Mechanics

Oil in Coolant

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Step by step guide on how to troubleshoot and repair an automotive oil in coolant problem, this information pertains to most vehicles.

Difficulty Scale: 4 of 10

Begin inspection with the vehicle on level ground, engine "OFF" and cooled off, in park with the emergency brake set.

Step 1 - Many automotive engines utilize an oil cooler in which radiator coolant passes through to remove heat from the oil. When these coolers fail they allow engine oil to enter the cooling system and into the radiator.


Oil Cooler

Step 2 - To check for this condition wait until the engine has cooled off and remove the radiator cap to inspect, the oil will float on top of the coolant and is easily detected.


Oil in Engine Coolant

Step 3 - Automatic transmission equipped vehicles utilize an automatic transmission fluid cooler located inside the radiator.


Transmission Oil Cooler

Step 4 - When this cooler fails it allows transmission fluid to enter the coolant portion of the radiator causing transmission fluid levels to be low while creating overheating problems for the engine.


Transmission Fluid in Radiator

Step 5 - Occasionally the head gasket can fail forcing oil pressure into the cooling jackets of the block or cylinder head. If all other testing has failed, the cylinder head must be removed for inspection of the gasket.

Helpful Information

These fluids may exchange places when these items fail due to pressures inside each system. Because engine and transmission oil pressure runs at about 40 psi and cooling system pressure runs about 18 psi oil is more likely to enter the coolant system rather than coolant entering the engine or transmission through the cooler.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2014-08-11)