temperature problem

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I am having a problem with the temperature on the Grand Voyager.
It will warm up, but then I turn on the heater, the temperature drop back down to cold. I am thinking the thermostat, but I am not sure. The coolant level is full.

There's been quite a few posts about heating problems, winter must be here! Lol. Anyhow if the temp gauge is reading lower than normal, then it could be the thermostat. Be a good place to start, or it could be a clogged heater core, from what I hear is a pain in the ass to get at. Good luck.

There's been quite a few posts about heating problems, winter must be here! Lol. Anyhow if the temp gauge is reading lower than normal, then it could be the thermostat. Be a good place to start, or it could be a clogged heater core, from what I hear is a pain in the ass to get at. Good luck.

I replace the thermostat, there was daylight visible when I pulled it out of a cold engine, definately a bad thing. Also I replaced the radiator cap. I discovered it didn't hold pressure when the engine was warm. Problem solved.