QuestionGreetings Howard.
I am returning my 1987 XJ6 Vanden plas (102000 miles) to service after being in storage for 5 years. I was out of the country. When I stored it I filled the tanks 1/2 full and put stabilizer in the gas. It started up with no problem after that long. I took it for a drive and tried to put 5 gallons of fresh gas in both tanks.
The left side took it with no problem but the right side only took about a half a gallon before overflowing as I had the fuel dispenser fully engaged. I thought that was odd because the right side tank indicated a lower level than the left. I didn't think to much about it. The next day I went to get it an emissions test and it passed with flying colors.
After I left the smog station I stopped at a friends and left it running while he and I talked. After about 5 minutes I looked over and noticed gas spilling from my vapor canister at a pretty good rate. I immediately turned off the engine and opened both tanks to alleviate any vacuum or pressure in the vapor system.
The left side gas cap let out a nice sucking sound but the right side immediately started pushing fuel out the gas cap and down the side of the car. It eventually stopped flowing out cap and I looked down the cap past the flapper plate with a screw driver and could still see gas as if it were topped off.
Before I went to have the emissions test I drove it around for about 10 miles switching from both tanks without a noticeable problem.
Howard can you tell me what could cause this problem? Is the fuel being sucked from the left had tank into the right hand via the vapor lines and then pushing fuel to the vapor canister? If so what is causing this and why would the right side indicating only a third full yet gas is present in the filler neck?
Will replacing the canister fix this problem? Not sure about the right tank sending unit. I am perplexed.
Thanks for any support you could offer.
Michael
AnswerHi Michael,
When the canister gets soaked in gasoline it is destroyed and must be replaced. However, that was the result of a failed return valve system, not the cause.
The fuel system with two tanks has a switching system and the return system also switches at the same time.
There are three valves in the system. One is the fuel tank switching valve in the bottom of the trunk. It is spring loaded to the left tank and when electrically operated, it switches to the right tank.
The return valves are located under a panel on the lower edge of each rear wheel well. The right return valve is spring loaded to the closed position and electrically operated to the open position. The left tank return valve is spring loaded to the open position and electrically operated to the closed position.
Most of the fuel that is pumped forward to the fuel rail returns through the AC system to be cooled and then back to the return valves. If you have the right tank selected, power is applied to all three valves thus drawing fuel from the right tank and returning it to the right tank. when you select the left tank, all three valves receive no power, thus the left tank supplies the fuel and the fuel can only return to the left tank.
The system was designed so that if you have an electrical failure on any of the three valves, you can run normally off of the left tank only. There is an exception to this and that is a sticking valve.
To test the system you need to turn the ignition key on but not start and select the right tank. Then test all three valves for power. Then put your finger on each valve as someone selects the left and right tank and listen and try to feel a "Click" of the valve. Another test is to start the car on the left tank and open the left gas cap and take a long screwdriver or such and open the flap in the filler neck while you look in with a flash light to see the fuel returning to that tank. Then check the right tank. (in your case you will need to drain some of the fuel out of that tank first.
From what you say, the most likely fault is a failed or disconnected trunk switching valve that is drawing fuel from the left tank while you have the right tank selected, thus running on the left tank but returning to the right tank.
When you correct the system and replace the canister, you will need to blow out the vapor lines as they are full of fuel and will destroy the new canister. Note that there are one way 3 lb pressure valves in the vapor lines.
Also, it was not vacuum in the tanks, it is pressure.
If you want a wiring diagram of the system let me know and I will post one on my web site for you to copy.
The tank sending units are located behind each tail light assembly but the tanks need to be a low level to remove and test each. It also helps to jack the rear of the car up high to remove them. When setting for a long time they tend to rust and thus stick. I have removed many over the years and usually am able to save about half of the failed ones by WD-40 and carefully work the arm up and down.
Howard
