changed T-stat, still not warming up right

So I swapped in a B18B a couple months ago 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?

the heater core acts as an radiator. when you turn the heat on, the warm coolant from the engine is used to heat the inside. since your engine/coolant is barely warm, when the heat is on it will just get cool. it happens to me too =/ i just let it warm up fully b4 turning on the heat.

I don’t understand why it has never done it before though. Car is completely the same as before, just a new longblock and T-stat. And I understand what your saying but if the needle is like a 1/8 of the way up on the guage it really shouldn’t go all the way back down. That almost has me wondering if the new thermostat is stuck slightly open or something. I’ve owned quite a few DA/DB’s and never had one do it like this.

I guess if I can’t fix it it’s not really that big of a deal but considering it never acted like that until after I changed thermostat that has me thinking something isn’t right.

hmm i wouldn’t really know since i have the fan blowing as soon as i start my car. however it takes way longer to warm up when its freaking cold and i have the heat on trying to not freeze my ass. but as stated above if you turn your fan on while the engine is warming up the coolant temp will drop down a bit before it starts to pick up again.

as long as once its warmed up it stays warm and theres heat that comes out the vents i wouldn’t worry to much about it.

Did you leave the jiggle pin on the t-stat facing up?

you didn’t mention this- did you bleed the cooling system?

You may have an air bubble in the system. Check out your Haynes on how to bleed. Also like stated above… the jiggle pin has be “up” on the thermostat when it is installed. Good luck.

Yeah, the jiggle pin is facing up. The way the seal is you would have to be retarded to put it in any other way because the seal totally wouldn’t sit right if the jiggle pin wasn’t in the little cutout in the seal.

And I’ll try bleeding it. I never use the bleeder screw when filling up the radiator, just let it idle at keep topping it off as the fluid level goes down. I also give the hoses a few good sqeezes to try and get any air in them out. Ive never had a problem with trapped air before doing it that way but I’ll give it a try anyway.

But I still can’t help but think the T-stat is slightly stuck open. Anybody else think that might be the problem?