Jaguar Repair: too much pressure on right fuel tank on 1985 Jag.xj6, fuel pressure regulator, manifold vacuum

Question
Car has 21,000 miles. took a short drive and car was wanting to stall when accelerating more than 20 mph. Few weeks later, poured some Seafoam into left tank. raised right tank cap and gas was boiling over. Decided a week later it would run well enough to get it back to my house, but completely quit after one block. Checked your site and was told it was changeover valve.Could not get the wire off of the valve so banged it with side of wrench to see if maybe could unstick the valve. Car started quick. Let it idle for about 10 mins. with the right cap open, no boiling. Closed cap and pressure started to build.Maybe one of the three valves is stuck. Noticed that the fuel pump was making a humming noise, never noticed it before. Much appreciate any suggestions. I have minimal tech understanding.

Answer
Hi Gary,

To diagnose any problem with the fuel system, you need to know how it operates on a two tank system. The switch on the dash changes to each tank but it does several things.

When you switch to the right tank the switching valve in the bottom of the trunk receives power and OPENs the fuel supply from the right tank and CLOSES the left tank fuel. At the same time power is supplied to the right return valve at the bottom of the right wheel well which OPENS the return valve. At the same time power is supplied to the left tank return valve which CLOSES that valve. Also the right tank level sending unit is connected to the fuel gauge.

Now at this time fuel is drawn from the right tank and pumped forward through the fuel filter in the trunk to the fuel rail and to the injectors. Fuel pressure is built up to about 32 PSI at idle and when the throttle is opened and the manifold vacuum is lowered and a fuel pressure regulator mounted on the fuel rail restricts the return flow raising the pressure to about 42 PSI on the system. Only a small portion of the fuel is used and most is returned through a can mounted on the AC compressor to cool the fuel and then returned to the return valve and on to the upper portion of the tank near the filler neck.

When the left tank is selected ALL power is disconnected from all three valves, the switching valve and both return vavles. This makes the switching valve only draw fuel from the left tank and the right return valve will spring load to the CLOSED position and the left tank return valve will spring load to the OPEN position. This is a "Fail Safe" design so that in the event of any electrical failure in the valve system, it will operate on the left tank normally.

However, it is possible for any of the three valve to stick in the wrong position and give faulty operation.

Here is the tests I do on the system to see if everything is working as it is suppose to.

Go into the trunk and have someone turn on the ignition and switch to the right then to the left and back to right a few times while you have your fingers on the switching vlave. You should feel it "Click" also put a 12v test light or volt meter on the power wire on the switching valve when they switch the valve and note that it is powered when the right tank is selected and no power when the left tank is selected.

Now, open both filler caps and with a flashlight look into the neck and open the spring flap so you can see down into the neck and have someone turn on the ignition and start the engine. Then switch tanks and you watch to see if the left tank is selected there should be fuel returning to that tank and go around and look in the right tank to see that no fuel is being returned to the right tank. Then have them switch to the right tank and note that fuel is returned to the right tank but not the left tank.

As for the air pressure build up in the tanks, that is controlled by the tank vent pipes that are routed under the parcel tray behind the rear seat and up to the right front fender behind the head light to a plastic pressure valve (about 3 PSI) Later models had a separate valve for each tank. After the pressure valve/s it goes to the charcoal canister behind the right front head light.

If you hear the pump running while inside the car it is an indication that a pump may be failing. (but that has nothing to do with the switching valve system) A partially stopped up fuel filter can make a pump hum too or a failed fuel pressure regulator or pinched fuel line.
Fuel pressure MUST be tested at that time.

Let me know the results of the tests. Print this and follow it exactly and you can't fail to find the problem.

Howard