I posted this in the electronic section to but I thought it might get more views here...

I always seem to get the weird problems that no one has ever seen before. My fuel gauge works fine with the ignition on. As soon as I start the engine it pegs over to full...

I can't seem to find anyone who's ever encountered something like this. Including my mechanic. And I can't think of a single thing that could cause this...

That is an odd one. If your fuel gauge is like most analog style, the float inside would vary the resistance as it moved up and down. On my PWC the "full" reading would be 33 ohms and "empty" would be 240 ohms. So if you're getting "full" when the engine starts, you have little to no resistance being measured. It sounds like something is grounding out your fuel sender wire going to the gauge.

Maybe take a peek behind your dash and see if any wires have come loose and might be touching the fuel sender terminals.

Since it is ok till start up I would think something related to charging system, and first check would be grounds. Just a guess

If gauge pegs to full, prob is usually on the sender (pink wire usually) wire side.

Do you have other items hooked (wired) to this gauge ?

First, I would unhook sender wire from gauge. Measure ohms from wire with key on. Then measure with engine running. Same reading ?

As a note: all fuel level Sender sends signal in ohms.

The back of the gauge will have "I" for ignition for when you turn on the key, "G" for ground black wire, and "S" for sender pink wire.

From “Hull Truth”