Hey Reddit, I drive a 2012 Civic Si and my coolant level has dropped over the course of two years. When my car is cold, the coolant reservoir is empty but the radiator is full. Once the car is warmed up the fluid makes its way into the reservoir. If I add coolant to the reservoir while it’s cold, will I cause an air bubble to stay stuck in the hose somewhere between the reservoir and the radiator? I have to imagine when it cools the coolant is stopping somewhere between the reservoir and radiator. I don’t want to introduce an air bubble and have my car overheat on the way to work.

Any input on how I should handle this or fill? Additionally, do I top up with premix coolant or distilled water? I’ve read that it’s the water that evaporates off but not the actual coolant. My mechanic however told me to not use distilled water and to top off with Type II coolant which is what is currently in it. Not quite sure what I should do and don’t wanna mess anything up lol. Any advice on how I should proceed?

Thank you for any help!