I'm the new owner of a 1999 XJR with 94,000. The motor was replaced at 45,000 with a steel sleeved motor and fresh chain tensioners. The temp gauge climbs towards the red about every third day of driving, usually after coming off highway speeds into slow traffic. The "LOW COOLANT" light comes on and it drips a small amount of coolant from the area of the overflow tank. Overall, it has not lost much coolant. When the gauge says it's hot, I can pop the hood and some of the small hoses are hardly warm at all and none of the hoses are too hot to touch. Both fans seem to run when they are supposed to, and when I shut the car down at normal operating temp, the fans don't run, which seems fine.

In the last four weeks, I've:
- replaced the water pump twice with the new version.
- replace the thermostat twice.
- replaced the hoses under the supercharger.
- radiator removed and cleaned out.
- replaced the temp sensor.

Anyone got a theory?

 

Good suggestions. The car is now on its 3rd week at the dealer: they thought they found a heater hose in the back of the engine that had a slight split...then it busted and dumped coolant...put on a new hose, drove it, and guess what...still overheated. They think the electrical aux water pump is fine. I'm running out of parts to replace nearly as fast as I'm running out of money.

Could it be as simple as a bad radiator cap? Do hoses ever collapse on these cars? I've simply never heard of not being able to diagnose an overheating problem.

 

Yes, I've lately wondered what happens to the plastic blades of the water pump if they "melt."

The saga continues: today the dealer said even though it passes the pressure test, it may be a slight leak in a head gasket. Of course, any diagnosis after 2 1/2 weeks seems suspect. It seems 90% of the time a blown head gasket is the result of overheating, not the cause of overheating. It doesn't have water in the oil or oil in the water, it doesn't burn white smoke and it runs perfectly. I would think that a bad head gasket would need to eliminate coolant as the cause of overheating, but it hasn't been low on coolant, other than the slight loss from the overflow tank. If the car heats up because of a head gasket, I'm surprised that I can let it cool for 2 minutes, drive on, and then not have the it overheat again for 2 days.

Has anyone had experience with a micro-leaking head gasket where exhaust gases heat up the coolant causing overheating on an intermittent basis? Man, that sounds way out there...

 

Oh, and I like your concern about the fans, but I recall them working correctly. I suppose it's possible that they work on high speed after it gets hot and I open the hood to check, but maybe they are not behaving correctly at normal temp, causing the overheating...

 

Sorry to hear of your overheating. Hopefully, you have a simple fix instore since the water pumps in these cars are notoriously bad, and the thermostats are a bit touchy, too.

I'll finish the story on my XJR overheating problem, which I hope won't keep you awake at night: we finally determined via chemical analysis that exhaust gases were in the coolant, indicating a bad head gasket. My mechanic replaced both head gaskets, put everything back together, and the car instantly spew coolant out the overflow tank even before it got up to operating temp. It then quickly overheated. Clearly, the very hot exhaust gases were still being introduced into the coolant at high pressure. We gave up, and the next solution was a new motor. This one works fine and the car is again terrific. My mechanic talked about how strange (loose) the head bolts felt when torqued down. After taking the bad motor apart, we discovered the leak was all along the head gasket. These motors don't suffer overeating very well, true of all alluminum motors, I suppose. The fact that the factory does not authorize the dealership to tear down a motor is an indication of how tricky these things can be.

I bought my XJR from local individual who is a master jag mechanic and he stood behind the car (1999, 92,000 miles) and ate the costs to make it right. He's a terrific guy, obviously.

Best of luck.

 

I have a 99 XJR with 125,000 miles. It starts and idles fine for about 4-5 minutes and then begins overheating whether it is sitting or moving. I've changed the water pump and the thermostat, any other ideas guys?

Thanks in advance.

 

I burped the cooling system and the vehicle is no longer overheating at idle. I can let the vehicle sit for an hour and it will not overheat. As soon as I start driving it, it immediately overheats.

Any ideas?

 

Ok, similar problem with overheat at stoplight so how do you burp, belch a cooling system? yes 97 xj6 vdp
thanks