Odd question about cylinder head temp

Based on a couple of poorly-controlled experiments, it seems to me that when my car is cruising on the highway, turning *off* the air conditioner seems to cause the cylinder head temp readout to rise on that center dashboard display. Turning the AC back on again causes it to go back down again.

Normally with the AC on in FL, I see around 185 degrees. With the AC off, it usually hits 200-205. My car is 100% stock with about 9k miles on it.

Am I just losing my mind, or is that the way it should be? In case it weren't obvious, I'm not terribly familiar with the finer details of car engines.

The AC turns the fans on.

PewterCam said:

The AC turns the fans on.

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That would definitely explain it, I just would have figured the fans that cool the engine would always be running, not tied to the cabin cooling system.

The fans only run when they need to, not all the time. 200 to 205 isn't bad. Nothing to worry about.

I'm in Northern California and I see around 187-189 most of the time, though occasionally it spikes to 205-210. Always weirds me out, though it still seems fine so I haven't really looked into it. Didn't think it could be tied to AC though.

first_timer_FL said:

That would definitely explain it, I just would have figured the fans that cool the engine would always be running, not tied to the cabin cooling system.

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Yup it functions like a household AC system. In the car when the AC is on and the Compressor is running the cooling fan also runs to pull air through the AC condenser which is usually in front of the radiator. It can have the added effect of cooling the engines coolant as well.

What type of speeds do you mean by on the highway? It's possible the grill shutters are closing with the AC off to try and save more fuel as well.

Banjo said:

What type of speeds do you mean by on the highway? It's possible the grill shutters are closing with the AC off to try and save more fuel as well.

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Good point! It could even be the shutters are opening more and allowing more air when the AC is active. If the condenser doesn't get enough air the AC won't work at all. I did a bunch of logs when I first got my car and found the shutters stay closed or almost completely closed a lot.