Jaguar Repair: 1984 Jaguar XJ6 Fuel vapors and over flow, jaguar xj6 fuel, 1984 jaguar xj6

Question
1.   Here is the latest tale in the ongoing saga with gas fumes. I read something on the net about a central switching Valve located under the trunk. I have heard that sound in the past and have not heard it recently now that is mentioned. The  second thing I noticed is on my charcoal canister part# (CAC 3814)…On the bottom of this canister, as it is mounted in my car, has a larger rubber hose line coming out that runs the length of the canister and then bends across the top and connects to nothing? This seems odd. When I plugged it off with my thumb and the engine did seem to run a little smoother…This leads me to believe I have the wrong part. A hose that runs to nothing and discharges at the top? What is the correct part number for the correct charcoal canister or am I inaccurate?
2.   On two different occasions to day once I shut the car off the driver’s side tank started over flowing with gas and pouring on the ground. I have taken the small plastic valves off near the canister and blown air into both with a compressor. I reassemble and put it out in the driveway and it overflows again? I google this issue and come up with some guy talking about a bad Central switching valve under the trunk. It says I should hear a click when I push the tank. I have heard it in the past but have not in some time now….Is this the problem?

Answer
Hi Curtis,
The switching valve in the trunk only switches the fuel supply of either the right tank when it is electrically powered by the dash switch and defaults to the left tank when the power is switched off (by the dash switch). It has nothing to do with venting.

The fuel return valves (located in each rear wheel well at the bottom) are also operated by the dash switch. The right return valve is default closed and opened by electrical power from the switch when the switch selects the right tank. The left return valve is default to the open position and only closed when the electric is powered to it when the switch selects the right tank. This system is set this way so that any and all electrical failures of the return valves or the tank switching valve, you will be able to operate on the left tank until a repair is made.

A sticking valve can up set the system but an electrical failure can not. You can have a overflow of the left tank with an electrical problem by trying to operate off the right tank.

Both tank vents are connected together and one hose is run all the way to the right front fender to a pressure relief valve which is set at about 3 PSI. As the pressure exceeds 3 PSI it is vented to the charcoal canister which vents to outside air through a large pipe extending up and over the top of the canister and loops over so as not to allow water from the front tire to enter the canister. A vacuum hose and a purge valve is also attached to the canister.

The simple test of the system is to remove the hose from the small plastic pressure valve on the tank side and open both tank filler caps and blow through the hose to see that it blows into both tanks. Then take the pressure valve off of the hose going to the canister and blow air into the canister to see that is it open to outside air to the large hose looped over the top. Then test the valve with a vacuum pump like a "mighty vac" to see that it will open at 3 PSI or less. (BE CAREFUL NOT TO GET THE VALVE TURNED AROUND BACKWARDS. AIR MUST BE ABLE TO PASS FROM THE TANKS TO THE CANISTER AFTER OPENING THE 3 PSI VALVE)

The only way the left tank can overflow is for you to have been operating on the right tank with a failed electrical system which over filled the left tank. Each tank has an expansion chamber so that the tank can not be over filled when refueling unless you retracted the filler neck and kept filling it. Each tank is designed to handle a 10% fuel expansion rate.

Howard