I have just recently bought a 1999 Jaguar XK8, it has only done 58k and was running fine until I decided after looking online to have the following changed including a full service, replacing the gearbox and read diff oils.
Engine timing chain and tensioners, fan belt, thermostat and housing (metal one), radiator and all engine coolant hoses, radiator expansion tank cap (O-rings were missing)
The work was carried out by a Jaguar Mechanic but now after changing all this there seems to be a lot of pressure in the cooling system that eventually caused the water pump gasket to leak and a cracked engine water coolant pipe. The system was bled before the water pump failed and no water leaked from the recovery tank which would lead me to think the cap was bad and not releasing the pressure but it’s brand new. I removed the expansion tank but can’t see any faults there and checked for any blockages in the tank pipes and blew through the recovery tank hose which doesn’t seemed blocked either. The radiator fans are working fine as well so I am really not sure what the problem is, I live in the Middle East and it is very hot at the moment but the car was fine until I changed the parts listed. Any help on this would be greatly appreciated?

 

I am making the assumption that the coolant pipe is the one at the top front connected to the thermostat housing and radiator. If that is the one that you are referring to I notice that it was not in the list of replaced parts. It should have been considering all the other stuff replaced. The missing 'O' rings in the reservoir could have been masking an already failed pipe. That pipe is a common failure point. I have one on order myself.

The water pump was not in your list of replaced items though it is referenced as failed. If it was replaced perhaps the 'O' ring was not well seated. If new I would remove it and reinstall with a new 'O' ring. If not new just replace the pump. After that a system pressure test would identify any leaks and only leave the pressure cap as suspect if, indeed, you even have a pressure problem.

There are two sources of abnormal pressure in the coolant system; the reservoir cap and a blown head gasket.

 

I think I remember reading this on one of the threads in this forum. Some of the replacement water pumps come with a paper, rather than a metal gasket, and the paper ones tend to leak. Not sure where I read it, but you should be able to search it out

 

Test point's reference to the missing o rings could be very relevant. Did you replace the radiator or just the hoses, if not replaced ave you back flushed & cleaned the radiator, if it has restricted flow even by a little then in your climate it will not take long to over pressurize.

 

Thermostat. Use an IR temp sensor to verify open/close cycles. You should see at the thermostat housing a rise then a fall in temperature around 195 degrees.

 

What controls the coolant pressure? The reservoir cap seems to be just a cap. It has no springs or such to control the exchange of coolant to/from the expansion tank.

I am chasing down minor coolant leaks as well. They started with the cap. A new cap seems to have fixed that. Then it moved to the thermostat housing bleed hose where it connects to the reservoir. Tightened the clamp a little - fixed. Then the leak moved to where the radiator bleed line connects to the reservoir. I couldn't get that wacky little Jaguar hose clamp off, so for now, I just added a regular hose clamp next to it and that seems to have fixed the problem.

Despite all this, I still saw more than a puff of steam driving to work yesterday and got a Low Coolant alert that afternoon. However, that evening after the engine had cooled, I took the cap off and the reservoir was full to the brim! This would drive me crazy if I wasn't already "there". ;-)

So back to original question: What controls coolant pressure and the expansion tank exchange?

Thanks! - Jim

 

Replace that fragile plastic hose line running between the top of the plastic coolant outlet duct and the coolant expansion tank. It deteriorates and warps over time and heat cycles, and I found that it could weep coolant at its connections even with the factory Norma cobra hose clips properly in place. I replaced it with a section of 5/16-inch rubber fuel line hose, threw away the two factory Norma cobra hose clips, and replaced them with two properly-sized worm-drive clamps. That ended the coolant weepage issue from the old plastic hose. But I still had to replace the factory plastic coolant outlet duct because they also get brittle and warp over time and heat cycles and tend to leak at the thermostat housing cover. Once I installed the new plastic coolant outlet duct along with the new 5/16-inch rubber fuel line hose between the coolant outlet duct and the coolant expansion tank in late November 2014, my wife's XK8 has not leaked or lost a molecule of coolant since. I suspect her car is still running its factory coolant expansion tank so I watch it like a hawk as well. At nearly 10 years old now, I know it is probably the next cooling system component to give up the ghost....

So replace that fragile plastic hose first. If you still find leaks, you should run a pressure test on your cooling system to identify the next weakest link in the chain. It could turn out to be your coolant outlet duct, your coolant expansion tank, or both....

 

The reservoir cap controls the system pressure and manages the transfer of coolant to and from the recovery container.

 

What am I not seeing? The cap looks like single piece or solid assembly (no moving parts) of molded plastic with one or two O-rings. How does it control anything?

 

Look on the side of the threaded portion above the 'O' ring. Mine has a hole that allows the coolant to come and go. As far as how it works, that is FM.

 

Looking down into the center of the (inverted) cap, we see a round, copper-colored metal thingy. Not visible, but there's a spring behind it. In the presence of 15 psi or so it gives way, allowing fluid to flow past it and from there on to the recovery tank.

Push on it with something soft, a wooden dowel or similar, and it gives.

 

Thanks Dennis. Just before reading this, I took a closer look at the cap and saw exactly what you've described. I guess it is a more complicated piece than I thought.

I also got the Norma-clip-thing released. I wrapped a few turns of black electrical tape around the hose to add a little more diameter, put the norma-thing back on and hope that it'll provide enough squeeze to seal now. If not - off it comes and a normal hose clamp goes on.