HELP!! '07 Mazda3s Thermostat Replacement problems

I changed the thermostat on my gf's car this morning. The old one was stuck open and the car was running cold causing a CEL. I used a Beck/Arnley replacement which turned out to be Motorad.

Anyway, I get it replaced, get the car up to temperature w/ heater and fan on high and refill the coolant. First thing I noticed was no heat. I took the car for a 10 minute round trip drive to the store. Using Torque to monitor temperature, it got up to 228*F when on the throttle. Backed off and it would cool down. Heat would be a furnace when on the throttle and go away when off the throttle.

The rad. hose on the driver's side of the engine is hot and the big hose coming out of the thermostat is cool. The smaller hose is warm.

I tested the new thermostat in boiling water before I put it in, and it opened and closed fine. Once in the car though I think it's defective. I'm going to have to put in the old thermostat (at least it is stuck open) to get her to work tomorrow if I can't get this working right.

Any other suggestions?!?!?! Thanks!

Sounds like you installed it backward.

Impossible to. It's a molded piece. Only fits one way.

Boiling hot at cruise, no heat at not? Almost sounds to me like some air got into the system somehow, or perhaps you lost some coolant due to burn off when the old thermostat went to E36 M3..... Or i suppose your water pump could be failing, and as a result it cooked the t-stat.....

You need to bleed the system, you've got an air bubble in there somewhere.

Bled system twice. No change. I put the old thermostat back in. Its at previous cold running condition now but wont overheat on her at least. Need to order OEM thermostat tomorrow. New one definitely defective. I've read that the aftermarket thermostats cause problems in the 3. But i did it anyway. Aaarrrggghhh. Piece of E36 M3!

Sounds like air is trapped somewhere. When my Speed3 needed a thermostat I got the dealer one. The thing is a bitch to change. I didn't want to do it twice because of an aftermarket part that fits but doesn't work quite right.

OK. I got an OEM thermostat installed today. No issues. I learned my lesson on that one. There are just some things that you need to get from a dealer.