Fuel and Temp Gauge Question??

Sounds like a problem in the gauge/ voltage limiter circuit to me. Time to do some real troubleshooting.

Pull the cluster back out and get them working before you put the thing together. Add a permanent pigtail ground from the cluster to the body.

Next, take your meter and see what voltage you get with the key on to the two gauge units. You can easily see where the PC board traces go from one place to the other. The "power" side of the two gauge units are both hooked to the same point---the output of the limiter. You should bet about 5V there with key on.

If so, ground the sender side and see if the gauge heads for the "up" scale end.

I'm betting you have a bad connection right there in that circuit, or you have a defective (or you damaged) the limiter unit.