I am having a fuel management issue with a 1979 Series III XJ6. with 70K actual miles.
We haven’t driven it regularly several years but maintained it a casual use driver.

The problem:
The fuel gage indicates the right tank is full, in fact it is pegged past full, however the tank if not empty is pumping the fuel to the left tank. Depending on the amount of fuel in the tank the car will stall in a few minutes as the tank is emptied into the left tank and the engine is starved. When the dishoard switch is pushed to indicate the left tank, the fuel gage indicates the left tank is empty however the tank depending on the amount of fuel in the right tank fills with the contents of the right tank which is pumped until dry. This initially seemed like a simple enough problem if I ignore the gage.

The attempted repair:

1. The fuel pump works, the fuel rail is pressurized and the engine runs normally.

2. Even after checking and determining that the change over valve (CAC8000 CAC 3941) seemed to be working properly switching back and forth when powered and then static without power. I replaced it anyway. The condition of course was the same.

3. The next logical component after looking at the service manual to me, was the non return valve.(CAC2027) no test was available so it to was replaced without success.

4. The fuel line was disconnected from the changeover valve and the engine ran normally on the right tank. When powered by the switch with the valve worked normally with the left tank.

5. The dashboard switch is switching from right to left by supplying power to the crossover switch.

I have been reluctant to remove the fuel level sensors and I have not been able to determine the location of the air bleed vale nor do I know is this could be part of the problem.

Since I have owned this Red Cat more than 25 years, I own all of the associated service and parts manuals. I believe and have reviewed the associated trouble shooting procedures. I want to believe this is not an electrical issue but, have yet to rule out the prince of darkness because the valve and pump seem to be working normally and logic says that the condition I am describing is not possible. The gage however should not peg full on the wrong tank anyone have any ideas?

 

You didn't mention anything about the left and right return valves.

Sounds to me like fuel is being drawn FROM the right tank but being RETURNED to the left tank. When running on the right tank the right return valve should be open and the left return valve should be closed and vice-versa if running on the left tank.

The return valves are not getting voltage or they are electrically defective or they a mechanically defective/clogged/sticking

This might help
http://www.jag-lovers.org/xjlovers/x...changeover.htm

Note that the "non return valve" is not the same thing as the left and right return valves

Hope this helps

Cheers
DD

 

Thanks Doug,
This may be the root cause. The service manual fuel schematic that I have doesn't show the fuel return valves. I checked the Parts catalog and found their location. I am assuming the cover and shields will not be easy to remove given its location. I will pull the wheels this weekend and will give you an update.
Regards Kenn

 

moved you over to the car forum...regional section won't get as much exposure

 

Well…………… I have removed the sending unit on the right tank and checked the resistance with an ohm meter. The apparent resistance across the terminals is 3 to 4 ohms. It does not vary as the arm is moved. The guage when grounded moves from empty to full. All indications( in my mind) is a faulty sending unit. But the left sending unit when checked is an open circuit., the tank when selected reads empty. I haven’t pulled the change over switch because it does switch the solenoids. What are the odds of both sending units going bad? Does anyone know what the range of resistance should be on the sending unit? Should I just replace both of them?
Thanks

 

I would say the odds on both failing is pretty good with a '79 car. Quality was not high in that year or the year of my car, 1980. Having said that I never had the fuel tank senders fail, only a lot else !

The way these work is the float arm tracks along a resistance track to vary the resistance. So as you have no variance in resistance on one unit, it seems to be a failure. Other unit is open circuit, also a failure. So it looks like you need to find replacements, as I don't think these things are repairable, but who knows, it may be possible, but certainly Lucas did not intend them to be repaired.

 

I should have mentioned that fuel returning to the wrong tank is nothing to do with the fuel level sender units, (although the dash changeover switch switches from one sender to the other), this is caused by the fuel changeover valves not working, either through failure, or by wiring faults, such as bad earths (grounds).

 

Thank you Fraser, 2 new sending units are on their way (10 days or so).
Kenn

 

Good! After you get the gas gauge working properly you can then go to work on the changeover problem....which I suspect is one or both of the return valves behind the rear wheels.

Cheers
DD