broken petrol gauge

[dave.gsi]

the gauge itself goes up when the car is turned on and goes right round as far as it will go and goes back down when the car is turned off but it never goes down as you drive (i did 200 miles in two days and the fuel gauge read the same). is it the sender unit or the actual gauge thats broke and lastly can i nick the bits from the 1.3sr thats rusting away on my drive and fit em to my gsi???

[Jules]

It's probably shorting out somewhere before it get's to the dial, to test:-

Have ignition switched to stage 2 (Red lights on),
Jack the rear of the car up,
Look under petrol tank and find the signal wire that's connected to the send unit,
Pull it off,
Have a look at your petrol gauge - If it's going (gone) down then it's a short somewhere.

Might be worth trying a new sender unit as it might of shorted inside, not normally the case as when the sender unit fails it normally gives no reading (Or a partial reading). Get one from a scrappy on the cheap, fit it and see what happens, if it still stays 'full' then it's the wire shorting out somewhere.
I'm not sure if the SR sender will fit in the GSi tank but for testing purposes then it should work, put the signal wire from your GSi on to the SR sender, hold the metal casing of the sender unit on a good earthing point and hold the sender unit arm about half way up, get someone to have a look at your petrol gauge (You'll need ignition on for it to work - stage 2 again) and see if the postion of the senders arm relates to the postion of the fuel gauge needle.
If it does then it means that it's a short somewhere along the line, be it in the wiring loom or the sender unit itself.

Hope that helps!!