Fuel Level Dilemma

A few years ago I purchased a nice flying RV-6A. The plane has been a joy and very trouble free. However, about a year ago BOTH fuel level gauges started acting ?screwy?. These are standard Van?s gauges and sender units. My big problem is I cannot get to the back of the gauges or even start to replace them without completely or at least partially pulling the instrument panel which will be a LOT of work. I am considering just living with this for another year since my fuel flow computer is very accurate. But I cannot imagine what is wrong other than both gauges have just gone bad. I am open to advice. Here?s what I am seeing:

RIGHT TANK:

Tank drained indicates zero with a measurement of 247 ohms across the sender.
Tank full indicates 12 gal with a measurement of 31 ohms across the sender.
With sender main wire disconnected, gauge indicates zero.
With sender main wire grounded gauge indicates 12 gal.

LEFT TANK

Tank drained indicates 5 gal with a measurement of 259 ohms across the sender.
Tank full indicates pegged, (gauge full scale) with a measurement of 34 ohms across the sender.
With sender main wire disconnected, gauge reads 5 gal.
With sender main wire grounded, gauge is pegged.

You mentioned that your resistance measurements are "across the sender".

To do an accurate check, you need to measure between the screw on the sender and airframe ground. It is possible to have the sender producing the correct resistance but have a high resistance between the sender flange and cover plate, or cover plate and fuel tank root rib. This is a very common problem (search the forums).
A simple fix if you have the problem is remove one screw from the cover plate and one from the sender flange (after lowering the fuel level. Re-install each screw with some tank sealant and an internal tooth lock washer. The washer will produce the electrical bonding needed between the two parts being joined.

Ground problem

Yes, I keep thinking it has to be a ground problem. But, I have tried everything I can think of. Each sender already has a separate ground wire from a mounting screw to the airframe, and my measurements to that grounding point on the airframe come out the same. I jumpered from the point where the gauges are grounded to the sender unit ground but this showed no change in reading. I guess I will just have to pull the whole panel.

You also need a very good ground at the gauges. Run them direct to a good firewall to battery ground. Don't share the ground wire with any other gauge.