Could someone please advise the correct cap for my coolant tank reservoir.
At the moment it is fitted with a 15lb recovery type cap and I have noticed there is still pressure in the tank after 24 hours.
I am thinking the cap should be a non recovery type.
Thanks

 

I assume you are referring to the steel expansion tank attached to the inside of the left wheel arch within the engine compartment. This should be fitted with a pressure cap. There is also a blanking cap fitted to the thermostat housing that is used when filling the system. A pressure cap must not be used at this location, the pressure cap on the expansion tank is where the pressure cap must be fitted.
If you still have pressure in the system after 24 hours, I suspect you have a head gasket problem caused by a cracked block. Cracks between the bores was a very common problem in the 4.2 long-stud engines, and very expensive to cure.

 

The header tank or reservoir is similar to this one and there is a plastic overflow bottle behind left headlights in wheel well

 

OK, so you've got the later, plastic, header tank, the one on the XJ6 I had was the old steel tank. That should have the pressure cap on it. Looking at drawings on the SNG Barratt web site, (thank you SNGB !), the header tank drawing shows the pressure cap. There is also something callled an "atmospheric recovery tank" with its drawing that appears to be something that would collect coolant getting past the pressure cap. This second tank is just a collector, and is not pressurised. Why this is fitted I know not, but probably something to do with pollution.

I also noticed that there are two versions of the thermostat housing. The one on my car was the earlier one that has what is referred to as "radiator filler cap", which I called a blanking cap,. It just allows for easier filling of the coolant. The later housing seems to have no means of easily pouring coolant into the engine, so I assume it has to be done via the header tank.

Anyway, the key fact in your post is pressure remaining in the coolant jacket when the engine has cooled down. This is a classic symptom of head gasket failure, so I suggest you get the coolant checked for combustion products. Most good workshops will have an electronic tester for this.

 

The atmospheric tank is really just an overflow tank. When/if the brown tank reaches capacity any excess goes into the atmospheric tank. On cooldown, it is drawn back into the brown tank....so a 'recovery' type pressure cap is needed on the brown tank.

In this later arrangement the brown tank serves as both an expansion tank and a header tank, with 'header' being a generic industrial/auto/marine term for a place where liquid is filled/added

The brown tank is typically filled about half way as is the atmospheric tank. More than halfway generally results in the atmospheric tank overfilling and coolant dumping on the ground. Actually, if you don't mind a small mess you could fill both and normal expansion will push out excess. In this way the system seeks it's own level, so to speak


I tend to agree, but pressure remaining for a full 24 hours? I guess it's possible. I'll have to mull that.

Cheers
DD

 

Thanks for the replies.I have been filling the expansion tank right to the top and there is definetely half a tank in the over flow bottle in wheel arch.
I will fill the expansion tank half inch below the top and see how it goes.

Note..I am not having any issues with the engine or temperature.

 

At the top of the tank ae two vapor pipes. Just for giggles...are they clear?

Cheers
DD

 

Hi Doug,
The 2 vapour lines are clear,in fact I blew all the pipes out with compressed air and there are no blockages.

 

When my engine began playing up in the late 80s, there was pressure in the coolant jacket 24 hours later. It was a chap I had bought a boot lid from, who also rebuilt XK engines, that alerted me to what was causing my loss of coolant. My engine was the long-stud block with liners to cover the cross-block coolant passages. It had cracks between the bores and was only cured with another, uncracked, block. Riviera's engine may, of course, have the slotted block, in which case just a clean-up after removing the head, and a new gasket is the cure. Slotted blocks don't suffer the cracking, but even so head gasket failure is fairly common from about 85k miles onwards, but may be earlier.
The slotted blocks were a typical Jaguar engineering spatchcock, (aka a quick fix !!) to enable the engine to carry on for a bit until the AJ6 aluminium engine could be fitted. It was, of course, never fitted to the XJ Series 3 cars, only the XJ40 series of XJ6 saloons from 1986 onwards.