High fuel tank preasure V12 1986

After short drives i have had very high pressure in the fuel tank, i can hear it flex back into shape when I slowly release the fuel cap. First happened a month ago. Car will not restart when stopped untill multiple tank venting and cooling engine, like I have vapour in the fuel lines. Then starts after 10 to 20 mins. Engine runs good other wise.

I have read Kirbys book, and past posts on here. I dont have the service manual, where can I get one please?

So far I understand it has somthing to do with the charcoal canister and valve in the front left wheel well. Does anyone have a schematic of the tubes I need to cheak? Good pics of things will help me.

Any advice on how to proceed to solve this issue :thinking:? Many thanks from Oslo.

Peter Aves youtube video seams to show a lot.

Very typical problem…what I did was found a 12volt air valve that opens when energized…then made a bypass around the valve that controls the carbon canister…hooked the electric wires to ground and to a switched hot wire so that when I turned ignition on it immediately opens line to the canister…this stopped the booming noises which is your tank wanting to collapse in and out… perhaps someone that can do graphic can put on a schematic… hopefully the tank hasn’t broke a seam

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If you open up the wheel well, which isn’t too bad, you’ll see a plastic valve right next to charcoal canister. It will have 2 fuel vapor connectors, and one vacuum connection.

You can easily replace it and they can be found for about $40 I think? Rochester valve.
Here is the one I bought: AC Delco 214-552

But they easily get clogged. My new one worked for two years, and then I had similar issue. I have since decided to just delete the Rochester valve. All that does is allow gas vapor to exit tank into charcoal canister after shutdown. So far, I haven’t had any gas smells sitting in garage after any long drive, so the charcoal canister seems to be handling the vapors.

Kirby suggests putting little fuel filters on both sides of it, but I decided to just simplify.

And how it works…when engine is running, the vacuum line opens the valve so that the tank can vent. There is a vent hole in the charcoal canister where the tank will draw air from to fill the void left from the gas. Otherwise, you get tank implosion.
When engine is shut down, no vacuum means valve is closed. So any vapor in the tank cannot get out. But the valve has a built in 2psi (2? not sure exactly) so that if the tank exceeds that pressure while sitting after shutdown, it opens and releases into the charcoal canister.


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The problem isn’t pressure building up in the fuel tank, it is vacuum. Your problem is a very common one that has been posted about many times here on Jag-Lovers. If you search the Jag-Lovers archives for “Rochester valve” you will find lots of helpful information about what others have experienced and how they fixed it.
I removed the Rochester valve (circled in white in the first attached picture) at the front of the left front wheel well in our 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) and tested it based on information I found in the archives about how it is supposed to work and found that it was stuck closed and so it wasn’t venting vacuum from the fuel tank as fuel was consumed creating a vacuum that increased as fuel was consumed. I replaced the original Rochester valve with a Standard CP112 valve, shown in the second picture, that I purchased at a local auto parts store. Although both of these valves function the same and have their three ports labeled as “CAN”, “TANK” and “VAC”, the orientation of these ports on the Standard valve was different so I had to reroute the hoses so that they went to the correct ports. After installing the new valve my vacuum issue inside the fuel tank was fixed. The third picture shows the new Standard CP112 valve in place with the slightly different hose routing to accommodate the different port configuration.
BTW, if you haven’t already done so you should download a copy of Kirby Palm’s “Experience in a Book” from Jag-Lovers and read it from cover to cover to learn about your XJ-S and see the helpful information about the Rochester valve (named for the company that originally made them).



You need a copy of the Jaguar XJ-S Repair Operations Manual (ROM) as shown in the attached picture.

I purchased my hard copy on eBay in 2005 shortly after we purchased our 1990 XJ-S convertible.
The vacuum diagram decals on the underside of the bonnet for your car should be all you need. Emissions control systems varied from country to country so best to go with the decals on your car, if they are still there.


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Turely greatfull for all the help and advice… Found time today while listing to the start of Le Mans.

Here is what i found… Charcoal canister had no rochester value, maybe it had been rmoved as Greg sergested. I could not see any vacume hose from the engine bay. Just two hoses on the bottom, one small coming from under the car next to the break lines, from the tank (I presume) and a bigger one going into the engine bay.

What I did. Took the canister out and hoses inside the wheel well off to test them by sucking and blowing air through them. Canister flows air both ways and so do the hoses and pipe up to the canister from the tank (?) Inside the well. But the metal pipe that runs under the car i could not blow down even with gas cap open, or suck petrol fumes out of…

Form this I am assuming the pipe from tank to canister has a blockage somewhere back of the front wheel well, either inside the metal pipe next to the break lines or further back from that, as it makes it way up and into the tank. - does this sound right? So it is not my charcoal canister or non existing rocheater valve…

Any tips on further diagnosis, I will look to see where the line goes up and in tomorrow.


Different models have different ways of evacuating the charcoal canister to the intake manifold. The vacuum hoses control when. You don’t want it always adding fumes into intake, and you don’t want it after shutdown.

Regarding line to tank, it’s metal all the way back, behind tank, and then to a hose that goes to upper/right side of tank, a little alien looking thing with a few other vent hoses to tank. That thing could be clogged.

You could get lucky and just blow some compressed air back.

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Emissions equipment varied from country to country so it is possible (likely) that your car was equipped differently from my North America (USA) market car. You should have decals on the underside of the bonnet that detail the system required for the market that your car was originally sold in.
I agree that you need to sort out why that pipe leading back the fuel tank is blocked. It might be blocked by debris, or possibly crushed by a shop that placed a floor jack in the wrong place or placed one of the arms of a two post lift beneath the car. Good luck with your search for the blockage. If you do a visual inspection and don’t see anything, you might try using a vacuum pump or compressed air to see if you can dislodge the blockage. But I recommend that you open the fuel filler cap first to avoid creating problems in the the fuel tank.



I followed the big hose line out of the canister into the engine bay, it comes up behind the air intake manifold into the fuel line, looks like there is a value at that point with electrical connection. Hard to get pics. Maybe this is where the engine controls the intake of fuels from the can. There is no vacume hose outlet on the front of the B bank either, so not vacume controled. Will get some canned compressed air and find the other end by the tank…