OK this has been driving me nuts for a while now, so excuse the LOOOOOOOONG explanation , just want to give as much detail as possible.

I've visited three mechanics including a radiator shop and a Jaguar mechanic. I have a 1988 V12 XJS, the needle goes all the way to the top ( as far as it can go) within 20 mins.

Shop 1: General mechanic - Said it could be bad guages the car is not getting hot? They replaced the thermostats on a previous visit about a year ago. They also suggested driving the car with no thermostats ( I declined)

Shop 2: Radiator specialist and general mechanic - Replaced the two radiator caps and said he was stumped why the car reads hot. Been in business for about 60 years. Owns an XJS himself ( without issues). Referred me to shop 3 after saying he was 'stumped' but could pull the radiator for $500.

Shop 3: Referred to this Jaguar specialist by shop 2. Lots of Jags there and I was told the mechanic previously worked for the dealership. Guy is from England and talked a big game about how he knew everything about these cars ( and my H & E was the worst Jag ever made etc). Promised to diagnose it for $60 looking at it for an hour. Car sat in his shop for nearly a week. He changed the temperature sensor, a $20 part and charged me $150 total. Needle still goes all the way up but he said the car is NOT overheating!! Said he could just start replacing parts, look at the just replaced thermostats etc............. for more money. No solid diagnosis, but again he said the car is NOT overheating.

So I'm back to square one after taking it to 3 places. Symptoms are the needle gets all the way to high. No coolant leaks or excess coolant from the expansion tank. When I stop and shut the car off the auxilliary fan turns on after driving the car for 20 - 30 minutes. It shuts itself off after cooling the car for 5 minutes or so.

Could it be a short? T-stats ( which are new), clogged radiator?? What are the tests for diagnosing these?????

 

*IF* we assume for the moment that the engine *isn't* overheating then we must look to the temp gauge. They are notoriously innaccurate. These cars are well known for problems with dirty contacts and bad grounds at the rear of the instrument cluster. The usual fix is to remove the cluster, clean all the contacts and connectors, and add a redundant ground wire.

In some cases the temp gauge itself is faulty and needs replacing but over the years I've concluded that the above procedure usually fixes things up.

You already have a new sending unit so you should be OK there.


Next, I don't know any mechanic nowadays who doesn't own one of those hand held infra-red thermometers. One of these can be used to very easily determine if the engine is actually overheating. Readings can be taken at various area of the cooling system to look for hot/cold spots. If the temp readings at the thermostats are about 190ºF your temp needle should be about in the middle of the "N".

If the readings show the engine IS running too hot then new thermostats (*correct* thermostats, I might add), a professional radiator boil-out, new fan clutch, and *properly bleeding the system* will usually fix things up. A good XJS mechanic will know to clear the steam/vapor hoses as well as the hose to the header tank as they're often clogged.

Personally I'd begin with the hand held infra-red thermometer thing and get an idea of what your coolant temp actually is and go from there. (Well, these won't tell you *exact* temp....but close enough to know if there's a problem or not)

Other will chime in. What I've written just scratches the surface of Jag V12 cooling system discussion :-)


Cheers
DD

PS- SaMoHi Class 1976

 

what doug said.
I removed my instrument cluster the other day and plugged it in and it initially didn't work properly. I plugged it in correctly and when i turned on the lights the two middle gauges pegged and the defrost light came out.

Needless to say the wiring back there can be weird and get crossed up.

A simple temp reading indicating whether it was hot was all you needed, from then all you are looking at is different parts of the system that take the temperature. Half the places you looked sounded like they went on about stats and radiators, but you can tell that within 5 seconds of using a IR thermometer, what a joke

 

Thanks Doug, I guess the next question is removal of instrument cluster and re-installation. I can do a search. I'm curious if you knew my old friend Booby DeSouza? He was a samohi alumni. First guy I knew in Santa Monica, bit of a long shot?!