Do I Put Coolant In My Car While Running?

If you are putting it directly into the radiator then you put it in with the motor running. If you are putting it in the coolant reservoir you put it in when the engine is off and cold.

The best way to add coolant to your car is to add it to the overflow tank and when the radiator cools, it will draw the coolant to the radiator...if you add it while running you run the risk of it splattering back on you causing burns and a mess under the hood.