How to Fog a Boat Engine

How to Fog a Boat Engine

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Fogging a boat engine is a part of the complete winterization project. Putting in the effort at the end of the boating season will be well worth it the following year. In colder climates winter's plummeting temperatures will easily freeze any remaining fuel residing in the engine. This could crack the part and result in an expensive overhaul. Outboard motors have easier access and tilted out of the water when not being used. This makes them more popular for recreational boats. This article will cover fogging an outboard engine. Before beginning read the owner's manual for particular engine fogging advice.

Items you will need

  • Fogging oil (sold as CRC Marine Engine Stor Fog Fluid)

  • Fuel stabilizer for gasoline engines such as STA-BIL or Fuel Guard

  • Fuel biocide for diesel engines such as Bio Keen

  • Outboard flushing attachment kit (fits most engines)

  • Oil or lubricant as noted in individual owner's manual

  • Marine fuel

Pull the boat out of the water and bring the engine to a vertical position for no less than 10 minutes. This will allow water to escape from the block and passages.

Remove the plug from the engine that keeps the water out. Attach the flushing attachment, as indicated in owner's manual, to the engine and flush out any remaining water. Hand crank the engine several times to make sure all water is gone.

Fill the engine with a combination of marine fuel and fuel stabilizer or biocide (depending on engine type). It need only be enough to run through the fuel lines and the engine, although some people like to run the engine for 15 minutes or so.

Pull the fuel line from the engine while it is still running. This will allow the engine to run itself empty. Just before that happens the engine will run rough by making loud sounds. At that time spray the engine thoroughly with fogging spray. Be aware the engine will smoke during this process and might attempt to stall. Should this occur, apply extra power on the throttle. Make sure the engine turns off a few minutes after spraying or the fogging oil will wear off the freshly sprayed parts.

Locate the spark plugs and remove. Spray the fogging spray into the spark plug canisters. Complete coverage requires turning the flywheel several times. Inspect the sparkplugs for wear before reinstalling and replace if necessary.


  • To guarantee all the internal parts and air intakes of the carburetor are well lubricated spray the fogging oil especially heavily. Some opt to do this by spraying multiple cans of fogging oil at a time.
  • An individual owner's manual will advise if removing the air boxes (near carburator) or fogging ports to gain better access is necessary.