How to winterize an outboard

Properly preparing your outboard for offseason storage will help ensure that it’s ready to go as soon as the weather starts to turn next season. Freezing weather can cause costly damage to an engine that isn’t winterized properly, but even if you live in a milder climate where that’s not an issue, the end of the boating season is still a great time to take care of annual maintenance that will keep your outboard running reliably.

To help you through the process, the outboard service experts at Mercury Marine outlined the basic maintenance steps to take prior to offseason storage. Their advice applies to most outboard brands. 

Do it yourself, or take it to a dealer?

When it’s time to winterize, you have two options: Do it yourself, or have a professional handle it.

If you have your outboard winterized by an authorized Mercury dealer or another marine service professional, you can count on the skills of a trained technician and avoid the chore of rounding up the required supplies and parts. The tech winterizing your outboard might also spot and correct unrelated issues.

“When we are winterizing engines, we might see things that are wrong with them that our customers might not catch,” said Scott Klein, president of Wendt’s Marine, an award-winning marine service shop in Van Dyne, Wisc. “It could be something minor, like a throttle or shift cable out of adjustment, but that can affect how the motor performs. We’ll also check for bigger issues, like loose transom bolts or a bent prop shaft, which can cause real trouble down the road.”

Many marine service facilities will bundle engine winterization with other offseason storage services at a discounted price. The offseason is also a great time to have a dealer install new electronics or accessories, to refresh canvas or upholstery and to send propellers out for repair.

If you go the DIY route, start by consulting the procedure for offseason storage as outlined in the outboard owner’s manual, and collect the tools and maintenance products you’ll need. The manual will list supplies and tools specific to your engine, but here are the essentials:

  • Engine oil
  • Engine oil filter
  • Fuel filter
  • Fuel stabilizer
  • Spark plugs
  • Marine grease
  • Fogging oil
  • Gear lube and pump
  • Rust-inhibitor spray
  • Basic hand tools
  • Grease gun
  • Spark plug gap tool
  • Motor flushing muff and garden hose
  • Prop wrench or appropriate socket
  • Funnel
  • Waste oil container

If you don’t have a copy of the manual for your outboard, you could order it from a dealer or download it from the manufacturer’s website. Many late-model Mercury outboards also include a label under the cowl with a QR code that can be scanned with a mobile device to access how-to videos produced by Mercury.

With the supplies gathered, here are the basic procedures that must be completed:

Stabilize the fuel

Treating the fuel in the boat and getting that treated fuel into the engine are among the most important storage tasks, especially if you have an older outboard with carburetors. Fuel that is left untreated will begin to oxidize and form a gum-like substance in the engine’s fuel system.

“Most of the issues our dealers run into are caused by stale fuel,” said Mercury Marine Dealer Team Lead Tim Hurney. “When possible, at the end of the season try to run your boat fuel tank almost empty, and then add fresh fuel before storage. If the fuel in your tank is more than a month old and it’s too late in the season to use it up, have it pumped it out, and replace it with fresh fuel.”

If you have an older boat with a vented fuel tank, it’s a good idea to fill the tank with fresh, ethanol-free fuel to keep condensation from forming in the tank. Stop when the tank is about 95% full because extreme temperature changes over the winter can cause the fuel to expand, potentially forcing gas out of the vent. Then add the correct amount of fuel stabilizer. The fuel tanks in newer boats can’t freely vent to the atmosphere and won’t collect moisture from the air, so they don’t need to be filled, but any fuel left in the tank should still be treated.

“The best time to treat fuel is when you pump it into your tank, either during your last fill-up of the season or when you replace stale fuel during winterization,” Hurney added.

He recommends treating fuel for storage with a marine-specific additive such as Mercury Quickstor, one of the products in the Mercury Fuel Care System, which can be used in any brand of outboard motor. These products are engineered to work together to optimize fuel, remove deposits from the engine and protect the fuel system over the winter months.

It’s important to get that treated fuel into the entire fuel system by running the engine for about 10 minutes, either in the water or while connected to a garden hose (follow owner’s manual instructions if using a hose).

Finally, replace the fuel filter. Now your engine’s fuel system will be ready to go next season.

Change the oil and filter

The engine oil and filter of a four-stroke outboard should be changed every 100 hours or once a season, regardless of how many hours the engine was used. Storing the motor with old oil can expose internal engine components to moisture and acidic combustion byproducts, which can cause corrosion.

Never use automotive oil in a marine engine! The line of Mercury Precision Lubricants includes four-stroke engine oils for any outboard brand, specifically formulated for the unique needs of the marine environment. Follow the oil-change instructions in your owner’s manual, and always dispose of waste oil properly.

