Ive searched with no specific results found. So I will now ask my question..

When I start my car and let it warm up everything is running beautiful. As soon as I turn on the heat the temp gauge slowly goes all the way down. If I turn the heat back off, the temp gauge takes forever but slowly rises back to normal. I dont know what is wrong. The thermostat has been replaced twice. Anyone have any suggestions?

still sounds like the thermostat to me. everything else stock? no ginormus radiator or anything? double check that you don't have any air bubbles in the system but my guess is that you have just had some shitty luck with thermostats.

another thought is that you might have some junk floating around in your cooling system that's getting lodged in the t-stats causing them to jam open, maybe try doing a very thorough flush of the cooling system

Thanks. I will. I guess another question would be what would turning on the heat have anything to do with my engine temp?... as far as it making it go down or not? I ask that because if I dont use my heat then it stays fine. But its winter... lol. Way too cold to not have heat.

well if anything I would think that with the heat on the engine temp would go down, but not much.. id say flush the crap our of the system and fill it back up and burp the lines see if that helps.. i dono good luck

Originally posted by cms
what would turning on the heat have anything to do with my engine temp?... as far as it making it go down or not?

Think about it, When you turn on your heater you are sucking cold air across the heater core to produce heat in the car. That heat is being taken from the engine. If the engine is not producing enough heat to keep up with the cooling taking place across the heater core fins then the engine temperatures will drop below normal. Unless you live at the North Pole this should not happen. It still sounds like your thermostat may be stuck open or somehow passing anti-freeze to the radiator when it should not. Either that or there is a big air bubble around your water temp gauge sender (unlikely).

When you first start the car, feel the upper radiator hose and see if it gets hot quickly. It should not be circulating fluid to the radiator until the car has been running awhile. Compare the temp of the upper radiator hose to the heater hoses or pipe. At start up, the heater hoses should always warm up way before the radiator hoses.

Would a new radiator cap be a remedy for this problem? It seems unlikely that I would run into two bad thermostats in a row. Also, it only does this if I turn on the heat. It will not do it for any other reason. It does however take longer sometimes for the vehicle to warm up. Just depends on the day. I did notice a couple that when the temp went down due to the heat being on that if I boosted a couple of times the temp would correct itself.. but later go back down if I kept the heat on. I dont know if any of this makes sense. Im trying to go with the most simple fix first as its winter and I dont have access to a garage to play around with it in.