1987 xjsc needle up to over 3 quarters

Just bought car today drove in 20 miles home and needle was at over 3 quarters. I shut car off and heard fans kick on. I checked oil no milkshake. Also no smoke out of exhaust. I turned car over and it started right back up no drag etc. Car only has 5300 miles on it and looks like time capsule. What’s best easiest way to diagnose this . Thanks in advance George

First step is probably to buy one of those optical temp sensors and start pointing it at various parts of the engine. See if the car is really running that hot, or if the gauge is just flaky. Usually you can tell if it’s really too hot by the gurgling and hissing sounds from the engine.

If it’s really that hot, drain the coolant and remove the radiator. It’d be good practice to take it to a shop and have it “boiled” or otherwise have the innards cleaned out, but chances are pretty good you’ll see the problem as soon as it comes out: the outside of the core is probably completely caked up with crud. If air can’t get through, it can’t cool the engine.

Install coolant filters in the upper radiator hoses before you turn the key to start it after that radiator service.

Whatever you do, don’t drive it any more until you get to the bottom of the temp issue.

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either gauge/temp sender is wrong, thermostat(s) are stuck closed, or radiator is clogged.

I’d start with thermostats and work your way up to radiator.

I’ve used the optical sensors, and they are quite close to actual. Point it at the coolant pipe where CTS is, and it should be within 10F of actual coolant temps.

Car idles high. And does not feel like it’s as powerful as my other xjs. I took it for a spin around the block a bunch today. It heat up to 3 quarters and stays there. It was hunting a little bit. Then it evened out. Could it be overheating because of other issue beside cooling system.

Typical symptoms of failed auxiliary air valve. Quite common problem on these engines. Can be rebuilt though :wink:

Yes, high idle on mine was AAV not closing all the way. Common issue.

Regarding temp, go buy a $30 optical temp sensor and determine if your car is really running that hot. Because if it is, you run the risk of head gasket failure and dropped valve seats.

If not, you either have a bad gauge or temp sender.

If so, replace thermostats and go from there.