I think my Series 1 XJ6 4.2 might not have a thermostat fitted as it takes an age for the temp gauge to move and when it eventually does, it's no more than 1/4 of the way across the gauge.

Is this suspect and were should the gauge be once the engine is up to working temp?

 

in a normally operating cooling system, in hot weather, with a/c on, a little above normal. (center to right, all the green area of gauge is normal). If the automatic fan comes on when it should, (past the green area), it will bring it down to normal and the fan will run as long as it needs to, to bring the temp down according to the Thermal Switch mounted on the right bottom side of the radiator.

it could have a stuck-open thermostat, best thing is to replace it but be careful, the thermostat housing studs tend to break when undoing the nuts. Better treat them with Penetrating oil and Rust Blaster for a few days before attempting to loosen the nuts. And even then, go easy, put the wrench on the nut and start tapping the wrench gently. otherwise if a stud breaks, you'll disable the car until you extract the broken stud and replace it.

if everything goes well and all nuts are removed without breaking the studs, you'll need a new gasket and thermostat, and you can apply anti-seize to the stud's threads, adding a stainless steel flat washer and a lock washer before the nut for easier removal next time.

 

I completely agree with Jose - my 73 (which has a new aluminum radiator and thermostatic electric fan - orginal mechanical fan removed) takes just a few minutes to come to operating temp in the summer and stays absolutely pegged in the middle of the gauge at highway speed. If your thermostat is stuck open (or not there) consider yourself lucky because when they stick closed it can be catostrophic.

It is also possible you're having a gauge, wiring, voltage stabilizer or sending unit issue. When I first got my car the gauges were a little screwed up and I found that pretty much all of the wires to the various sending units were badly heat damaged close to the engine.

Definitely follow Jose's instructions and check that thermostat though - a bad gauge won't blow your head gasket.

 

thank you Mike. I have seen one Series 1 with the same wiring issues you mention. I would try to drive the car for a while and then check how hot the engine feels when you open the hood. If not too hot (Jaguars run hot, I have two), then it could be assumed the thermostat is either not there or stuck open. (almost the same thing).

I know that in very hot countries, people remove the thermostats to keep the engine cool. Considering this car is carburetted, I don't see an issue with removing the stat. If it was a fuel injected Series 3, then there would be issues.

99.9% of people complain about overheating!

 

Definitely Jose. When I lived in Florida I used to always come across old cars with the thermostat removed but in the Jag (as I recall) not having a thermostat at all will allow the bypass to remain open at all times which isn't the best situation and it will allow the coolant to leave the head way too fast which won't let it reach normal operating temp. A better way to go if you don't want to use a thermostat is to take the functional innards out of it so it's basically just a restrictor... still doesn't solve the bypass issue though.

Personally I've done the no-thermostat thing a few times in FL although that was with cast iron headed American engines... but in my defense I was young, impetuous and even poorer so the cheap fix was the fix I could afford.

 

You can buy a laser temperature reader at AutoZone, etc., for $30 and read off the thermostat housing and read exactly the temperature while running. Then do thermo accordingly.

 

Thanks Guys, I've got something to work with now.

I don't think it's a wiring issue as the temp does rise to the 1/4 of the gauge, but I'll do the temp check on the housing and go from there.

 

If you have the repair manual there should be a temp vs resistance chart for the temp sensor. Compare the resistance read at the sensor against the chart and a temp reading from an infrared thermometer to confirm the state of the sender.

 

the shop/service manual will also tell you to substitute the Temp Sender with a new one, and that installation is the reverse of the removal.

the service manual does not tell you to add Teflon tape to the sender's thread to eliminate potential and future leaks!.