The first question I have is that is it okay to just run on one tank in these cars? Also I notice the valves may be not working in the switchover. Im getting the common problem of one tank getting more fuel then originally what was put into it after switching tanks. Is this an expensive repair if it is the valves that need replaced. Car wants to act up sometimes and stall if there is gas in both tanks but will run smooth on one.

 

Can run on one tank for the short term if you want, left tank is probably best as it is the one operating when the solenoids are passive/unpowered so less to go wrong. Not sure of your logic though if you are having fuel return problems. Assume this means you only see that on one side.

Solenoids are about $100. This page may help diagnostics Fuel Tank Switchover Checklist, Series III XJ6 also many good threads on here if you use the search function.

 

Running on one tank means the range will be very short, like about 120 miles. Each tank holds about 8 UK gallons, slightly bigger than the US gallon, and with a fuel consumption of about 16 mpg, it's a matter of arithmetic. You definitely don't want to empty the tank so a 1/2 gallon needs to stay in.

The changeover valve in the trunk can stick,(I used to have to occasionally tap it with a spanner !), and the return valves can eventually give up the ghost, so it is a matter of finding the ones that have failed and replacing them, LH and RH are not interchangeable, as on is open on volts, the other closed on volts.

If I were you I'd sort it out. The changeover switch on the dash can also fail, but can be taken apart and fettled to work. Doug Dwyer on this forum has done so, I believe. He is your Mine of Info on XJs.