Why Is There Oil in the Radiator Fluid?


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.

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