Jaguar Repair: 1986 XJ6 Fuel Problem, charcoal canister, fuel injection system

Question
Hi Howard,
I have read most of you posts about fuel smell and leaking fuel tank on the XJ6. I have replaced both sending units on my tanks but the right tank still leaks. If I pop open the filler cap it will not leak while she is parked. Earlier you posted

"Another problem is the vent system which runs plastic vent hoses forward all the way to just behind the right front head light bucket where there is a charcoal canister and a pressure valve that applies up to 3 PSI on the fuel tanks. This too can be easily tested by driving the car for a distance and then open the filler caps. This should cause a large rush of air from the tank when you open the cap. I believe that 3 PSI is too high and causes a lot of problems so on one car that was a constant problem I removed the spring from a 3 lb valve and adjusted the spring so it only had 1.5 PSI which solved that car."

Why do the tanks need to be pressured? Do you think this is my problem too?

Judy

Answer
Hi Judy,

If the sending unit leaks you should closely examine the surface of the tank and the sending unit and install a new rubber seal (probably a good idea to replace both).

I have not read any statement from jaguar as to why they pressurize the tanks but there are a couple of reasons I am sure are factors. Fuel tends to get hot as most if not all gets to make a trip to the engine compartment and back again many times when driving (even though the fuel is cooled a little by the AC unit under the hood before returning to each tank it still gets hot. If you add the heat from the road and the heat of the day in the summer plus the heat of the fuel pump as the whole pump is cooled by the fuel, you tend to boil the fuel and turn it all into vapor. Fuel vaporizes at a much lower temperature then water does. Raising the pressure on the tanks raised the boiling point of the fuel.

A second factor is that the fuel system pressure needs to be in the low to mid 40 PSI at times on the fuel injection system and no lower then the low 30's PSI and a pressurized tank helps the fuel pump in maintaining that pressure.

On my car, I had a problem with the sealing of the tank to filler neck which is just a thin "O" ring so I lowered mine to solve that as aligning the tank did not correct it. (Tanks have to be aligned)

A leak at the sending unit should be an easy fix and the problems that could result from eliminating the pressure system could be very extensive and even dangerous, especially if you garage your car.

Howard