Why does my marine diesel engine blow white smoke at cruising speed?
I have noticed that when I push my boat up to cruising speed, the engine starts blowing a bit of white smoke from the exhaust. It seems to run fine otherwise, but the smoke has me worried. What should I be looking for?
Answer
White smoke from a marine diesel at cruising speed is usually steam from a restricted raw water cooling system or unburnt fuel from a faulty injector. To fix it, you must first determine if the exhaust dissipates immediately like steam or lingers over the water with a sharp diesel smell.
Identifying Steam or Smoke
Look at the exhaust plume behind the boat before turning a spanner. Steam evaporates and vanishes within a few metres of the transom. Unburnt atomised fuel hangs in the air, drifts across the anchorage and stings your eyes with a distinct smell of raw diesel. Keep your hands clear of moving belts and hold a piece of cardboard near the exhaust outlet. If the cardboard gets wet with water droplets, the engine is producing steam. An oily residue points to fuel.
Checking the Cooling System
If the white cloud is steam, your engine is lacking the raw water needed to cool the exhaust gases. The engine block might not be overheating yet, but the exhaust temperature is high enough to boil the reduced water flow. Check these components in order:
- Raw water strainer: Look for weed, plastic bags or jellyfish clogging the basket.
- Seacock: Ensure the valve is fully open and free of marine growth.
- Impeller: Remove the raw water pump cover and inspect the rubber impeller. If vanes are missing, you must find them downstream in the heat exchanger core before they block the copper tubes.
- Exhaust injection elbow: This is the most common fault on engines over five years old, particularly Yanmar YM or Volvo Penta D-series units. Carbon and salt build up inside the elbow and restrict water flow into the exhaust. If the metal feels hotter than normal to the touch, the elbow is blocked and needs replacing.
Testing for Unburnt Fuel
If the smoke lingers and smells of diesel, the cylinders are failing to reach the temperature required to burn the fuel completely. At cruising speed, the engine should be well up to its operating temperature of 75°C to 85°C. Cold running is rarely the cause under load.