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Solved problems · Propulsion · Crew answer

Why is my marine diesel blowing black smoke?

Black smoke means fuel isn't burning completely: the engine is short of air or overloaded by a fouled bottom, a fouled or over-pitched prop, or a blocked air filter. Here's what to check.

Symptom

Dark gray or black smoke from the exhaust, especially when you push the throttle up or motor into a head sea. The engine may also fail to reach its rated RPM at full throttle.

Likely cause

Black smoke is soot from fuel that didn't fully burn. There are two broad causes. Too little air reaches the cylinders, from a clogged air filter or intake, a restricted exhaust or poor engine-room ventilation. Or the engine is overloaded, so the governor adds fuel that can't burn, which happens with a dirty bottom, a fouled prop, the wrong prop, or poor alignment. Worn or dripping injectors come next.

Stop now if

The engine is overheating, the exhaust temperature or overheat alarm sounds, or exhaust gas or smoke is entering the cabin. Ease the throttle, and stop the engine if it is safe to do so. Fit and test a carbon monoxide alarm if exhaust can reach the accommodation.

The fix, in short

  1. Back off and watch. Note whether the smoke clears at lower throttle. Smoke only at high load points toward overloading or an air restriction rather than a fuel-system failure.

  2. Check the air side. With the engine off, inspect the air filter or intake silencer and clean or replace it. Make sure engine-room air vents are open and not blocked by stowed gear.

  3. Clean the bottom and prop. Growth on the hull and propeller adds load quickly. Have a diver clean them or haul out, then retest.

  4. Test whether the engine reaches its rated RPM. In calm water with normal cruising load, briefly run to full throttle. An engine that falls well short of the rated RPM in its manual, with a clean bottom and prop, is likely over-propped. Talk to a prop shop about pitch.

  5. Look at the exhaust path. With the engine off and cool, check for a collapsed or soot-clogged exhaust hose or a scaled exhaust elbow, both of which choke the engine under load.

  6. Have the injectors tested. If air and load check out, have a diesel shop test the injectors and injection timing. Worn nozzles that dribble fuel cause black smoke even at moderate load.

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

Equipment involved

  • Engine: Inboard marine diesel, air intake and filter, wet exhaust, injectors
  • Drivetrain: Propeller, shaft and hull bottom

Related guides

Sources

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

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