White smoke coming from the exhaust

Symptoms

• Thick white smoke that doesn't stop after a few minutes of driving
• Sweet smell from the exhaust
• Coolant level dropping with no visible external leak
• Engine may run rough or overheat
• White, milky residue on the oil dipstick

Likely causes

• CommonBlown head gasketA failed gasket lets coolant leak into a cylinder, where it burns and turns to steam.
• PossibleCracked cylinder head or engine blockA crack can let coolant into the combustion chamber similarly to a blown gasket.
• CommonNormal condensation (brief, cold start only)Water vapor in a cold exhaust system briefly looks like white smoke and clears in a minute or two.
• Less commonFailing injector seals allowing coolant intrusion (diesel)On some diesels, injector cup seals can leak coolant into a cylinder.

What to do

1. Watch whether the white smoke clears within a couple of minutes of driving (likely condensation) or continues (likely coolant).
2. Check the coolant reservoir level and look for a sweet smell around the engine.
3. Check the oil dipstick for a milky or frothy appearance, which suggests coolant mixing with oil.
4. Stop driving and let the car cool if it also shows signs of overheating.
5. Have a shop pressure-test the cooling system and check for combustion gases in the coolant (a head gasket test).
6. Have the head gasket, head, or block repaired based on what testing finds; don't keep driving on a confirmed coolant leak into the engine.

Tools and parts

• Flashlight

When to call a pro

This is a job for an experienced mechanic; head gasket and engine block repairs require specialized tools and testing to diagnose correctly.

Safety