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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?

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If the system is cold, even as cold as you are stating, there won't be any worries of pressure build up. If the coolant expands to cause pressure issues during freezing weather, you have much bigger issues on your hands (IE: possible freeze plug damage or worse). Really, your concerns should not be concerns at all in this case.

As for testing the coolant, get a refractometer to test the antifreeze levels. I'd suspect even automotive parts shops in Sweden will have these available at a reasonable price.

As far as which coolant to use, I believe Glysantin G30 is the standard, but I'm not positive about it (Note: G30 should be a deep red in color. If your coolant is of a different color, then I've got this wrong). You can check your owner's manual to see for sure. There's several different makers of the coolant, but I'd agree with you to add straight coolant (not 50/50 mix) to bring the mixture level up to a maximum of 70/30 antifreeze to water mix. 50/50 mix is good down to -34ºF (-37ºC). 70/30 is good down to -84ºF (-64ºC). If you can get the antifreeze percentage up even 5% (55/45 mix), it should give you better protection. Every 5% of increase in antifreeze should give you another ~7ºC worth of protection, for a maximum of -64.4ºC at 70% concentrate.

To get it to the 70% concentrate, you'll need to bleed off some of the coolant from the system (obviously). There should be a drain plug at the bottom of the radiator. Unstop the top as well so you don't have a vacuum effect keeping the coolant in the system. I don't know how much coolant your system holds, but I'd suggest draining out a liter or so at a time, then refilling at the top to replenish with pure antifreeze. Once you have the new liter put into the reservoir, you'll need to run the system until the thermostat opens so as to mix the solution. Once it's fully mixed, use the tester to check it again. This is going to be a laborious process, but you want to ensure the mix doesn't get past 70% or other issue can arise. You want to ensure coolant is fully mixed or you'll get a false reading of the concentration.

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