I did nothing out of the ordinary the last couple of weeks and suddenly after filling both tanks with 91:

1. the left tank starts leaking like crazy on my way back home. Leaking from the back but on the RIGHT side?

2. After all the fuel had leaked out, i ran some errands (running on the right tank).

3. Left my car overnight, and next morning the vapor canistor is leaking.

Obviously now the car doesn't start because the vapor/carbon canistor is filled with liquid == ruined.

I fear that replacing the canister will just ruin the next one, where do i start troubleshooting?

The fuel plug at the bottom of the left fuel tank is not leaking nor is the fuel system in the trunk.

Even if i find a crack in the next couple of days on the left tank, why would it leak from the right side and why would it ruin the vapor canister?

Thank you guys

1987 Jaguar Xj6 4.2 VDP

 

The cannister can be disconnected and the line to the engine plugged. the car will run,

It is not a crack in a tank. The lines may be closed so that the fuel is drawn from one tank and the excess returned to the other.

The vent system one way valve is stuck open.


The tank seection valve can be stuck.

Start by checkking the drains on each tank to see ifd one or the other leak.
Tehen the valve for operation as dorected by the dash switch.

Then line by line for leasks or stoppages.

Aye the cannister is ruyined., it can be fixed. but replacements are plentiful..

GM cars used the same device.
Carl

 

I suspect the fuel tank isn't leaking but, rather, overflowing. Fuel is being drawn from one tank but being returned to the other tank....causing the other tank to overflow. The overflow can find its way into the vent pipes and then into the canister.

Open the gas caps and start the engine. Use a screwdriver to push open the spring loaded flap in the gas tank filler neck. Select the left tank. You should see a stream of gas flowing into the left tank...and none flowing into the right tank.

Now flip your gas tank switch to the right tank and check again. You should see gas flowing into the right tank and none flowing into the left

A small flashlight helps.

If you have gas returning to the wrong tank, post back

Cheers
DD

 

No leaks from drain/no cracks! No woosh either (So that vent works)

It's definitely overflowing. That makes a ton of sense. Thank you!

Fuel is going from the left tank, to the right tank (seemingly no matter what tank i choose in the cockpit - seemingly).

I have to replace the fuel tank vent valve on the right side in the rear wheel well - correct?

 

The venting/leaking is a symptom, not the problem

More likely the valve in the boot floor that switches both the tank feeds and the overflow, sounds like its stuck and not switching the overflow.

The guys you have been talking to will respond as their time zone and availability come around.

 

That's what I would do although some report that the return valves are sometimes just mechanically stuck and respond well to a soaking in carb spray or similar.

If the valve has failed electrically the only practical solution is replacement.

Cheers
DD

 

Sticking of the change over valve near the spare is a classic problem of the fuel injected series 3 XJ. Almost all of us who have owned a series 3 for more than a few years have met it. This valve does the selection of which tank the fuel comes from. Apart from the fact that it's not the greatest device in the world, the valve motion can be blocked by particles that come out of the tank due to either dirty fuel going into the tank or corrosion of the tank. The valve can respond positively to a well judged blow from a small hammer or shifting wrench. It's not difficult to swap - you need to clamp the three hoses. However, it has been rumoured that, like a lot of other parts, modern ersatz replacements aren't as good as the Tecalemit originals from thirty years ago - I think some upgrades have been suggested in other threads on this subject. Also, it's worth having a look at the condition of the fuel tanks. Drain some fuel off and check it for particles of rust. The failures that mine have suffered over the years have all almost certainly been caused by rust inside the fuel tanks or dirty fuel.

 

I have owned S3 XJ6s for over 30 years

The fuel return valves are the culprits. The fuel tank changeover valve simply selects the appropriate fuel tank. They can be troublesome but what you are experiencing is a failed fuel return valve.

