Oil in the coolant usually shows up as a brown, milky or oily film in the coolant reservoir. The most common causes are a failed head gasket, a leaking engine oil cooler that lets oil cross into the cooling system, a cracked cylinder head or block, and, on vehicles with a transmission cooler in the radiator, transmission fluid leaking into the coolant. Stop driving until you know which, because mixed fluids can destroy an engine quickly.
What oil in the coolant looks like
- A brown or tan sludge or film in the reservoir or under the radiator cap
- Oily droplets floating on the coolant
- Coolant that looks milky or reddish brown
Common causes
| Cause | Clues |
|---|---|
| Head gasket failure | Overheating, white exhaust smoke, bubbles in the coolant; see signs of a blown head gasket |
| Engine oil cooler failure | Oil in coolant without overheating; common on engines with an oil-to-coolant cooler, including many diesels |
| Transmission cooler leak in the radiator | Reddish, oily coolant and transmission fluid that turns pink and milky |
| Cracked head or block | Persistent mixing after the gasket and cooler test good |
Oil in coolant but no coolant in oil
When engine oil pressure is higher than cooling system pressure, oil is pushed into the coolant but coolant does not enter the oil, so a clean dipstick does not rule out a leak. An oil cooler or a head gasket leak between an oil passage and a coolant passage often behaves this way.
Subaru engines
Oil in the coolant on Subaru boxer engines is often linked to head gasket problems; see Subaru head gasket problems.
How to diagnose it
- Check the engine oil and transmission fluid for milky contamination.
- Pressure-test the cooling system and watch for pressure loss.
- Run a combustion leak (block) test for exhaust gases in the coolant.
- Test or inspect the oil cooler and the radiator transmission cooler.
- Run a compression or leak-down test if a head gasket is suspected.
After the repair
Flush the cooling system several times until the coolant runs clean, because leftover oil degrades hoses and blocks passages. If coolant reached the oil, change the oil and filter promptly. If transmission fluid was contaminated with coolant, the transmission may need service or replacement; see transmission fluid color.
Can you drive with oil in the coolant?
Not for long. Contaminated coolant cools poorly, and coolant in the oil wipes out bearings. If damage has already reached the bearings, see how to tell if an engine is blown and compare replacements on our engines page.
More guides: milky engine oil · cracked engine block.
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