I've done a major cooling system overhaul (koyo, new WP, new tstat) in the past 2 weeks, and today I got everything finished up. Filled car, ran it with the cap open (s5) and the car heated up nice and quickly, but then did not cool down. Basically, my tstat refuses to open (I think). Its mazda part no. 8AF6-15-171-9U on the box, was sent from Malloy mazda, and looks alot like the ones posted here (reffering to tjeter's thread comparing the stant and oem ones), but it doesn't look quite the same as the one I removed. The system seems like it has pressure (no leaks that I can find), but the car can get up to 205F (aftermarket temp gauge in stock location) and I still have nothing (pressure, water, whatever) in the top radiator hose. The above is true with or without the radiator cap on.

I pulled the tstat housing off to check it, and I lost a whole pile of coolant as soon as the bolts broke lose, so it seems like there is plenty of heat and pressure behind it. Please help, I'm supposed to take this thing to auto-x novice school tomorrow and I am STUMPED.

 

If you want a sure way to see if your T stat is opening or not. Take it out. throw it in a pan.. put it on the stove and boil the water..

 

Just checked Tstat, cracked open at ~180, fully open by the time pot boiled.