Changing the coolant flushing the heater core.

polomints

First time I took the car out the engine almost overheated and the heater didn't work at all. I then carefully loosened the cap to the expansion tank and a load of air came out. I'm guessing I should have ran the engine till hot with the expansion tank cap off for a while to remove any air? Anyway I've loosened the cap a few more time when the engine has been hot and now, not only does the heater seem to be working now, but I can modulate it much better, it's not either on or off any more.

There seems to be a lot more pressure in the system now as well.

Oh and this is what came out.