Driving down the other day, I noticed the temperature on the
my gauge was slightly elevated above the normal half mark,
on my 2003 XJ 8. I watched for a moment and it rapidly went
back to normal. A while later same thing. about 90 minutes
later I'm almost home and it starts going towards the red. I
pull in my driveway and pop the hood. Everything is normal
and nothing is hot. No smell of hot coolant. At this point I
was thing a stuck thermostat. The next day I ran it around
the area to see if it would still elevate. It finally
started to look like it was going to get hot. This time I
turned the heat on full. The gage went back to normal within
30 seconds. When I got close to my driveway I allowed it to
go up again. This time it went into the red as I pulled in.
Turned on the heater again and opened the hood. Again
nothing was hot, no smell, or boiling over. The gauge went
from 100% hot to normal in 30 seconds. After letting it cool
for a bit, I checked and coolant level is was normal.If I
had a bad thermostat or water pump, running the heater
wouldn't cool anything down that fast, so I'm guessing temp
sensor. Any thoughts? Am I missing something?

 

Might also note the electric fan was running hard after shutting the car off and running longer than normal, with no signs of the car running hot. At that point the gauge was still at normal all the time. This was happening for a couple of days before the gauge stated to "elevate".

I've ordered a new sensor and will have it installed in the morning. Hope this takes care of it...

 

This is EXACTLY what happened to me some months ago.

I noticed the fan running when coming back from some short trips, something never happened before.
I checked as well, but no smell, no extra heat, nothing.
Then, couple of days later, on a longer, more mountains ride, the temp went up like crazy, and hell broke loose.

Please note the temperature gauge inside the car is no use, it will stay at the half mark most of the time, then shoot up like crazy if the coolant reaches 220F.
You are playing with fire if you rely on it, as the margin of error becomes very very small once you reach 220F, and the consequences of overheating on your car are huge.
The only thing which prevented overheating of your car already is probably the much colder temperatures in Wisconsin you experience.
I should not ride it till you solved this issue.

Back to the problem, mine was a stuck thermostat, something which is not uncommon on the x308.
Replaced it, and the temps are back where they belong (190-210F).
I have a OBD2 readout, which reads the real temperature in F or C on my phone, a much better indication of whats going on.

You can read a bit more here, I got my problem solved with the help of this forum as well.
https://www.jaguarforums.com/forum/x...eating-167043/

Eric

 

Even after it shot up to hot, there was no sign of overheating in the engine compartment. I've gone through all kinds of overheating problem in my cars including 4 Jags, and there was no sign of it. No smell, steam, engine running normally. Temp went back to normal in 30 seconds of cranking the heater....

 

My thinking, if it is the sensor at fault, the opening of the heating should not make a difference.
Still think you might run high temperatures ( , yeah, I know I can be stubborn ..).

As you can read in my post, while I was in it, I reverse cleaned the heater core.
After that, I have up to ~5 degrees difference between the heating off or on.
That might make your difference.

Most easy thing, buy an OBD2 Bluetooth device from the net, you can get them below Usd. 20, and install Torque or Torque pro on your mobile.
It gives you instant confirmation on the temperatures, and takes out any speculation.

EDIT: While you are changing the sensor, it is just 15 min more to drain some of the coolant, and take the thermostat out.
Check it in boiling water, and you will see if it opens or not. Easy reassurance.

 

As eric said, you are playing with fire. There is an overwhelming number of threads on the V8 forums started by members who tried to outguess or ignored the symptoms you have observed.
Ron Jansen

 

I would do the following: reread Eric's suggestion; trust the gauge (you are overheating); change the sensor, maybe the cross over pipe with it; change the thermostat; change the coolant.

An OBD ii reader will read the temperature in real time while you drive; buy one that gives you data and use it to tell how hot you are getting. If you are still overheating, change the water pump, check the head gaskets.

 

Your symptomes are how my XJ8 reacted last year to a dead thermostat...

Fan was belting. It would run even on the coolest of days. The interior heater would be sufficient enough to cool the engine down to a useable temperature. My gauge went up and down like hell... Worst was of course it was middle of summer and we had 35°C in the shade... My car had no AirCon... GREAT!

I thought a dieing water pump or a blocked radiator would be at fault. None of which. It was simply a cracked thermostat. I replaced it against a cooler one, which opens a few degrees earlier... Since then, the old bugga' is running nicely (temperaturewise)

 

I'm pretty sure you're all pretty much right and I'm replacing the thermostat this afternoon.

 

Problem solved. It was the thermostat. Thank you for all of you great advice and insight. I have ordered a bluetooth OBD-II dongle and have downloaded Torque.

Thanks again, Mike

 

Now that is very good news, glad you fixed it.

Not sure on Torque, but in Torque Pro (just a couple of dollars) you can set an alarm on a reading.
I did so on the coolant temperature at 105C (220F), it gives a bit extra confidence right after the repair job.
I just hit it once (long mountain up in the usual hot tropical conditions here) and it is loud, scary loud, nothing to miss .

 

You should Install a RealGuage kit too. Or something that tells you the real temp of the coolant. The OBD readers are nice, but I like seeing the guage and beliveing what it is telling me. I don't boot up Torque everytime I get in the car either.

Our coolant guages are really "idiot lights" that stay in the middle until it gets really hot, then they shoot to the top.

Keeps granny/wifey from worrying about variations in temperature, but it can destroy your engine.

XK8 / XKR / XJ RealGauge - TheJagWrangler
Vector

 

My temperature sensor, or the temperature sensor metal collar embedded in the crossover pipe, kept leaking. I tried to reseal but to no avail. I ordered a new sensor and it came with a copper o-ring instead of the fibrous-type material on the old one. I carefully tried to tighten but the metal collar embedded in the crossover pipe came right out. Probably from that area being weakened.

I have ordered a new crossover pipe. It looks to have a new temp sensor with it. And of course I have the new one I ordered.

Can someone give me the torque specifications for all tightening points on the crossover pipe? Especially for the temp sensor.

Also, can someone indicate what is the "proper" o-ring that should come with the temp sensor? Copper, fiber, rubber? Should a sealant or Teflon tape be used.

Thanks.

 

I just replaced the crossover pipe with the new sensor and plugged it in.

 

i want to bring this back, it is happening to me, exactly as stated
i replaced the thermostat, coolant, water pump and cross over plastic pipe with new o rings and sensors attached, about 6,000 miles ago and less than two years.
could this be it, a defective thermostat that randomly sticks and then un sticks after the engine is turned off a few min? or i drive it for a while and it's fine then it goes beserk, could that be a thermostat that has just been replaced 6,000 miles ago, coincidentally going out at the same time as the expansion tank sprung a leak?