Car is an S5 T2. The fuel level gauge behaves oddly. With the key in the ON position and the car not running, it will display what is assumed to be a mostly correct fuel level (nearly full after a fill up, around half at 150 miles, etc). With the car running, the gauge displays a much lower level, for example, right now it displays a quarter tank after only 75 miles from full. Leave the key in ON with the engine not running and it will go up above half (there has been lots of idling on this tank with burping the cooling system, so that seems reasonable).

When I first noticed the issue, I cleaned up the terminals at the fuel level/fuel pump connector at the hatch. This seemed to help for a bit. Eventually it came back and I then replaced the terminals themselves on the chassis harness side (they looked the most corroded). Unfortunately, I can not remember if I also replaced the terminals on the other side of that connector.

Things I have checked that have all come back in spec:

• resistance at fuel level sender connector in hatch
• gauge operation when hooked up to a resistance box and giving resistances given in FSM

One other thing I have done is inspect the ground point behind the instrument cluster. I cleaned it with Deoxit and a brush. That was this morning and the problem still presents itself.

The last ditch effort I just tried was a different instrument cluster altogether. Problem persists, so I think the one thing I can hopefully rule out is the gauge itself.

Any thoughts?

 

its probably the sender unit, they do this thing where the full part will read ok but as it gets lower it stops reading so it will bounce between actual level and empty, and then at some point it will just show empty all the time

 

That would certainly stink. That sender barely got any use. I replaced it into a new empty tank in around 2015(?) and it's only seen about a year and a half of actual use.