Celica Forums General Discussion Engine and Performance Overheating Mystery

So, to cut a long story short, I have a 52 plate Celica T-Sport that is overheating.
There has recently been installed a new water pump, radiator and thermostat.
There is no hot air coming from the heater and the temp gauge will go all the way up to hot, indicating that the thermostat isn't opening as it should.
Tried running without a thermostat for about 500 miles and it runs fine, although taking longer to heat up and only warm air from the vents not hot, which is understandable.
Put a new thermostat back in and the overheating starts again, as if there is something stopping the thermostat opening; forcing it shut.
Has been tested for head gasket problems and came through with no signs.
Other than some air still being in the system causing the thermostat to malfunction, does anyone have any idea what might be causing this?

They can be a bit of a bitch to bleed. I had to hang the reservoir from the hood latch with the bleed screw open to get mine to bleed. The exact instructions are in the manual. You will know you have it when the heater is working. Also as it gets hot squeeze the radiator hoses back and forth to tries and push coolant on to the thermostat to get it to open. Wear gloves and always be careful when working around a hot radiator.

My toyota seems to have this same exact problem! What did you find out?

Well I had the thermostat replaced with an official Toyota one and the system bled once again, and since then I haven't had any problems. The heaters still aren't working 100%, they're hotter sometimes and colder others, and they never all seem to be outputting the same temperature... I think this may be due to some air still in the system, but the engine temp hasn't gone above the third bar once in about 1000 miles since the new thermo was fitted