So I swapped in a new motor a couple months ago (97' B18B in a 90' DA) and not long after the swap the thermostat got stuck open not allowing it to warm up. So I went ahead and replaced the thermostat with a new OEM one and did a flush and everything seemed great at first. But recently I have noticed once the car starts to heat up and the needle starts moving if I turn on the heat it will go back down to cold. It only happens when the car is just starting to warm up though, after it warms up to like a 1/8 of the way or more on the guage it's fine. Like you know how the needle is well below the bottom mark on the temp guage when it is completely cold? Well when it starts warming up and the needle reaches the bottom of the guage if I'm to turn on the heat it will drop the needle back down to completely cold. It will do this up until the needle reaches about a 1/8 of the way on the guage, after that it doesnt do it anymore and everything seems normal.

It's not really a big problem now but it is something that has been bugging me because it is winter and I'm usually trying to get heat out of my vents as soon as possible. So it's really gay when you see it's starting to heat up then you put the heat on and it drops back to cold again. And I know this isn't normal so I want to fix it. So does this sound like a bad thermostat again? If not, any other ideas?

 

why not just wait till its warmed up enough to turn the heat on? it is a new thermastat, so maybe its supposed to do that. if the gauge isn't 1/8 the way up the gauge i would suppose that its not warm yet.

 

I realize I could wait another couple minutes for it to heat up and it would be fine but it shouldn't be doing it in the first place. I have owned this car for many years and I always would turn the heat on as soon as the needle would start to move, it never once made the needle drop down. So obviously something isn't working 100% and I would like to fix it.