Fuel Tank Cap seals

[oldtimer]

Hi All,
An advertisement in the July 2014 JEC Magazine (page110) under the heading, Ken Jenkins, tools and spares, shows the picture of a Fuel Filler Cap Seal.

The reason I mention this, is that it is not listed as a separate item in the Jaguar Parts book and I doubt if the complete cap is available anyway.

I fitted two brand new tank cap assemblies at least 20 years ago and now find that the seals have split radially at about 1/8 inch spacing all the way around.

The seals differ slightly between the Series 1 and 2/3.
Series 1 - OD 65mm ID 45mm
Series 2/3 OD 65mm ID 40mm

So at the monumental cost of £2 .00 each I have ordered a couple today.

Whilst on the subject of Tank Caps you may be interested to know that it contains two valves, one to let air in and one to let air out.

Have you ever noticed a rush of air when the cap is opened up for a refill, this is air rushing in and at the same time the side of tank grumbles as it returns to the correct shape. A tiny relief valve is contained inside the domed part of the cap. An air relief hole can be seen and the valve is located inside which has a flat rubber face that sticks to it's seat. This sticktion can normally be released by breaking the seal with a suitable size drill. Stuff a lump of rag down the neck of the tank in case the drill is dropped.

There you are, not everybody knows that

I hope that is of use to somebody

Oldtimer

[mike020150]

FWIW my local MoT tester has filler cap seals on his list of favourites for failing cars brought for test. Consequently, over the years, my series 2 coupe has had a couple of sets of seals replaced to avoid this pitfall! Mike.

[khamsin]

I am a returned (after a long time) member and have just found this post. I have bought a neglected S1 XJ12 and have a fuelling issue; after a few miles she runs out of fuel, and splutters to a stop. If I open the filler cap I get a rush of air. I assumed this was caused by a partial vaccum in the tank causing the pump to sruggle pushing enough fuel through. I stuck a small drill through the hole in the under-side of the cap and 'unstuck' whatever is in there, I havn't tried the car since. My question is; as the valve in the cap can be pushed upwards this must be for releasing pressure rather than vacuum, so how would a partial vacuum be released?.

[mike020150]

Hi Khamsin,

The tanks 'breathe' via a small bore pipe which can be seen underneath the car just forward of the rear axle. It's the same pipe which, on US emission cars, continues forward to the charcoal canister. For UK spec cars, it finishes in a location which can become plugged with dirt. Poking it out is best done with something like the plastic inner from an old electric aerial. However, also make sure you check that the car is returning fuel to the same tank it is pumping it from. Failures of the tank valves or even a dodgy dashboard switch can cause related problems. Usually, the first signs are a smell of fuel as it is pumped from one tank into the other already full tank causing it to overflow. Mike.

[khamsin]

Mike020150. Thanks for that, but I would need convincing. My filler necks do not have a tube facing forward (underneath the hinge of the cap) which would on North American(?) spec and perhaps later cars be connected to the charcoal canister and associated emissions pipework. There is a drain hole towards the rear of the filler neck but that I think is just a drain for rain water and exits beneath the fuel tank, it is outside of the cap closure so could not vent the tank unless it where connected to some other pipe. There is a vent pipe which exits the top of the tank (I have not removed the tanks) but that I presume is blanked off on a UK car. If there were vent pipes why would the cars need a valve in the filler cap? I am tempted to buy one and cut it in two to see what goes on inside. What do you think?

[mike020150]

I haven't a good pic of the V12 carb petrol tank to hand to confirm it beyond doubt but since both the 6 cylinder cars and the later V12 PI cars definitely have the small bore vent pipes, I would be very surprised if the V12 carb cars don't have them. They are located approximately 60% of the way along the length of the tank top measured from the filler cap end (i.e. not in the tank neck). There are slight variations in the pipe runs, model to model, from that point but you should be able to see where the vent pipe emerges eventually under the car. There are indeed water drain holes below the filler caps but they have nothing to do with tank venting. Mike.

[khamsin]

This one?

[oldtimer]

Hi Khamsin,
Oh heck, where do I start!

I have a '73 early Series 2 V12 on carbs which I have owned from almost new.

Twenty years ago I fitted new tank caps.

I have in front of me one of the original tank caps covered in 'orrible wax oil. This cap does NOT
have a vent pipe connection by the hinge - nor the ones currently fitted to the car.

As I said from the outset, The tank cap with the spring loaded dome which seals the tank when the lid is closed contains a TWO WAY VALVE allowing air in as the fuel level drops and air out as the ambient temperature rises.

I have just been sat working out exactly how the two way valve works. I needed to sketch it out on a piece of paper to get the function correctly orientated. My previous attempt was not correct or complete.

The valve comprises of a single one inch diameter disc Attached to the lower face of the disc is a flat 1/16 thick rubber sealing disc of exactly the same diameter. This is held in place by a coil spring above the disc pressing downwards and keeping it closed. With a rise in ambient temperature the pressure of expanding air and fuel in the tank is released via this valve to atmosphere.

In the centre of the disc referred to above is another valve of much smaller dimensions. On this occasion the function is in the opposite direction in that air is allowed in as fuel level drops or temperature reduces. This valve is a 1/4 inch diameter disc attached to a spindle which passes through the main disc and is held in place by a very lightweight coil spring. On close inspection I could not fathom out how air was able to pass through this valve. All was revealed with the use of a magnifying glass, only to find that the spindle is a square rod passing through a round hole. How about that !

