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Solved problems · Propulsion · Crew answer

Why is my diesel blowing white or blue smoke?

Some white smoke on a cold start is normal. Smoke that persists can mean unburned fuel, water in the cylinders or burning oil. Here's how to tell them apart and what to check.

Symptom

White or bluish-gray smoke from the exhaust that doesn't clear once the engine warms up, sometimes with a fuel or oil smell, rough running, falling coolant or rising oil use.

Likely cause

White smoke is usually unburned fuel: a cold engine, poor compression, worn or sticking injectors, late timing or poor fuel. It also shows after long idling, such as charging batteries at anchor, and should burn off under load. White vapor that smells sweet or comes with coolant loss can mean coolant entering a cylinder through a head gasket. Blue smoke means engine oil is burning, from worn rings, valve guides or seals, or an overfilled crankcase.

Stop now if

Coolant is disappearing, the engine is overheating, the oil looks milky, or the smoke smells sweet. Stop the engine and don't run it until the cause is found. Never open a hot cooling-system pressure cap. Coolant can erupt and scald.

The fix, in short

  1. Warm it up under load. Run the engine at a moderate load for 20 to 30 minutes. Smoke that clears was likely cold-start or idle buildup, which is normal on sailboats that spend a lot of time idling.

  2. Check coolant and oil when cold. With the engine cold, check the coolant level and look at the oil on the dipstick and under the filler cap. Falling coolant or milky oil points to an internal leak and needs a mechanic.

  3. Track oil use. Note how much oil you add per engine hour. Rising use along with blue smoke points to worn rings or valve guides. Also make sure the crankcase isn't overfilled.

  4. Rule out fuel and glow plugs. Drain the water separator and look for water in the fuel. In cold weather, check that the glow plugs work, since poor preheating causes lingering white smoke.

  5. Get a compression and injector test. Persistent white smoke on a warm engine with good fuel calls for a compression test and an injector test by a diesel shop.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Engine: Inboard marine diesel, cooling system, injectors and glow plugs

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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