When replacing coolant in a vehicle while conducting a scheduled maintenance coolant flush, it is good practice to leave the radiator cap off of the radiator as you turn the vehicle on. The effect of this technique is that once the vehicle reaches the specified temperature, the water thermostat in the vehicle will open allowing circulation of the coolant and allowing any excess air to escape through the cap opening on top of the radiator. Please make note of this, if you keep the radiator cap off for more than 4 minutes the radiator will begin to boil over and the effect will be reversed.