mk e wrote:
I did read somewhere that the heater core can freeze when the AC is on so maybe that is the reason?
Bullshit.
If the A/C is functioning correctly, that should not be possible, as the evap won't get below freezing. Evaps that get below freezing are actually a problem as they will ice up and block discharge air.
My dad's V8 Datsun used the old-school DA6 (IIRC) compressor and ended up with a little too much refrigerant, and it would ice up the evaporator... but that's not exactly working correctly.
Also, even if the evaporator were below freezing, there is ZERO chance of bringing 195 degree coolant down below 32 degrees in a single pass using HVAC discharge air. *MAYBE* a car that does not have a full-flow heater core would be more susceptible to this, but that's lining up a whole lot of unlikely events.
I @$$ume the 308 uses a full-flow heater core like GM cars do. I know the E30 and E34 BMWs (and probably most similar vintage BMWs) control the heater core flow with solenoid valves, but that's extra complexity of the kind Germans love, while building a full-flow heater core is simple.