Is my thermastat stuck open or closed?

[gabegt90]

Hey mangs I have a problem. Last saturday I installed a new radiator so when I bled the system when puting new coolent in it. the coolent system will not build pressure. the car is at normal operationg temp 200 F and it won't pressure so my first though it would be that the thermastat is stuck closed. but when I drive the car it does not overheat then I thought I could be stuck open.
when I'm driving the temp guage goes from 187-215 it goes back and forth sometimes but will stay at 200. my question is do you think that there is still air in the system or the thermastat is stuck open or closed. I allready pressure tested the cap and the system there good. the weather has been in the teens around here so do you think that's why it's not overheating if the thermastat was stuck closed any info would help I really don't know what the problem is

[Blaze86Vic]

Your cap is what regulates the pressure. Go get a new 16psi cap, and have a good day! If the temp is fluctuating then that means the thermostat is opening and closing so it's working.

[gadget73]

Might still be air in there. Keep coolant in the overflow bottle and top off the radiator as needed. I have one of those lever vent caps on mine, and I can burp the air out after getting it hot simply by lifting the lever. Bubbles come up in the overflow bottle, and as it cools, coolant is sucked back in. I've also seen some aftermarket radiators that the neck for the cap was formed slightly strange, and the stock cap doesn't seal quite right. Might need to replace the rad cap if you're using an old Motorcraft one.

[gabegt90]

I just got a new cap last year but Will get a new one i guess I also need to put a new hose for the overflow bottle

[Lincolnmania]

get the air out, replace the radiator cap, make sure there are no leaks