Winterizing Your Marine
Diesel or Genset


By Tom Burden, Last updated: 9/21/2017


Changing the oil is an important part of winterizing this Yanmar diesel.

Winterizing your diesel engine(s) is a step-by-step process. When you’re done, your engine will have clean oil, stabilized fuel and a cooling system that’s either completely drained or filled with antifreeze. You will have inspected key components, covered all openings, left big reminder notes and listed all required repairs. Your engine will be ready for spring commissioning with a minimum of hassle. Your diesel marine generator also needs to be winterized, and most of the steps are the same ones you need to take to winterize your boat’s diesel propulsion engine.

Easily done. Let’s get started!

Stabilize the fuel

Check your fuel-water separator for sediment and water, and clean or replace the elements as needed. Fill your tank 95% full with clean diesel and add a fuel stabilizer. I use Biobor JF and Star Tron Diesel Additive, as recommended by Practical Sailor. We sell lots of other excellent products like ValvTect BioGuard and STA-BIL Diesel. Run your engine to distribute the stabilizing additives throughout the fuel system.

Change the oil

Change the engine’s oil and replace the oil filter. Why do this now, instead of waiting until spring? Because the old, dirty oil contains contaminants and acids that will harm the engine during the lay-up. Clean oil is a must for the health of your engine. Change the oil in the transmission too, for similar reasons. Be sure to first run the engine up to operating temperature and change the oil while it’s warm. This helps drain all the contaminants and gunk away with the oil.

Coat the cylinders with oil

Spray some oil into the inlet manifold. Then crank the engine over a few times (without starting it) to coat the cylinder walls with oil.

Winterize the cooling system

Freshwater-cooled engines with a heat exchanger: Replace the coolant in the freshwater side of the system, a 50-50 mix of water and antifreeze. This is an important task to perform now, because the antifreeze contains additives to fight corrosion. While the antifreeze will not wear out, these additives do.

If you’re using an antifreeze that needs to be diluted, like our Pure Oceans Antifreeze & Coolant, you should dilute it first, before adding it to your freshwater cooling system.

Raw water cooling system: There are two acceptable choices. Your first alternative is to drain the entire system, being careful to not leave standing water in any low spots. Draining an engine must be thorough, as small pockets of water can crack important engine parts. To drain, check the owner’s manual for the location of all petcocks and open them. If water fails to drain freely from a petcock, it may be clogged with rust or debris. Remove the petcock and free the opening with a tool or coat hanger. Don’t forget also to drain the sea strainer, seacock body, and water lift muffler, if installed.


This diesel generator gets the same treatment as the main propulsion engine.

The other choice, which we prefer, is to fill the cooling system with antifreeze. This is better for the engine and less likely to result in freeze damage. To winterize the engine with antifreeze, you’ll need a five-gallon bucket, and enough antifreeze for your engine and related plumbing (at least two gallons). If your engine takes a lot of antifreeze, you might also need a helper to add antifreeze to the bucket as needed. To circulate antifreeze through the engine:

  • Fill a five-gallon bucket with undiluted non-toxic antifreeze. Have additional antifreeze handy if you know your boat takes more than that or if you’ve never winterized before. You can use any leftovers in the boat’s plumbing systems.
  • If the boat is in the water, close the intake seacock.
  • Remove the raw water intake hose from the seacock (this is sometimes difficult, as hose clamps and hoses may be mechanically frozen in place) and insert the end into the antifreeze in the bucket.
  • Start the engine and run at idle until antifreeze discharges from the exhaust outlet for 30 seconds or more.
  • Secure the intake hose back on the seacock.Secure the intake hose back on the seacock.

Two types of antifreeze

There are two types of antifreeze. One is the common automotive antifreeze like Prestone. It’s great for cars, but not good for outboard engines. Because automotive-type antifreeze is highly toxic, we’ll need to use a non-toxic variety for this application. That way it can be used more safely, in case some of it ends up in storm drains or the water where you use your boat.

This less-toxic antifreeze is made from propylene glycol. West Marine’s Pure Oceans brand antifreeze is premium “virgin” antifreeze, meaning it has not been recycled. It also has corrosion inhibitors to protect the engine and cooling system.

Three levels of concentration

With -50, -60 and -100 degree antifreezes available, which should you choose? For use in winterizing your engine, we recommend the highest concentration available, the -100. Why use this product when the temperature never dips below -50°F? Simply because there’s always some leftover residual water inside the engine, and that dilutes the antifreeze you pour in. What goes in as -50 may not come out as -50, so that the resulting freezing point may be much higher than the rated temperature.

A final note about non-toxic propylene glycol antifreeze concerns its “slush point,” which is plus 11°F for the -50 antifreeze. Slushing is OK and will not harm your engine or other systems. For more technical info about understanding burst point and freeze point, and about our West Marine Antifreeze, click on the link at the bottom of this article.

Some experts advise you to remove the impeller from the raw water pump, lubricate it with Vaseline petroleum jelly, and put it back in the pump. Then replace the cover to the pump housing, but leave the cover screwed in place loosely. This prevents the impeller from sticking to the pump housing. You can also remove the impeller until spring, so that its vanes do not “take a set.” Either way, leave an easy-to-see note to remind yourself to replace the impeller and/or tighten the cover in the spring. For more on how to service the impeller in your water pump, click here.

Check the exhaust system

Break the exhaust loose from the water lift muffler or exhaust manifold and inspect for carbon buildup or corrosion. Take the raw water injection hose off the injection nipple and see if it has become blocked by debris or scale.

Inspect hoses

Check all hoses for evidence of softness, bulging or cracking. Pay extra attention to the hot side of the exhaust and cooling systems. Check all of the hose clamps to be sure they are tight, and look for signs of corrosion.

Seal engine openings

Seal all of the openings that go into the engine to prevent the damp air from getting inside. Make caps out of plastic containers and tape to cover the air inlet, transmission and crankcase breathers and exhaust outlets. Leave a note for yourself listing all the openings you have sealed, so you’ll remember to uncap them in the spring.

Miscellaneous jobs

  • If the boat will be stored in the water, you should tighten the stuffing box to eliminate all dripping, and of course leave a prominent reminder note so you’ll remember to loosen it in the spring.
  • Inspect the motor mounts, and check their flexible rubber elements for softening caused by leaks of oil or diesel fuel.
  • Lube all grease points on the engine.
  • Remove control cables from their housings and coat them with grease.

Note all damage you found in your inspections, or needed repair or maintenance. Get the repairs done before spring, if possible. Touch-up paint, remove rust, clean up grease. These small items are easy to neglect in the rush of recommissioning!