Engine Surge w/ Air blowing cold when set to full temp.

[Delta-Tango]

Got a petrol 2006 1.8L MR2 Mk3, with no mods. roughly 63k miles

When idle after driving for a few mins, the engine pulses, or surges.

Rev counter is normally at 900 - 1000, but drops to around 700 and back up again (all at the beat of either 120bpm yesterday or 200bpm this morning if that's relevant).

This happens regardless of the air blowing cold (when set to full heat temp.) if I leave it off after a while, it's hot for less than a minute.

Had a full+ service from toyota dealership (they clean the engine and replace all liquids - apart from petrol haha) and passed MOT all clear back in December 2017.

Noticed the engine coolant level is wayyyyy above the full line but not had any problems until yesterday.

Please advise.

[Delta-Tango]

Engine is fine now.
Blowers only blow cold air when idle (900rpm). If I speed up or keep revs above 1900rpm, hot air comes out.
What do I do?

[Slushbox]

Coolant air lock.Take off expansion tank cap when cold, start engine and run till hot.

Otherwise, bleed coolant system.

[Delta-Tango]

took it in, mechanic said its either heater matrix or water pump. engine won't overheat cos fans are working.
im stuck

[Delta-Tango]

All sorted - my radiator had been completely worn away. Replaced it and the coolant, works fine now