So now we know, the big valve allows air out and the little valve allows air in. Although a drill passed through the vent hole will release the large valve the unit will need to be dismantled to gain access to the small one.

It's getting past my bedtime so I shall not say too much about vent pipes that Mike refers to.
I have studied my Series 1, 2 and 3 parts manuals. The parts picture posted above says not CAN/USA I can only conclude this is an error 'cos the Series 2 parts manual says CAN/USA only and the Series 3 book says CAN/JP /USA only. The parts book clearly show the vent pipe connections on the tank top and the filler cap which are linked together but not for the European market. Apart from all that they are not fitted to my car.

I hope this has helped, do let us know how you get on

There could be other things in the fuel system to look at but not tonight

Oldtimer

[khamsin]

I have freed the stuck fuel cap valve, replaced the engine bay fuel feed hose, the existing one had an aftermarket filter in it, and took the shed out for a spin. Seemed to pull ok and was going well for a while but then started to lose power. Got home ok, before she would have come to a stop, but on much reduced power. She kept running but would not accelerate. Got out and opened cap, still a whoosh of air but much less so. Better all round, but not right. I have not yet looked at the pumps. I did notice that the left hand manifold heat shield is missing; is there a possibility that the fuel in the left hand carbs is vapourising? I bet one of those is easy to find.

[oldtimer]

Hi Khamsin,
Have just read your thread in which you refer to releasing the valve in the tank cap.

It would help me to understand, if you could amplify exactly what you did to both tank caps.

Did you remove the dual valve assembly from the main cap and carefully prize open the lugs which holds the unit together to gain access to the second (small) valve). ?

If you did, you will have seen the square operating spindle passing through a round hole which gives adequate clearance for the passage of air - providing it is clean of course.

Your reference is to only one tank, what happens to the other one.

There is a superb AC Delco fuel filter in the back right hand corner of the spare wheel tray.
Have you found this and replaced the filter ? Replacement filter cartridges are still available.

You did not mention how far you drove the vehicle. If you repeat the same trip with both tank caps released at least you can determine if your problems are tank vent related.

I think that is sufficient to be going on with

Oldtimer

[khamsin]

An update. I have replaced one of the pumps with a new one. I drove the thing for about four miles, saw seventy at one point, then the same problem; loss of power, keeps running but will not accerlerate. Next I fiddled about with something else and then repeated the exercise with the filler cap open; exactly the same result. So tanks flushed and filters replaced, filter in boot replaced, main underbonnet hose replaced, new pump. Next I shall measure the flow from the pump and if not to spec look to the back to front fuel pipe.

[oldtimer]

Hi Khamsin,
Interesting to read what you have done so far but like you confused as to what is going on.
A fuel flow test is no bad thing. These twin solenoid pumps deliver 30 gallons an hour. A short while ago I serviced both of my pumps and achieved that figure.

I presume you appreciate that there is a supply pipe from the filter in the boot that runs up the off side chassis rail through to the carburettors and a return pipe back to the boot running along the near side chassis rail. Just by the front nearside carburettor is an inline relief valve in the return fuel line. When the float chambers are full the surplus fuel returns to the tank that it was drawn from. In a quiet environment this valve can be heard functioning when the ignition is switched on and the engine stationary and at the same time the pumps can be heard clicking over and pumping. Two solenoid operated fuel valves are mounted in the boot amongst the pumps and pipe work. their purpose is to direct the fuel back into the tank selected at the time.

I would hope that you are able to switch from one tank to the other and back during your flow checks to ensure the complete fuel system is functioning correctly. At least you can be satisfied that there is a reliable supply of fuel before exploring further.

I trust you will take all the necessary safety steps during this task.

Finally, from your comments earlier , in that after four miles you suffered a loss of power, I am scratching my head - is it a heat related problem

The joys of old Jaguars

I hope this helps a little, do let us know how you get on

Oldtimer

[khamsin]

Today I have carried out a simple flow test by running the fuel hose into a one litre bottle while using a stopwatch to time it. One litre in twenty seconds, approx 180 litres per hour; whatever it is it isn't too little. I am surprised.

I think I mentioned that the exhaust manifold heat shield is missing on the left-hand side, I purchased a second-hand, later one, as a stop gap but needless to say one fixing bolt has been snapped off and the other doesn't want to budge. What does anyone think about my petrol vapourising theory?

[oldtimer]

Hi,
Fuel Pump flow of 80 litres an hour means nothing to me but converts to 40 gallons per hour which I believe is rather optimistic as the spec is 30 gallons per hour . The point is, that your fuel flow test proved that an excellent fuel flow is available.

Your workshop manual describes that this vehicle has a recirculating fuel system, when the four float chambers are full and the float valves are shut, surplus fuel is returned to the tank from which it came. Whilst the engine is running under a range of throttle openings the float chamber valves and return relief valve work in concert. Open the tank cap of the selected tank and fuel can be heard to be returning providing the engine is stationary and the ignition is on.

The V12 engine under bonnet temperature always seems a bit overwhelming, it is perhaps the sheer size of it all. Although having an exhaust manifold heat shield missing is not ideal, with ambient temperatures of recent months I find it hard to believe that vaporisation could possibly be an issue especially in view of the re circulating system described above.

Oldtimer

[richardpierce]

The breather on an XJ12 S1 is in the filler cap

They clog and after a fuel miles this causes a vacuum, no fuel and car stops, the car will run perfect with the cap open short term

Exact same happened to mine

Long term give it a good blow, that sorted mine

Any photos of the car

Richard