On my 1998 Jaguar XJ8 with the AJ26 engine, I have an overheating problem. It started when my thermostat housing went bad and I had a leak, note that the car had never overheated on me before. I replaced the housing with a new aluminum one. At the same time I changed the thermostat. After doing this the car started overheating, but only sometimes. It heats up and then rapidly cools back down.

The first time it overheated, I drove an hour away and while on the interstate it started to overheat, after I started to panic the car cooled back down again and I was able to make it to my destination. I checked coolant level and it seemed fine. On the way back home it did fine.

Driving there a second time, it did the same thing. The level was fine again. On my way back this time though, it overheated and cooled rapidly again. I checked the fluid level and it was low a very slight amount. This made me think it was a faulty thermostat so I changed it out.

Later that week I had to drive to a job interview and it did just fine, however when I drove to my sisters house only 10 minutes away it overheated and rapidly cooled both on the way there and on the way back.

When I say overheated, the temperature gauge goes to the red and stays for a few seconds and immediately drops. I have tried to bleed the car more times than I can count, using instructions on the forum. When I have it in my driveway I have tried to leave it running to see if I can add fluid then, and it did not overheat and I can't seem to put any more fluid in it. I think it has an air pocket, but I have not had any luck with getting it out. I have searched this thread and tried to find something, anything that will help me find out what's wrong, with no avail. Sadly, my X-Type is no more. I'm tired of getting rides from family, and am about to start a new job.

If someone could please advise on what to do. I have not had this issue with any of the other jags in the family and I am at a loose.

Thank you for helping me in advance.

 

Sounds like your temp sensor is going bad, or there's a wiring fault somewhere. I don't remember with these if the resistance goes high when temp goes high, or the other way around. If it's high resistance/high temp, a bad connection could make the computer think the temp suddenly went way high. That's just a guess though... I feel like coolant wouldn't jump a couple dozen C going from operating temp to red range, then back to operating temp in the amount of time you're talking.

 

I was thinking the same thing.

 

Hi,
Did you ever had the opportunity to determine the effective coolant temperature by datalogging using a OBD scanner or via IDS?
Provided that the engine control module passes through the signal of the engine temperature sensor instantly and without delay to the instrument cluster, a faulty or at least intermittent faulty engine coolant sensor (ECTS) comes to mind. It is also possible that you (still) have air trapped around the area of the thermostat tower, ECTS and water outlet pipe. You tried to rule this out, but what if the bleeding process is impossible due to a blocked bleeding line? Did you check this? Since the area around the tower/ECTS/water outlet pipe is almost the highest point of the coolant system, all air will be trapped right there.

What I've learned from reading in the forums and my own experience, the temperature gauge does not rise on a linear basis with the effective coolant temperature. This allows the presumption that there's some kind of preliminary processing of the ECTS signal before getting to the temperatur gauge in the instrument cluster. And that leads me to the take an intermittent faulty instrument cluster into consideration. There is no way that the increase and sudden drop of the gauge needle reflects the actual temperature - not within seconds.

What about the coolant that you used? Is it the right specification for your car and/or is it the same as was used by your preowner? The point I want to make is that mixing up different types of coolants end up in producing some kind of gunk in the coolant system which can lead to blocked bleeding lines amongst other very unpleasant things.
Cheers, Alexander

 

By the way, please find attached the details concerning the ECTS signal to help you with diagnosing it. Admittedly, it's quite difficult to determine the issue if you cannot reproduce it.

 

Thank you for the suggestion, I've been pulling my hair out trying to figure out whats wrong.

I don't believe the engine is actually overheating because I have not seen any signs that it has done so. No burnt smell or milky look to the oil or the antifreeze. I have an OBD scanner I will use to see if it happens again. I have also been able to feel the part the sensor goes into when has "overheated" and it feels about the same temperature as it does at normal operating temperatures.

I cleaned the connectors to the sensor in case that could be an issue. If it happens again I will change it out with another. I will update this either way.

Thank you all for your help!

 

The symptoms you are experiencing are very similar to others who have found the sensor to be failing.

The crossover pipe has probably not been changed, and they deteriorate much like the thermostat tower. This is a good time to replace the pipe, which will come with a new sensor, and also do the short connecting hose.