My 2001 XJ8 is overheating, and I can't figure out why. At first, it was overheating when I wasn't in movement, and then we discovered the fuse for the second fan was blown, and replaced it. That seemed to fix it, except now, it overheats when I drive it for more than 15 minutes with the air conditioner going, and if I turn it off, the temp drops again. Without the air conditioner running, it will not overheat. What should I do next?


Also, the turn signal will not make its sound unless I set my seat in position 2. THEN the sound will come on. Why is this?

 

The turn signal problem is a very unusual one.
I can offer some advice for the overheating. The most likely culprit is the Fan Control Module. When the air conditioning is running the fan module is supposed to turn on both fans and place them at high speed. It sounds as though that is not occurring.
The ECM tells the fan control module that the A/C is running at the module steps up the fans.

 

Did you top up the coolant after replacing the fuse?

 

The coolant was at the right level after changing the fuse.

 

I can hear both fans running so I am not sure the module is to blame. Is there any way to test this?

 

What I suspect is that you have chronic overheating, by which I mean it's always running too hot. The temperature gauge will hide a lot of problems, as it is heavily processed and will read normal over a wide range of temps. So what is happening is the engine is running hotter, hotter, and hotter, and then suddenly tips over the point where the gauge is programmed to indicate overheating.

That's why it appears to overheat with A/C on, and not when it's off. The A/C is the straw that broke the camel's back.

This constant but hidden overheating is very damaging to the engine, especially the heads and head gaskets.

Number one rule of Jaguar ownership : don't drive it broken. Stop driving it until you have verified the problem, and fixed it, or it will join the legion of X308s that overheated, blew the head gaskets, and are for sale for parts.

If you take it for a drive with a code reader or something like the Ultraguage, that will show you the real operating temp.

PS : Forget about the turn signal for now, as the health of the engine is a far greater concern.

 

As Mark says, it's critical to determine the actual temp. Stop running the engine until you have a reader to monitor the temp.
It could be a stuck thermostat or a failed water pump. When you are able to run it again touch the hoses on both sides of the radiator. Is one hot and the other cold?

 

I am coming back to thank you for the help- we ended up doing the hoses test and it was the thermostat, which is strange because it was just replaced last year, along with the housing by a professional mechanic. I am worried though that since the thermostat was broken it was quietly overheating and not lighting for a while before we figured it out and maybe has damage of some kind? How would I know if it did suffer damage, and what would be most likely to be effected? Should I watch for anything?
We still have no idea about the turn signal. It just does whatever it likes, pretty much.And the door lock button is also a sometimes working thing.
Thank you again, I doubt we would have figured out what was going on without the help.

 

Thermostats are a weak point for some reason. If the car was overheating to a serious extent a dropped valve seat or head gasket failure are among the likely problems.

The valve seat would make a tapping sound and probably cause a misfire, so you would be aware. A head gasket could start with a small coolant leak not easily detected until you get a low coolant message.

The best way to protect the engine from overheating is to install the Realgauge modification marketed by whitexkr on his thejagwrwangler.com site, or an OBD reader like Ultragauge or a smartphone app. Search code readers for some ideas.

 

There is now a little tapping sound but it is like the sound my other car used to make when it had cheap gas. Would it sound anything like that? It is new. I am trying to figure that out too.

 

With all the possible noise makers in the engine it's impossible to give you an idea over the internet ... the valve noise can be localized with a stethoscope. Nothing can be done to reverse the damage if done .. time will tell.

Using the correct thermostat is so important --- it's a shame .... I have seen so many Jaguars and MB's done in by a $35 part. The Jaguar OE is about $35 .. property ordered in the aftermarket it's about $10 cheaper.

 

Here is the link for RealGauge: XK8 / XKR / XJ RealGauge - TheJagWrangler

It has saved quite a few cat lives.

 

You can do a test for exhaust gases in the coolant, but if it has no other symptoms of head gasket failure I wouldn't bother (They are : overheating, constant need to top off the coolant, misfiring, boiling over, cloudy oil).