How to De-pressurize a Coolant System

Always de-pressurize a car's coolant system before attempting to check the coolant level, adding coolant or attempting any repair on the system. Modern car coolant systems work under pressure to allow for the high temperature under which the system operates, about 250 degrees. Since the mid-1970's car manufacturers have used overflow reservoirs to contain the overflow when the internal pressure exceeds the recommended PSI and bleeds off a small amount of coolant. through the radiator's pressure cap.

Instructions

Difficulty: Moderately Easy

1. Realize that a pressurized coolant system filled with extremely hot coolant is very dangerous. Releasing the radiator cap under these conditions can cause serious injury. Depressurizing the system is a function of time and temperature. The only safe way to depressurize the coolant system is to allow enough time for the temperature to drop to a safe level, below 100 degrees.
2. Resist the temptation to open the hood and remove the radiator cap as soon as you stop the car. Releasing the pressure suddenly results in hot steam and coolant spewing all over the place and scalding you. If steam is coming out of the engine compartment, don't even open the hood until the steam dissipates. When it's safe to open the hood and while the system cools down, look around the engine to see if you can locate the source of a coolant leak.
3. Avoid pouring or spraying water over the outside of the radiator to hasten cooling. Cold water on a hot system could damage the radiator and other hot components, further damaging the engine parts.
4. Test the temperature by carefully touching the radiator cap and the top of the radiator. When you can comfortably rest your hand on the cap and radiator top, it's safe to remove the radiator cap. The drop in temperature depressurizes the system by itself. A cooled down system is a depressurized system.
5. Add water or coolant only when the system is cool. Refill the overflow reservoir, if necessary , to the level marked on the outside of the reservoir.

Tips & Warnings

• PSI (Pounds per Square Inch) is the measurement of pressure inside the car's cooling system. Each auto manufacturer lists the recommended PSI in the car's operating manual. The PSI may also be printed on the car's radiator pressure cap.