98 XJ8 overheats

[RonJ]

First time poster so I'd like to start with a Hello to everyone!
I just purchased a 98 XJ8 Vandenplas with 62,000 miles and am having overheating issues. When I picked it up it had a coolant leak at the base of the thermostat housing. A new O Ring solved that problem and all was fine for about 3,000 miles. Then the top radiator hose bew off after about 6 hours of steady driving. I checked the thermostat which was working properly, flushed the cooling system and replaced the spring loaded clamps with regular screw type clamps. It ran fine for another 200 miles or so then started to get seriously overheated and began blowing coolant out the overflow bottle at the left front fender! Now I can let it sit and idle for for extended periods of time and it will go through the proper cooling fan on and off cycle several times with no problem. I I even raised the idle to 1500 rpm and let it run through the cooling cycle a few times with no problem. As soon as I begin to drive the temp rises quickly and it overheats blowing coolant again!
The only thing I can figure is that I must have a blown headgasket?? I don't claim to be a Jag expert so I'm wondering if I am missing something else here???
Thanks for the help!

 

[sammyb]

Not a Jag expert, but the standard coolant things:
1) Replace your thermostat
2) Make sure the radiator cap is the correct psi -- the fact that it isn't venting properly leads me to think someone might have fitted an incorrect cap.
3) Your water pump might be toast -- moving enough water at low RPMs to keep it cool, but not nearly enough at driving speeds. Of course, more likely is that there's a major blockage in your radiator, so there's not enough flow to keep up with the water pump.

 

[Dave Russell]

Hi Ron,
A few checks that can be quickly & easily done by a radiator shop:

1- Check for combustion by-products in the coolant. This is a chemical test & can show a leaking headgasket or cracked part.

2- Run a pressure leak down test on the system. This can show if there is a leak either internal or external. It will check everything but the cap itself which has been mentioned above. Combined with the first test, it will narrow things down a lot.

3- Run a pressure test on the radiator cap. It will quickly show if it is ok or not.

At least, you will have some idea of where to focus your investigation, which is better than randomly replacing parts.
D

 

[RonJ]

Thanks for the pointers guys. I did check the thermostat, it seemed to open properly (the wife wasn't thrilled with me cooking it on her stove but that's another story). Even though it seemed to be okay I replaced it anyway.
Coolant flow is strong so that rules out the water pump.
I've got a $150 pressure system tester that naturally doesn't have the the adapter that fits the Jag. ggrrrr..lol
I don't have the equipment to test for combustion by products so I guess I'll have to haul it into the shop so it can be tested there.
I'm begining to resign myself to the fact that I may have to shell oout around $2000 to replace the headgaskets! I don't think that's something I'll be able to do in the garage where I don't have a lift or engine hoist.

Anybody want a Jag with an overheating problem and 67k on the clock for $11,000.....lol....

 

[John S Farrington]

You can make some tests to verify or discard your head gasket theory. Usually a leaking head gasket will allow water to enter the crankcase oil. That will make the oil turn milky looking. Conduct a compression test on your motor. Variances between cylinders (more than 10 psi) will be a sign of a head gasket leak. Further testing by pressurizing each cylinder with high pressure air while on the compression stroke will reveal a head gasket leak by allowing the pressurized air to find its way to the coolant chamber, the crankcase or even an adjacent cylinder. I would certainly do this testing before signing up for head removal. My personal feeling is that your radiator might be clogged up. Have a radiator shop check it and rod or recore it if necessary.