Diesel Engines - Winterising

A few months of idleness over the winter can do far more harm to an engine than any amount of hard work.

Oil that has done its cleaning job throughout the season lies in the engine, contaminated by the acidic by-products of combustion, while the upper part of the engine, now dry of oil, is exposed to the damp and possibly salty atmosphere. Things that are supposed to be flexible, such as vee-belts and pump impellers, may become stiff; mice and spiders hibernate in air filters and exhaust pipes; and if you’re unlucky, the air temperature may fall low enough to freeze the water in the cooling system, to crack the engine block or cylinder head.

Winterisation, then, is primarily about protecting the engine against corrosion and cold. It’s also an opportunity to do a lot of routine maintenance jobs.

It’s important to plan what you’re going to do, and when and how you’re going to do it: some jobs are best done afloat; others can only be done with the boat ashore, and although some can be left until spring, others should really be done in the autumn. The exact process will vary between different boats and circumstances, but the following routine is fairly typical:

Autumn: before lifting out

  • If possible, fill the fuel tank to reduce the air space in which condensation can form (but check that this won’t take the boat over the weight limit for the crane or boat-lift).
  • Drain the fresh-water cooling system. It’s often easiest to do this by disconnecting a hose from the circulating pump. Then shut any taps or drains or reconnect any hoses, and refill with a strong solution of fresh antifreeze.
  • Drain off any water and sludge from the fuel pre-filter, and change the fuel filter(s).
  • Run the engine to operating temperature.
  • Change the oil and oil filter while the engine is warm.

Autumn: after lifting out

  • Clean the raw-water fi lter.
  • Flush the raw-water system. There are various methods, but this is one of the most widely recommended:
    • a. Remove the thermostat.
    • b. Disconnect the raw-water inlet hose from the skin fitting or seacock, and extend it with an additional length of hose if necessary so that the end can be immersed in a bucket of water. Support the bucket at or near the boat’s normal waterline, and arrange for it to be kept topped up by a constant supply of fresh water from a hose.
    • c. Start the engine and let it run for a few minutes to flush the system. Tip antifreeze solution into the bucket, turn off the hose, and switch off the engine just before it empties the bucket.
    • d. Re-connect the hose to the skin fitting.
  • Remove the raw-water pump impeller. (Put it in a plastic bag and hang it on the engine controls or tie it to the keys so that you don’t start the engine without it!)
  • Remove the alternator and water pump drive belts.
  • Change the air fi lter, and seal the air intake with oiled rags and/or a plastic bag.
  • Seal the exhaust pipe with oiled rags or a plastic bag.
  • Inspect the gearbox/saildrive/outdrive oil, and replace it if it looks grey or milky.
  • Remove the propeller and grease the shaft (not necessary for shaft-drive boats).
  • Either remove the batteries to storage ashore or clean and grease their terminals and arrange for them to be charged once every six weeks or so. All lead-acid batteries lose their charge over time, and a fl at battery can then be irreparably damaged by freezing.
  • Spray the backs of switchboards, instrument panels and other electrical connections with water-repellent oil.

Spring: before launching

  • For saildrives: inspect the rubber seal around the drive leg, and replace in accordance with the manufacturer’s instructions (eg every five to seven years).
  • For outdrives: inspect the bellows between the drive leg and transom shield, and replace in accordance with the manufacturer’s instructions (eg every two years).
  • Test and re-fit the thermostat.
  • If your raw-water cooling system includes zinc anodes, inspect them and replace them if necessary.
  • Check that all plugs and drain cocks in the cooling system are shut.
  • Re-fit the raw-water pump impeller with a new pump gasket.
  • Inspect all hoses: replace any that are visibly cracked or that do not feel pliable.
  • Check hose clamps for security: tighten any that are loose, and replace any that are corroded.
  • Re-fit the alternator and water pump drive belts, and adjust them.
  • Unblock the air filter and exhaust.
  • Check the oil level in the gearbox/saildrive/outdrive.
  • Grease the propeller shaft and re-fit the propeller.
  • Reconnect the batteries.

Spring: after launching

  • Inspect for leaks.
  • Check the engine oil level.
  • Grease or bleed the stern gland as appropriate.
  • Run the engine to operating temperature, inspect for leaks, and check oil and water levels.


Adlard Coles Book of Diesel Engines

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