I am chasing an overheating issues with my teenage son's Volvo S60.

The car developed a coolant leak because the small plastic crossover coolant tube cracked. I drained the system, replaced the tube, and vaccum filled the cooling system with a 50/50 mix of Volvo coolant and distilled water.

Following the repair the vehicle overheated on the test drive. There was no heat coming from the heating system. Then right before I pulled over for a tow, the heat started blasting in the cabin and the vehicle temp came back to normal. Assumed it was an air pocket. Waited overnight for the car to cool and expected I would need to add some coolant to the reservoir becuase of the vacated air but the level was fine.

Since then I have taken 3 more short test drives. All of which had a high temp episode with a loss of cabin heat only to have everythign go right back to normal. Once normal I can let the car idle at home. The radiator is rejecting good heat and the fan speed is modulating.

The only item of note from the repair is that I could not get any drainage from the lower petcock on the radiator drain.

Almost seems like the thermostat is sticking but I wouldn't think that woudl impact the heater core for the cabin??

Any help woudl be apprechiated