The S3XJ6 uses a constant flow /pressure fuel delivery system. The injectors take what fuel is needed for whatever is taking place eg idling or flat chat on a highway. The rest of unneeded fuel returns to the selected tank.
However if a return fuel valve which is located in the rear wheel arches fails the fuel can return to the wrong tank. There are two valves. One in each wheel arch and they are specific for their location

I personally experienced this some years ago where a somebody pulled up beside me and told me I was venting fuel on the road. The immediate fix was to select the overflowing tank.

It sounds like your right hand fuel return valve failed.

Bill Mac
MK1
MK2
S3XJ6
X300
Previous 14 Jags. MK5 to X308

 

Bill, It's always been the opposite with mine. The return valves have switched correctly while the the change over valve that takes fuel into the loop has stuck solid. Though as I wrote above, in my car it's always been caused by particles of rust from the tanks.

 

Peter
As we lurch on into history with our now typically 30to 40+ years old series 3 XJ6 cars there is a major problem in the fuel system which is a result of old and deteriorating fuel tanks.
I have run into this many times as we have had numbers of XJ6 and 12 cars which included three S3XJ6s. Bits and pieces of rust and crud have got into the fuel pump and tank changeover valves.
The solution to this problem is to insert inline fuel filters at the fuel tank outlets prior to the fuel tank changeover valves.

However in my current 1980 S3XJ6 I have ditched the fuel tank changeover valve and have just a T piece connecting both tanks to the fuel pump. I do have a filter prior to the fuel pump.
The advantage to this is that the fuel selector and return valves no longer have any meaning because fuel can easily move between the tanks. No more overflowing! In addition the bloody expensive valves involved in the fuel system are not needed.
The only slight disadvantage is that if you are topping off both fuel tanks you may have to go back and forth a couple of times to get both tanks completely full ( if you can afford it).

After chasing fuel valve problems in a S3XJ6 a friend of mine who is a retired mechanic resorted to my solution and says he has never been happier.

Bill Mac
MK1
MK2
S3XJ6
X300 (x2)
Previous 14 Jags. MK5 to X308

 

Bill,

I know what you mean. Ten years ago, the last time that it caused me trouble, I spent some time trying to cook up a re-design to solve the 'change over valve problem' and couldn't think of anything better than what you've done: a simple T and filters on the tank outlets. I wasn't totally happy with the T because of fears of what might happen if the car ever ended on its side (it happened to my Mk2 at only a few mph through falling into a field about 2 metres below the road). Anyway, even that didn't last long as the smallest particles seemed to make it through all the filters and cause trouble with the injectors and the pump. In the end, we changed both tanks and all the fuel lines and had the injectors cleaned. Since then everything has been fine running the stock system.

Over the years, there's been a lot of discussion about what's wrong with the change over system and upgrading the valves (in Aus, people have used one from a Toyota Land Cruiser and in the UK ones from alternative fuel arrangements), but most often it's the fuel tanks. Because they are tall and narrow, there's a big area to collect condensation and go rusty if you don't keep them full. If you leave your car outside and the rubber seal on the filler cap and adjacent drain don't do their jobs, rain and dirt enter the tank.

 

Peter
Ahh, the tyranny of distance.

I note that English suppliers like SNG Barratt list new tanks at about 250 pounds. Sorry, but Australian keyboards no longer carry the pounds symbol. We have been on decimal currency since 1966 and only have the dollar $ symbol.
Anyway by the time the tanks make it to Australia they have increased to around $800 which on current exchange rates is around 400 pounds.
You would want to be very enthusiastic with a very nice S3XJ6 to justify spending that sort of money. I have seen S3 XJ6 cars sold for under A$1000 and they were not rusty wrecks. Just needed some mechanical fixes and TLC.

I have "saved' a couple of XJ fuel tanks by doing fibreglass repairs. Don't reel in horror because properly done fibreglass is quite OK around fuel systems. The wing tip fuel tanks in my Piper Cherokee 235 aeroplane are fibreglass.

I agree with your remarks about proper maintenance on the rubber seal of the filler cap and regular checking of the adjacent drain. Again and again I have discovered blocked drains on XJ6s.

Bill Mac
MK1
MK2
S3XJ6
X300 (x2)
Previous 14 Jags. MK5 to X308