Overheats when AC is on

[angusbn]

1989 535i - >400,000km - have owned this car since Summer 2006

First and foremost there is NO overheating problem with the AC off.

- Aux. fan runs perfectly when the AC is on - aux. fan resistor replaced 1 yr ago
- Main engine fan runs perfect when the AC is on
- New Behr rad and BMW blue coolant - both done in 2006 <9,000km ago
- Water pump and T-Stat - do not know the history of these two parts
- Temperature gauge in sitting at 2 o'clock, not in the red zone though
- City driving; backs off a wee bit on the highway

So no problem with AC off, somewhat of a problem with AC on.

Any suggestions - have I overlooked something? Something I need to live with?

Blair...

[nirvana19]

change the thermostat first, its cheap and very easy to replace (as long as you don't snap the bolts on the housing that is) and has a tendency to cause all sorts of weird cooling system problems. if the t-stat doesn't fix it verify that the fan clutch is good.

[bimmo_2007]

Check the radiator vent hose & it's hole in the expansion tank -tiny hole- as it could be clogged with grime which causes overheating issues.

[angusbn]

Perhaps I'll replace the t-stat - the fan clutch isn't the culprit if the engine fan is running OK?

[angusbn]

The rad vent hose is the so called "pisser hose"? If so, I'll disconnect and check - the tiny hole in the expansion tank wasn't clogged.

[bimmo_2007]

The rad vent hose is the so called "pisser hose"? If so, I'll disconnect and check - the tiny hole in the expansion tank wasn't clogged.

yeah it is the "pisser hose"