Bubbles in Radiator Overflow Tank With No Overheating

by Geoff Hineman

If you notice bubbles in your overflow tank, this may or may not be a problem. In fact, if your engine is not overheating, the odds are pretty good that you they are not problematic. Still, if this symptom signals a difference from how your cooling system normally works, it is worth investigating further, as it could lead to a large repair bill if the bubbles are indeed indicative of a problem.

How Your Cooling System Works

To understand where bubbles could come from, it is worth briefly discussing how a cooling system works. The cooling system in your vehicle is comprised of passageways throughout the engine block and heads, a water pump that circulates coolant, a thermostat that controls coolant temperature, a radiator to keep the coolant cool and a radiator cap to control the pressure of the coolant. Coolant picks up heat as it moves through the engine and is cooled down as it passes through the radiator. The cooled coolant is then returned to the engine to pick up more heat.

Overflow Tank

The overflow tank is also commonly referred to as an expansion tank, a coolant reservoir or an overflow canister. Regardless of what you call it, however, it is a part of every vehicle’s cooling system. The overflow tank is connected to an overflow tube that comes from the radiator. This tank provides extra storage space for coolant as it expands when the engine heats up. Without this extra overflow storage space, the coolant would expand and run out from the overflow tube and onto the ground.

Normal Bubbles

It is common to find a few bubbles in the overflow tank. One of the functions of the overflow tank is actually to remove bubbles caused by air in the cooling system. Automotive coolant tends to work much more efficiently without air bubbles as the lack of air bubbles allows the coolant to absorb heat at a much faster rate than coolant containing air bubbles, making it a more efficient means of keeping your engine from overheating.

Problem Bubbles

Though it is completely normal to find bubbles in the overflow tank while the engine is not overheating, bubbles in the coolant could be the sign of a leak at the head gasket. To test for a head gasket problem, check each cylinder of the vehicle with a cylinder leakage tester while the engine is off. If bubbles are present during the test, combustion gas is leaking into the cooling system and this will need to be repaired. If left untreated, it could lead to a blown head gasket, which could leave you stranded.

About the Author

Geoff Hineman has been a professional writer since 2001. His work has appeared in Dodge Magazine, The Ann Arbor Paper and online. Hineman holds a Master of Arts in writing from Northern Michigan University.

Photo Credits

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