Winterizing Your Engine

Reprinted with permission from 'Seaworthy', a publication of BoatU.S.

Engines don’t like to be idle, even for three or four months during the winter. Aside from changing the oil regularly, winterizing your engine properly is one of the best ways to assure its mechanical well being. Your efforts, or the efforts of your boatyard, will pay dividends as the engine gets older. A half-hearted effort, on the other hand, can result in anything from corrosion to a cracked block. Winterizing the engine is one thing that is truly critical; follow the steps below and consult your owner’s manual for specifics.

Oil and Fuel System

Gasoline and Diesel Engines

Step 1: Change the oil. Residual acids and moisture left in the crankcase over the winter can pit bearings and other vital engine parts, so it is important to change the oil just before laying up the boat. For best results, run the engine for a few minutes to warm the engine and lower the oil’s viscosity. Shut off the engine, change the oil, and replace the filter. Rubbing oil on the filter’s gasket will help it seat better.

Restart the engine and run for about a minute to circulate the fresh oil to internal parts. While it’s running, check to make sure the oil filter isn’t leaking.

Step 2: Replace the fuel filter and install the replacement seal. Like the oil filter, the fuel filter should be checked for leaks while the engine is still running.

Gasoline Engines

Step 3: Fog the Carburetor: Shut off the gas supply. With the engine running at fast idle, spray a coating oil (there are several products available for this purpose) into the carburetor air intake as the engine begins to sputter. This coats the internal surfaces - throttle plates and choke plates - to help prevent rust. Note: If you plan to winterize the cooling system by drawing antifreeze through the raw water intake, you’ll have to wait until the antifreeze is circulating to fog the carburetor.

Step 4: Cover the Carburetor. Use a plastic sandwich bag or duct tape to cover the carburetor inlet. This prevents corrosive moist air from having its way with the intake manifold and valves. This is especially critical when an engine sits directly under a leaky hatch. Don’t forget to take it off next spring.

Step 5: Other chores. Clean the flame arrestor in kerosene and reinstall. Linkage to the carburetor should be sprayed with a coating oil. If you didn’t fog the carburetor, you can remove the spark plugs and spay a light coating oil into the cylinders. Clean the plugs, and replace any that look worn. Remove the coil wire and then crank the engine over a few times to spread the oil. Loosen the alternator and water pump belts to prolong their durability. Finally, a coat of CRC or similar oil spayed over the engine help prevent corrosion.

Diesel Engines

For diesel engines, coating cylinders is not critical and should not be attempted unless specifically recommended by the manufacturer.

Cooling Systems

Gasoline and Diesel Engines

Step 1: Winterizing the freshwater cooling system. Check your antifreeze with a tester. If you top off the system (you shouldn’t have to do this very often), use a premixed solution of water and antifreeze. Many skippers simply add antifreeze which, over time, increases the percentage of antifreeze in the system. Using a solution that is more than 70% antifreeze can leave a residue that will damage your engine. A 50/50 solution works best. To winterize the raw water portion of the system, see below.

Step 2: Winterizing the raw water system. There are two ways to winterize a raw water cooling system. With the first, you simply open the various drains and petcocks (check your manual) in the engine, muffler, and sea stainer to drain the system. This system works on every engine but is the preferred system with larger diesel engines.

Be forewarned, however, that petcocks can be easily clogged with rust, debris, or sand. If water fails to drain freely out of the petcock (some seepage is not enough), unscrew the entire petcock and use a coat hangar to clear the passage.

Alternate Step 2: Run the engine at idle until it reaches its normal operating temperature, which will open the thermostat so that antifreeze can reach the manifold. With the engine running, close the raw water intake and place the intake hose in a bucket filled with non-toxic (propylene glycol) antifreeze with rust inhibitors. Keep the engine running until the solution flows freely from the exhaust. Note: Cold water may prevent the engine from reaching its normal operating temperature, which would prevent the thermostat from opening. If this happens, you’ll have to remove the thermostat in order to be assured that antifreeze reaches all areas of the cooling system.

Once the antifreeze squirts out the exhaust, shut down the engine and leave antifreeze in the system.

Water Pumps

The water pump’s rubber impeller should be removed for the winter so it doesn’t become set in the same position. Don’t forget to put it back before starting your engine next spring. (As a reminder, one savvy Boat/US member says he always stores the impeller with his ignition key.)


Check the dipstick. If the fluid looks milky (indicating water) or dirty, drain and add fresh fluid.

Outdrive Lower Units

This is a job best done on land. With the bow of the boat slightly up, lower the outdrive unit as far as possible. Drain the gear case and add fresh lubricant. Water or metallic shavings indicate a broken seal. Have the unit pressure tested by a mechanic if you suspect you have a problem.

Outdrives are expensive and have become a frequent target for thieves. Even if the boat is kept in your driveway, consider taking the outdrive off and storing it in your garage or basement for the winter.


If possible, take smaller outboards home for safekeeping. Like outdrives, outboards tend to be an easy target for thieves, especially in winter. To winterize, follow the instructions below and consult your owner’s manual for specifics.

Cooling system: Start the engine and flush the cooling system with fresh water until it reaches normal operating temperature. (Various devices are available to adapt a garden hose to the engine intake.) Make sure all gear housing drain holes are open. An alternative to draining the system is to add antifreeze using a gadget that connects the engine intake to a jug of antifreeze.

Fuel system and power head: Disconnect the fuel line from the tank, start the motor, and inject fogging fluid (a light lubricating oil) into the carburetor just before the motor stops. This procedure prevents corrosion of the powerhead parts. (Fogging fluid has rust inhibitors that are lacking in regular two-cycle oil.) Next, remove the spark plugs; clean and replace any that look worn. Clean the fuel pump filter. Lubricate carburetor and choke linkage, cam follower, starter spindle, throttle shaft bearings, and gears. Consult the manufacturer’s lubricating chart for specifics.

Lower unit: This should be drained and new gear oil added. When you loosen the drain plug, watch to see if water or oil comes out first. Water or metallic shavings indicate that you need to replace the seal. If you’re not sure, have the unit pressure tested by a mechanic.

Fuel Tanks

Top off the fuel tanks and use additives to inhibit fuel degeneration. There is a twofold advantage to topping off tanks: with gasoline, it is safer because fumes are minimized, and you will also minimize the possibility of condensation corroding the tank. When you’re filling the tanks, leave some room for the gas to expand.