My 1989 V12 decided to start to overheat intermittently, she would run for hours then for no seemingly good reason the temperature would climb slowly or sometimes just skyrocket. To me this smelt of a sticking thermostat, so these were replaced with new ones from my trusty Jag supplier.

This fixed the skyrocketing temp but I sill have an issue where the temp sometimes creeps up. Now for the bazaar bit. When the temp is at the top of the N if I turn the engine off and immediately re-start (so the engine is off for less than 10 seconds) the temp miraculously drops to the bottom or just below the N.

Has anyone else experienced this phenomenon, maybe the temp sender or gauge??????

BTW the system was filled and bled as per the ROM

 

Reading carefully what you wrote I reckon you got earth issues with the instrument binnacle. Well described in Kirby's book.

Failing that it may be the sender unit going off, or the wires to it, or even the earth straps/wires from the engine to the chassis, also a well known issue with these cars.

It could also be something sinister like a dud thermostat, it does happen.

 

Thanks Grant. I have just done a road trip from Perth to Sydney and I had the temp going up and down between N and C. Found the spade connector on the sender loose. If it was a bad earth on the cluster I would expect all of the other gauges to be playing up, what do you recon?

I was going to replace the sender next. I will pick one up on Monday. Thinking about it I will pull out the instrument cluster and check the earths. I need to fix the AC flaps anyway no air to the center vents. Didn't think about the engine earth I will check this tomorrow, earth loops/problems can be a pain and cause all sorts of crazy problems, thanks it's been a long time since I left the bench.

Before installing the new thermostats I tested them in a pan on the stove to make sure they operated simultaneously (close enough anyway).

 

I had the same symptoms as far as gauge readings went,she would be normal then creep up and after immediate restart,back to normal.I did the extra ground wire trick from Kirby's book to no avail, changed thermostats and sender and had the rad done.Finally just put in an auxilliary gauge with temp values showing 190-195 in all conditions.As we all know here Jags gauges not too reliable.
Good luck

 

I'm not disagreeing with anything that has been said but for some reason I have a nagging feeling that you may still have an air pocket in the system....which can cause peculiar gauge readings just as easily as the electrical faults.

When you bled the system did you get a steady, strong stream of coolant from the bleeder?

If there's any doubt, re-bleed. Just my two cents :-)

Cheers
DD

 

Right on Doug,in fact if memory serves...... you recently outlined the procedure in great detail.
And for the record, you're 2 cents has been worth more than a few dollars to several readers

 

Thanks for that, Les. I help where I can, as do we all, here and elsewhere.

Jags are quirky cars. I would've never survived my first Jag (ages and ages ago) were it not for the collective help from many others. I am 100% convinced that the internet has played a major role in keeping old Jags (and others) on the road.

Kirby's book is a terrific resource. It sometimes paints an overly bleak picture of old Jags but it has tons real world of info that you'll *never* find in a traditional service manual.

Cheers
DD

 

Thanks Guys, when I bled the system I used the procedure in the book so as soon as the air was purged and coolant flowed from the bleeder I plugged it up.

Doug, I have just read your writeup of antifreeze replacement on another thread so I will give this a shot. Next is to actually check the running temp with my trusty infrared thermometer.

 

Thanks for all your input guys. As usual this is not one problem but a culmination of a couple of small ones.

Findings
When stationary in traffic the temp creeps up with AC on. Much more stable with AC off.
The gauge is sticking ever so slightly (give it a tap and down comes the temp)
And Doug's bleed procedure reduced the temp creep in traffic.

I have had the fan clutch checked by my local radiator guy and it is ok. I think the temp creep is caused by the aux fan not engaging until the coolant temp is about 100degC. It does not come on with the AC compressor causing the temp to creep up in traffic and also poor flow through the condenser (great design Jaguar).

I don't think the rad is blocked otherwise the temp would not come down so significantly when under way (guys feel free to chime in and correct me if I am wrong on this one)

With the new 82deg thermostats the temp is just over 1/2 way between the C and N. It creeps up to middle of N ( it does go a little higher with the AC on and stationary for prolonged period) then at constant 60km/h comes down albeit with a not so gentle tap of the gauge.

So next I think is a set of Ford AU electric fans, but this will have to wait until my stuff arrives from Perth.

 

Acatully it sounds like you're in pretty good shape although it would be nice of the guage could be trusted a bit more. Cleaning the circuit board and adding a redundant ground usually helps although your sounds like it is physically sticking.

The 88-earlier XJSs were wired so the fan came on with the a/c compressor. Why Jaguar changed it in 89 remains a mystery. It wouldn't be hard to re-wire it.

The fan should come on at about 94ēC. Your switch might be a bit iffy.

Cheers
DD