Losing Antifreeze With No Apparent Leak

Updated April 17, 2017

Whenever a vehicle loses antifreeze, also known as coolant, the usual expectation leads the owner to check for evidence of leaks at all the coolant connections and components. Obvious coolant leaks can usually be found in the form of puddles left on the pavement. A more mysterious coolant loss problem involves no evidence at all of leaks or puddles, yet shows a definite reduction in radiator coolant levels.


If the head gasket on the vehicle has weakened or burnt, it can crack or blow open a water jacket that sits in proximity to the top of the combustion chamber. Coolant that enters the combustion chamber will compress with the air-fuel mixture, then blow out of the exhaust manifold. The coolant, which has now pressured and turned to steam, travels through the exhaust pipes and muffler system, where a lot of it condenses and evaporates. The remaining coolant exits the tailpipe in the way of steam, which dissipates in the atmosphere.

Radiator Cap

Radiators hold pressurised coolant in a confined area. A radiator cap, which has a circular gasket seal under it, keeps pressurised coolant from escaping from the top neck connection. A defective radiator cap gasket will allow coolant to escape in the form of steam. The steam disperses inside the engine compartment. With high engine temperatures, the steam evaporates or moisture collects on the engine compartment firewall and fender wells. Coolant loss results, without obvious puddles signs.

Overflow Reservoir

When a vehicle overheats, or after the engine has been turned off after a long ride, an expansion valve opens inside the radiator cap to allow pressurised coolant to enter the overflow reservoir. If the expansion valve becomes stuck open, it allows a constant flow of pressurised coolant to enter the reservoir, where it turns to steam. The steam, which has coolant in it, is forced out of the reservoir cap vent hole or loosens the cap. Steam and coolant are released into the engine compartment.


A head gasket that has a blown water jacket hole can allow water to enter the combustion chamber in profuse amounts. Water that does not pass through the exhaust valve and manifold gets compressed by the piston. Coolant is forced past the piston rings and enters the oil crankcase. The coolant mixes with the oil into a brown or cream-coloured frothy consistency. The radiator coolant level will drop perceptibly, while the oil level in the crankcase will rise. At this point, signs of exterior water leakage will show.

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About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.