Auto Service World
Feature   January 1, 2002   by Glenn A. Hunt

Back to Basics: Purging Light Vehicle Cooling Systems

Frequently asked Questions about Removing Air from a Vehicle Cooling System

Please use caution: Do not attempt this method on a hot vehicle or one that has a full cooling system. Never attempt to remove a radiator cap on a hot vehicle. Personal Injury could result.

Q: Why is it important to remove all air from the cooling system?

A: If air is trapped inside the engine block or cylinder head(s), it will cause an overheating condition. This could result in very costly engine repair or possibly engine replacement. A radiator or fill tank can look full but it does not mean that the engine block or cylinder head(s) are full.

Q: How does the air get trapped inside the engine?

A: Many newer model vehicle designs have lower hood lines and are more aerodynamic, which means that the engine is positioned higher than the radiator. While this promotes a more attractive, sleeker look, it creates a problem within the radiator. Basic physics tells us that water, which is part of the engine coolant, seeks a natural level. The coolant cannot be drawn into the engine efficiently because air exists between the upper portion of the engine block and cylinder head(s).

Q: How do you remove the air that is trapped in the upper portion of the engine?

A: You vacuum it out using air evacuation equipment, creating negative pressure within the cooling system (a vacuum). By creating this vacuum, it removes the trapped air and refills your cooling system while still holding your system in a vacuum. All fluid will be drawn from the coolant container into the system until it is full. (Ensure you have enough coolant mixture before performing this task.) Once the system is filled the vacuum will no longer exist because the coolant now occupies the void. No air, no problem!

Q: Air never hurt older vehicles. How can it hurt new ones?

A: Radiators in the older vehicles were positioned higher than the engine. It used to be easy to just fill the radiator and run the vehicle until the thermostat opened and then top it off, put the radiator cap on and drive away. With newer vehicles, most manufacturers specify to purge the air out of the cooling system. If air gets trapped in the engine block or cylinder head(s) and it is run for any period of time it will cause “hot spots”. This will damage head gaskets, the cylinder walls and the entire cooling system. Overheating and expensive repairs will be the end result.

Glenn A. Hunt is Product Development Manager for Mityvac/Prism Enterprises, Inc. Mityvac/Prism Entrerprises manufactures automotive test kits, cooling system test kits, specialty tools and accessories.

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