My 2002 VW Passat wagon does this. Even after blowing out the heater core (it was partially clogged) and also flushing the cooling system a couple of times it continues to do this occasionally! You would think by now all the air would be purged from the system!!

I believe the other guys are onto something. Idle it when jacked up or on a steep hill to try and drive the air out of the heater core and heater hoses. Think I'll try that myself. My car is much better than it was so no urgency here... Ideally you want the coolant reservoir tank as high as possible relative to the heater core. If you have enough hose slack you could also try and lift the coolant reservoir tank out of its cradle as high as possible while doing this.

Have not tried this, but there is an air bleeder hole under one of the heater hoses on the VW. Loosen the hose a little and it will bleed air out that way. Maybe worth checking into this?

Dear 94 945T,

Hope you're well. To purge air from the cooling system, park the car facing up a steep hill. Run the engine for a few minutes. Then, park the car facing down the steep hill. Run the engine for a few minutes. That should allow trapped air to migrate to the coolant overflow tank. I do this whenever I drain/flush cooling system, replace heater hoses, etc.

If you're in a place where there are no hills, then do as "Volvo from Heck" advises - jack up the front of the car.

Hope this helps.

Yours faithfully,

Spook

Never have I heard heater core cavitation or of it, not even after I forget to pre-fill the heater core after a complete flush including the heater core. Trapped air should find its way to coolant tank. Thermostat orientation correct? Stop-leak stopping fluid from getting out but not air from getting in? I have read of parking the car up hill, nose up to help remove excess air but no proof myself.

TOm