You should run the heater whenever you do a coolant change or hose change on a car to bleed the air out of the system. Once the car has the correct amount of coolant in it you can then check the temp out.

Has the radiator cap been replaced? They don't last forever.....

Cheers

ToranaGuy

Forgot to add 'new radiator cap' to the list. Havent taken it for a run since last night, will do later I guess. Is it normal for the heater core to suck 2-3 litres of coolant? What is the capacity of those things? I forgot to add that when the car is sitting in the driveway after a long (hot) drive, it will very quickly drop down to between the low end slash and the N and stay there as long as the motor is running, even in high ambient temps and with the AC on. This is what is leading me down the path of running with the seperate trans cooler as Im thinking it may be a problem with heat soak from the trans cooling lines running through the radiator.

Ok cool. I'd be looking at either the timing or fuel mixtures, if they are wrong it would idle & low speed run ok, but have issues at higher speeds, causing heating issues.

Cheers

ToranaGuy

Good idea, I'll pull the plugs out and check the colour tomorrow.

So now that it is tomorrow, Ive tracked the heating prob to a small split in the header tank around the filler neck, very p****d off as it is 2 months old. Dont buy aftermarket is the lesson here folks.