is fine. just try it it's free.

Tried two different PCM's with the same results. Its not standalone, but I do have the hardware to modify the stock tables. I have a friend sending me a wiring harness so if its not that, then it has to be something like the BCM. Thanks again for the input guys. Cant wait to get more than 20 miles on it.

I drive a Cadillac...still a GM vehicle and yeah it has a bunch of modules...IPC, IPM, DDM, RIM, PCM...well you get the point. GM uses a class 2 serial data line that all the modules communicate over. You could try to trace the problem through that...good luck it's a lot to go through. With all those mods has anything been done to the PCM to change it's performance, or are you running a stand alone unit and just using the PCM to keep DTC's from setting and all the other modules running? I had a similar problem in my 92 ETC the PCM was bad but the IPC would lose fuel data and the compressor clutch would fail to engage...I thought it was the IPC and paid 250 bucks for a new one and that didn't work. I then went to the PCM and tap tested it, every time I tapped it the problem would go away...replaced the PCM and boom problem fixed. Could be a bad solder internally to the PCM.