Q: How long can I drive on a cracked coolant reservoir in the winter until there’s damage to my engine?

The volume and pressure of the engine coolant increases as the engine warms up. A purpose of the recovery tank is to serve as a temporary holding spot for the overflow of coolant that occurs both during engine operation and after shut down (the radiator cap has valving that allows coolant to flow both to the recovery reservoir and from the recovery reservoir back into the engine cooling system). If the recovery reservoir has a hole in the bottom that allows the coolant to escape, eventually the coolant level in the engine will become somewhat lower but the level will stabilize at that "lower" level. Often that means there is still enough coolant to still keep the engine running at a normal temperature. Of course, you can just keep a watchful eye on the engine temperature gauge and occasionally, when the engine is ABSOLUTELY stone cold (for safety reasons) check the level in the radiator. But, due to all the risks and uncertainties involved, it is probably cheaper in the long run to just have the recovery tank replaced. The tank is not that expensive and if you request service from YourMechanic on a mobile basis, that is have the work performed right at your location, it should be economical enough. If you want to go that route, the service to request is coolant reservoir replacement. If you request that service, the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.