Why is my Yanmar diesel engine blowing white smoke, and what should I check?
My Yanmar diesel has started blowing white smoke from the exhaust, particularly just after starting. How can I tell whether it is normal cold-start smoke, a fuel problem or water getting into the engine, and what should I check first?
Answer
White exhaust from a Yanmar diesel is usually unburnt fuel from incomplete combustion or water vapour from the wet exhaust; a light haze at a cold start can be normal. Check exhaust cooling-water discharge and engine temperature first, as restricted cooling needs immediate attention, then see whether the haze clears as the engine warms.
Fuel smoke or steam
Watch the exhaust from a safe position outside without deliberately breathing it. Steam usually disperses quickly; unburnt diesel tends to linger as an acrid haze, often with uneven running or misfiring. These clues alone cannot confirm the cause.
A little vapour on a cold, damp morning can be harmless, particularly if the engine runs smoothly, temperatures remain normal and fluid levels stay steady. Dense white exhaust that persists at operating temperature needs investigation.
- Only after a cold start: incomplete combustion while the cylinders are cold is a likely cause.
- With rough running or difficult starting: investigate fuel quality, injector performance and compression.
- With reduced exhaust water or rising temperature: suspect a cooling-water restriction.
- With falling coolant level: investigate an external leak or coolant entering the engine.
Stop the engine if the temperature alarm sounds, cooling-water discharge disappears or heavy white exhaust develops suddenly.
Cooling circuit and fluid levels
On most yacht installations, raw water enters the exhaust elbow to cool the exhaust gases. Too little water can produce conspicuous steam before the overheating alarm sounds.
- Confirm that the seawater inlet seacock is fully open and compare exhaust discharge with its usual pulsing flow.
- With the engine stopped, close the seacock before opening the raw-water strainer. Inspect its basket, lid seal and inlet hose, then reseal it and reopen the valve before restarting.
- Inspect the seawater-pump impeller for damaged vanes and recover missing pieces from downstream hoses or the heat-exchanger inlet.
- If flow remains weak, check the heat exchanger and water-injection passages in the mixing elbow for blockage.
Check coolant only when the engine is cold. Never remove a hot pressure cap. Check the oil dipstick too: stop and investigate a rising level or milky oil, although clean oil does not rule out a cylinder-head gasket leak.
Cooling arrangements vary. Many older Yanmar GM installations are directly seawater cooled, while GM-F and YM engines generally have a separate coolant circuit and heat exchanger.