I have a 2003 MCS that I changed the thermostat and housing. When I got it back together and was putting coolant back into it, it overheated. I had it running for about ten minutes and then the fan kicked on. About a minute after that the fan got a lot louder and the coolant temp went through the roof so I shut the car off. No coolant came through the bleed screw on the top hose. So I took everything back apart to put in the old thermostat with the new gasket in to see if it was a faulty thermostat. When I took off the bottom hose the coolant was still cold but when I pulled out the thermostat coolant came out of there hot which coolant shouldn't have come out of the thermostat. Once I got the old thermostat back in it did the same except coolant came out of the top bleed screw and it took in more coolant. What could I have missed? I'm at a lose right now.

 

You had an air-bubble in the system...
These cars are tuff to get the air out of.
Simply re-bleed, and then check the fluid level a few more times...and rebleed....
It can take a few tries without a preasure bleeder.
You know it is right when the coolant level is stable, and when you first start your car in the am, and it is quiet, you don't hear a mini waterfall sound from the airbubbles trapped in the heater core!

 

So just let it run with both bleeder screws open? But shut it off and let it cool if it gets too hot?

 

First step...make sure you did not put the thermostat in backwards...it happens...
But an overheat after having the system open usually indicates an air bubble...
There are numerous threads on bleeding around..just have spare coolant to keep the reserve tank full, and don't worry about loosing a few quarts....it takes a while to get the air out. It should not overheat sitting, idleing, heat set to max, coolant tank full, and bleed line cracked open...the car must be allowed to get to full operating temp (fan on, no ac) to ensure the thermostat is open and coolant is flowing freely. The tank must be kept full, or it will suck in air...then start over...

 

That's what I have been doing. It will start to overheat before anything even comes out of the bleed screw.

 

Does the MINI thermostat have a little bleeder hole on the thermostat itself?

If it does, it needs to be installed with that hole facing up (towards the sky)so that bubbles can be expelled out.

Sorry, not had to put in a new thermostat yet, but maybe this might help the OP if this is applicable.

 

It seems to be working now. I took it all apart and then back together. It filled with coolant and finally came out of the bleed screw. I took it for a drive and the temp was good and the coolant level was good when I got back. I'm about to take it for a long drive now. Thanks for all the help.

 

I read in one of the threads about bleeding the cooling system... that besides the reserve tank, you can pour coolant into the bleed hole of the upper hose next to the radiator. Run the engine with the heater blower at max till the coolant reaches normal operating temp. Let the motor cool down, top off coolant and repeat.