I have a 1.6L petrol Peugeot 307 (2001) that normally is driven in the south of Sweden where temperatures are typically greater than -10 degrees Celsius, but am now in the north where temperatures can easily reach -25; it isn't impossible for the overnight temperature to go lower than -35 degrees. I purchased the car recently and am unsure on the ratio of the current coolant.
I am concerned about the pressure buildup due to the expansion of freezing water. The car doesn't have a block heater or similar. It isn't possible to park in a heated garage.
I won't be using the car for a few weeks. It won't be possible to do a coolant flush, but I may be able to replace some of the coolant with undiluted antifreeze to reduce the risk of freezing (I know Peugeot uses a "custom" coolant - any safe off the shelf alternatives? I can perform a full flush in a few months)
Question: To alleviate pressure inside the system, is it a good idea to partially remove the radiator cap? If it isn't a good idea and the problem is very real, what is my best option?