I have a working temperature gauge, for a change.

When I drive the car it runs normally at about 1/3 of the gauge, however when I turn the heater on to run my defroster, the engine temperature slowely rises to perhaps 1/2 deflection. When I turn the heater off, the temperature settles back down. It's blowing warm, and the radiator and overflow appear to be working correctly. No leaks.

I'm thinking there is too much flow being diverted to the heater core when I turn it on.
Thoughts?
Cooling the heater return too much and closing the thermostat? I put a 180° stat in, the radiator is new, the water pump is less than a year old, and the block was flushed this year.

It doesn't appear to be in the danger zone, but seems a bit odd.

CJ

What car are we talking about? Also, how fast does the gauge move? Will the gauge move when the fan is turned off and on? Heater fans draw a lot of power and if you have bad grounds and such you could just be influencing the gauge reading and not the actual engine temperature.

If your car is an A body built without AC I'm pretty sure the heater core gets full coolant flow full time. My van and pickup both had a valve connected to the heater hose operated by a cable to control heater output, but all my A bodies used an air mixing system and diverted air around the heater core to reduce heat output.

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Joshua

Yep. A body. I'll experiment with the fan. That could very well be it.

CJ