Wwhen idling or going slowly in trafic and always when I'm lost my Z3 overheats. Switching on heater and full blast fan ususally brings temperature down. Anyone any idea of cause?

Hi Grizelda

Welcome to the Forum

Overheating assuming you don't have a leak and fluid levels are correct, is normally either thermostat, or auxiliary fan related if your aircon is working so should the auxiliary fan. The auxiliary fan provides additional cooling when either the aircon is on or extreme conditions such as heavy traffic 

Get your engine driven fan checked and the auxiliary fan plus the thermostat.

Dave

Hi Grizelda

There is a possibility that in a 19 year old car, there is a build up of sludge / residues in the engine generally and particularly in the radiator.

Could be that a thorough flush out and refill with new coolant could solve the problem. That shouldn't be too costly and can be done at home by someone competent or by any (not back-street !) garage.

Hope this helps

John