Milky or frothy oil (coolant mixing with oil)

Symptoms

• Oil on the dipstick or under the oil cap looks milky tan or frothy instead of clear amber/brown
• Possible white smoke from the exhaust as well
• Coolant level dropping with no external leak
• Engine may run rough or overheat

Likely causes

• CommonBlown head gasketA failed gasket can let coolant migrate into oil passages as well as combustion chambers.
• PossibleFailed oil coolerMany engines use an oil-to-coolant heat exchanger; a failure there lets the two fluids mix directly.
• PossibleCracked cylinder head or blockCracks can create a path between coolant and oil passages.
• Less commonCondensation from very short trips only (usually not true milky mixing)In cold climates, short trips can create light moisture under the oil cap that's different from true coolant contamination; a lab test can tell them apart.

What to do

1. Check the oil on the dipstick and under the oil filler cap for a milky, frothy, or tan 'milkshake' appearance.
2. Check the coolant level for unexplained loss with no visible exterior leak.
3. Do not drive the vehicle if contamination is confirmed, since coolant in the oil badly reduces its ability to lubricate the engine.
4. Have the vehicle towed to a shop rather than driven any distance.
5. Have a mechanic perform both an oil analysis and a coolant combustion leak test to pinpoint the source.
6. Plan for a significant repair such as head gasket replacement or oil cooler replacement depending on the cause.
7. Have the engine's internal condition assessed, since running on contaminated oil for any length of time can cause additional wear.

Tools and parts

When to call a pro

See a mechanic immediately for testing and repair; this points to a serious internal issue that needs professional diagnosis before the vehicle is driven again.

Safety