“After you change the oil, it’s always a good idea to start the engine again to circulate fresh oil through the engine and to check for leaks,” said Hurney. “It’s not uncommon for the oil filter gasket to stick to the engine, and, if it’s not removed, oil can leak past the old gasket after a new filter is installed. I’ve seen engines ruined because the owner didn’t notice engine oil leaking out past a doubled gasket.”

Fog the inside

Ideally while the motor is still warm, treat four-stroke and conventional two-stroke engines with fogging oil to prevent corrosion within the engine. Remove the spark plugs and spray the fogging oil directly into each cylinder, following the directions on the can. Use a fogging oil product that is specially designed for use during winterization, such as Mercury Storage Seal.

For direct-fuel-injected (DFI) two-stroke engines such as Mercury OptiMax, Yamaha HPDI and Evinrude E-TEC models, instead of using fogging oil, squirt 1 ounce of DFI outboard oil into each cylinder through the spark plug hole. A small oil can with a long flexible neck works well for this task.

Hurney recommends putting a coat of anti-seize lubricant on the spark plug threads before carefully replacing the plugs. Use new spark plugs, correctly gapped, per the service schedule in your owner’s manual.

Refresh the gear lube

Any water in the gear lube can freeze and expand during cold-weather storage, potentially cracking the gearcase – an unpleasant and very expensive discovery to make in the spring. Changing the gear lube is part of the 100-hour/annual service for most outboard brands, but check your owner’s manual for the specific service interval. Refill the gearcase with fresh lubricant using a quality marine-specific product such as Mercury High Performance Gear Lube, following the instructions in your owner’s manual.

If the drained gear lube appears white or creamy, it may indicate the presence of water in the lubricant, often because the propeller-shaft seal is leaking. Water in the lube can cause severe damage to the gears, so you’ll want to get that gearcase checked by a professional.  

Inspect the propeller shaft 

Many boat owners remove expensive stainless steel propellers to prevent theft during winter storage. The offseason is also a good time to send your prop out to repair nicked or bent blades. Most marine dealers work with a prop shop for this service. 

Now is the perfect time to check the propeller shaft for fishing line. There might be a large thrust washer on the shaft behind the prop. Remove this washer. If there is fishing line on the base of the shaft it’s often coated with grease and hard to see. Use a sharp pick or a small screwdriver to pull at the area around the prop-shaft seal to loosen any line that might be present. The forward thrust of the propeller can press the line into the rubber prop-shaft seal and can actually cut through the seal, allowing water to enter the gearcase. It’s a good idea to check the prop shaft for fishing line several times a season.

Coat the prop shaft with a quality marine grease such as Mercury 2-4-C Marine Lubricant before reinstalling the prop, and tighten the prop nut to the torque specification listed in the owner’s manual.

Check the power trim fluid

Consult your owner’s manual for specific instructions on how to check trim fluid on your outboard model. The fluid inside should be bright red. Trim fluid that is pink or milky indicates the presence of water, which means there could be a leak somewhere in the system, and the trim pump should be inspected by a professional technician.

Check sacrificial anodes

Sacrificial anodes (or zincs, as they are often called) are designed to protect the engine’s other submerged metals from galvanic corrosion. Most outboards have several sacrificial anodes, and they can be located by consulting the owner’s manual. Find and inspect them. They should be replaced when they are 50% deteriorated.

Lubricate any grease points 

Get out your grease gun and hit any grease zerks or other lubrication points on the outboard. The owner’s manual should indicate where your outboard needs grease.

Coat the powerhead

Hurney, of Mercury Marine, recommends spraying down the powerhead with a rust inhibitor such as Mercury Corrosion Guard.

“This is especially effective for motors used in saltwater,” said Hurney, “but can also help in freshwater environments. It displaces water and forms a barrier to protect painted and unpainted surfaces from corrosion caused by saltwater either on the powerhead or in the atmosphere.”

Corrosion Guard can also be applied to prevent corrosion from forming on the lower unit and power-trim motor.

Service and maintain the batteries

It’s usually a good plan to take the batteries out of the boat to store in a cool, dry place for the offseason. Inspect the fluid level of lead acid batteries (if they are not sealed batteries), and add distilled water as needed. Check the date on your batteries, and consider replacing for next season if they are more than five years old. It’s better to replace them than wait for them to expire – and leave you stranded – next season. Make sure the batteries are fully charged before putting them up for storage. Storing the batteries on a maintenance charger over the offseason will keep them charged and fresh until spring.

Store the engine upright

The outboard should always be trimmed to a vertical position for offseason storage. This allows any water remaining in the engine to self-drain. Water that is trapped inside the engine could freeze, expand and cause serious damage.

With the offseason storage procedure complete, your outboard will be protected for months. Come spring, getting back on the water will be a snap. Just reinstall the batteries, if they were removed, reinstall the prop and turn the key.

For more boating and outboard maintenance tips from Mercury Marine, visit the Mercury Dockline Blog.

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