Why Is There Oil in the Radiator Fluid?

by Richard Rowe

Engine oil passages and coolant passages cross over a number of times in an engine. This means that there are a number of opportunities for fluid crossovers. The one common denominator in fluid crossovers is that in no case should they ever be ignored.

Types of Oil

Either engine oil of transmission fluid may be found in the coolant. It's important to diagnose which is which to resolve the problem.


When a sample of the coolant is removed and examined, the oil in it will separate. Transmission fluid is red and engine oil is amber or black.

Oil Leaks

Engine oil leaks can occur at the intake manifold gasket, at a blown head gasket, or within a cracked head or block.

Transmission Leaks

A transmission fluid leak almost always occurs within the radiator, where the transmission fluid cooler integrates into the water portion.

Temporary Solutions

Auto parts stores sell a number of stop-leak products. Transmission stop-leak, oil-stop-leak and coolant stop-leak are all available.


Though a fluid crossover may be fixable with nothing more than a stop-leak product, it almost always implies a larger engine problem that needs attention.

About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.

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