www.jec.org.uk

Good evening everyone, not sure if this is a strange fault or common but i seem to be loosing fuel from the right tank and it being added to the left tank. My jag is a late series 2 4.2.
Its been fine for a while but i noticed this the other day.
Any help would be apreciated.

Alan

The problem is caused by a failure of a change-over valve between the tanks in the boot. The precise arrangement of valves and hoses differs between cars with separate and in-tank fuel pumps. Test the valves by disconnecting the electrical connections and applying a 12v feed. You should be able to feel the valve 'clicking' in response. Mike.

Ah i did wonder if it may be somthing like that. Mine is a late series 2 car so has in tank pumps. i shall see if i can find the switch over valves.

Cheers

Well ive had it on the ramp today to see if i can trace the switch over valve and it seems as if it doesnt have one?
Ive traced the pipework from each tank and they run behind rear axle and join into a T piece that then runs to the front of the car. I assume where it is a late series 2 with in tank pumps it doesnt have the switch over valves, im a little baffled now unless the pumps have non return valves in them?
The right hand pump is new last year and the left one is original so im not sure if i may have a faulty left pump?

The pumps are electrically switched. If one's not operating then it doesn't allow fuel flow
There was a modification to that for the north American market that used a switch over valve
shown on p8 of the below. The unit shown for Canada and north America appears to have two
fuel ins but only one out

Here is a diagram of the fuel layout and connections on a XJ6 series 1, don't know any different for series 2 as
mine's a 1972 chassis.
https://www.jaguarclassicparts.com/cata ... ystem/#p=1

Any help?

For a 6 cyl car with submerged pumps, the non return valves are mechanical (ball & spring) units fitted inside the ends of the short angled rubber hoses emerging from the tanks at the back of each rear wheel. One end of the rubber hose attaches to the tank. The valve should be located in the other end of the hose. The ball ends of the valves should be fitted facing the tanks. Mike.

Ok thanks for that. I shall take a look at that tomorrow.

Ok thanks for that. I shall take a look at that tomorrow.

Sorry, my bad. I should've read the whole thread where you refer to having a late series II. Not sure what advantages were got from having in tank fuel pumps...
I'll shurrup now But do let us know how you get on?

No worries. Finding info on late series 2 fuel system seems to be harder than i thought..

No worries. Finding info on late series 2 fuel system seems to be harder than i thought..

Go to their older vehicles' locator: https://www.jaguarclassicparts.com/uk/o ... hicle-info
And then select your weapon. I guess your in-tank fuel pumps would be in the Series III section Fuel System.
Then you get the flip book

I'm only coming to this thread almost five years later...

I have a late model Series 2 and hadn't appreciated the fuel tanks are interconnected? Is it normal for petrol to transfer from the right hand tank to the (empty) left hand tank when running with the right hand tank selected? I'd always assumed the two fuel tanks/lines to be separate.

Advice appreciated - thank you.

Annoyingly, this problem seems to becoming more prevalent with modern fuels which may be damaging the Series cars' fuel changeover valves. There are some variations in the fuel piping, dependent on model (6 or 12) and fuel pump location, but essentially, fuel is taken, via a valve in the boot, from one tank and pumped to the engine compartment, then the excess returned to the same tank via a second valve. The valves are controlled via the dashboard switch. Failure of the valves or the switch can result in fuel being drawn from one tank and returned to the other. That's not disastrous unless the second tank is already full, in which case fuel is dumped across one of the rear silencers and onto the road - not recommended for all the obvious reasons. The smell of fuel whilst driving is usually the first indication of this problem. Often, the quickest way to diagnose a faulty valve is to hold it whilst someone presses the tank switch. The valves can usually be felt or heard to click when the switch is pressed. Mike.

Its worth watching 'Harrys Garage' on YouTube, he covered this problem in his 2022 round up.
There is also another thread on here about it.

Failure of the valves or the switch can result in fuel being drawn from one tank and returned to the other. That's not disastrous unless the second tank is already full,

...or the LH tank has a leak - caused by a disintegrated fuel pump seal - which is why I was using the RH tank in the first place...

So, thanks for explaining Mike, and to Poprivet for directing me to that YouTube clip from Harry's Garage. It's particularly annoying since I'm always careful to avoid E10 petrol, yet suspect increased bioethanol to be cause of this recent pump seal failure. The car's parked up for now until the weather gets warmer and then I'll make a proper investigation.

Lesser cars don't have the option of choosing a tank! Mike.