Hello!
I've recently bought a 2004 S-type 4.2 as my first Jag to see what driving and owning one is like.
However, I bought it for a cheap because the car has a problem - seller told me it "loses coolant".
I saw that the cap of the expansion tank is bad and I've changed it. The car was running fine and seemed to have no issues with leaking coolant.
I drove the car for 30-40km and then turned it off. When I started it again after an hour the temp gauge started to go above the middle and before it reached max I've turned the car off. I have noticed that coolant has been pouring around from the expansion tank cap all over the engine bay. Apparently the pressure in the system was high, because the cap was brand new and OEM.
My mechanic hasn't yet found time to see what the problem is. He says that the car works OK when it's idling.
What could cause high pressure in the cooling system? Blown head gasket/s are one possibility, but what else can it be? Water pump? Thermostat? Anyone had the same isssue?

Thank you!

 

Based on your description, it's most likely a sticking thermostat.

It's best to replace the entire water outlet, which contains the thermostat as it's made of plastic and can only withstand a finite number of cold to hot to cold cycles before failing.

 

Is it the N/A, 4.2 If so I would check the thermostat housing. (seat) It can break internally and cause your issue. The chip that is missing is usually on the top of the water pump.

 

The first thing I would do is to take the coolant cap and have it tested to see if it's releasing pressure when it should, the cap controls the whole coolant system. For a blown head gasket which is unlikely, the 4.2 has a thin metal gasket, have the coolant tested to see if there are hydrocarbons in it. If you are lucky, your cap isn't relieving pressure and you just need another cap. Just because the cap is new, it doesn't mean that it's working properly. FOMOCO caps only! I've had great luck using a cap from a modern Thunderbird with 16lbs pressure instead of the 20 lbs with an S Type cap. Less pressure, less stress on all the components. It might be the expansion tank that is cracked and leaking too. If the expansion tank is cracked and only leaking under pressure you can fix it by putting on some aluminum tape over the cracks when it is cool and dry so you can drive the car until the new one arrives. I've gone through two expansion tanks myself with the 20lb caps. Changing the thermostat housing is pretty difficult, but if it isn't opening, it isn't opening.

 

not how it works…110kpa cap vents faster in an overheat that’s it