Hi Everyone...

I have a 2004 XJR with 60,000 trouble free miles on it. Always well maintained, etc. Over the past week or so, a problem has crept up. The temp gauge slowly rises to it's normal temperature, as it always does, right in the middle of the range. It's never varied from there over the past three years I've had it. Over the last week, the needle fluctuates between normal, and above...never getting into the red. It doesn't stay there long, maybe a minute or so, then comes back to normal...then goes back up...bounces around above normal, then will come back to the normal reading for a while, and back and forth. It doesn't matter if I'm doing 70mph, 25mph, or sitting in traffic. It never goes into the red...but seems to fluctuate from normal to hot. Also...when it rises above normal, and stays there, it seems to automatically turn the A/C compressor off, and then when the temp gauge returns to normal, the a/c turns back on. I also ran it without the a/c on at all, and the same issue happened temp gauge wise. I have noticed the electric fans behind the radiator also seem to run a lot more than normal...both when the car is on, and when it's been off for a while...more than it used to...coincidental with the timing gauge fluctuation. Sending unit? Doesn't sound like a thermostat... Thoughts, could use some help here? Thanks very much in advance...

Adam

It's not the sender because you have increased fan operation and ac shut down, both of which indicate an actual overheat condition. My vote is for a lazy thermostat.
You have checked for coolant loss ?

I have, the coolant is filled to the top.

Thermostat, they stick alot, but normally partially open. The issue is more the plastic neck inside that holds the t stat in place weakens over time and comes apart. Either way you need to open and replace but br prepared to buy a t stat housing. Or you may want to do that to start with since they crack alot and they come with new t stat installed already

brutal,have you forgotten that all supercharged models have a metal housing...
also you like the labour times for replacing t stats

atweis,when ordering a new t stat have the last 6 of your vin handy 2 different t stats are used in that model year,also check that your hoses havent become soft,and once your done make sure the fan works,lately i have had more then a few fan failures not a common issue but check anyway

So, I have to now ask the question, in light of the housing being the metal alloy, as opposed to plastic, does that change your minds about my above problem being the thermostat?

Sounds like the first "mutterings" of a failing thermostat to me. Don't wait until if fails completely is my advice, or you really will boil up.

Thanks gentlemen, that did the trick. Changed the thermostat and did a coolant flush. Running perfectly now. Appreciate the help very much.

Adam