air lock??
 

had top rad hose develop a leak.....lost quite abit of water but car didnt get close to over heating as i spotted it.

replaced hose...ran car heater on full hot...while topping water up...

got no hot air enless i drive it fast ish...temp sits as it should as does water all fans kick in.

driving slow or in traffic stand still....heater blows cold air and temp goes rite up!!! soon as i put foot down hot air blows through and temp drops back to where it should.... checked water and still as i left it...

so....air lock or thermostat???

how do you bleed rads on these???

Most likely air locked. May just be easier to drain the system down then refill by the turbo water feed pipe.

total bitch to bleed...sheds loads of air but got there in the end...