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How to Winterize Your Outboard

For reliable performance next spring, follow these tips when preparing an outboard for off-season storage.

How-To
How to Winterize Your Outboard | Mercury Marine

Winterizing is Really Important

Correctly preparing your outboard motor for the off-season will help insure that it’s ready to go next spring. In cold climates, freezing weather can cause costly damage to an engine that isn’t winterized properly. Even in milder climates, the end of the boating season is a great time to take care of annual maintenance that will keep your outboard running reliably. Whether you do the job yourself or take your outboard to an Authorized Mercury Marine dealer for service, spending a little time and money today can save a lot of both come spring.

Should You Do it Yourself or Take it to a Mercury Dealer?

If you have your outboard winterized by an Authorized Mercury dealer, you can count on the professional service and skills of a trained technician and save yourself the trouble of rounding up oil and filters, gear lube, fogging oil and other required supplies. The tech winterizing your outboard may even 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, Wisconsin. “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 propshaft, which can cause real trouble down the road.”

Many marine service facilities will bundle engine winterization with other off-season storage services at a discounted price. These additional services may include maintenance of the boat’s water and waste systems, shrink wrapping and storage. The off-season 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.

The procedure for off-season storage is outlined in every Mercury owner’s manual, and with a few basic tools and the proper supplies, those inclined to do-it-yourself can usually tackle the task. In fact, many late-model Mercury outboards, including the new 4.6-liter V8 and 3.4-liter V6 models, have a label under the cowl with basic maintenance specifications and a QR code that can be scanned with a mobile device for links to how-to videos produced by Mercury.

If you go the DIY route, start by consulting the manual and collecting the tools and maintenance products you’ll need. If you don't have a copy of the manual for your Mercury outboard, you may order it from a Mercury dealer or download it from the Mercury Marine website. The manual will have information specific to your Mercury outboard model, but here are some basic instructions from the service experts at Mercury Marine:

Keep a Winterization Supplies List

See your outboard owner’s manual for guidance on supplies and tools specific to your motor.

  • 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

Treat the Fuel

You should always store your boat with fresh fuel that has been treated with marine fuel stabilizer. Fuel that is left untreated will begin to oxidize and form a gumlike substance in the engine’s fuel system. This is especially true for outboards equipped with carburetors.

“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 fuel to keep condensation from forming in the tank. Stop when the tank is about 95 percent full because extreme temperature changes over the winter can cause the fuel to expand, potentially forcing gas out of the vent. Because newer tanks can’t freely vent to the atmosphere and won’t collect moisture from the air, they don’t need to be filled. If possible, refill the tank with ethanol-free fuel such as REC-90, a premium blend formulated specifically for recreational engines. Then add the correct amount of fuel stabilizer to the fresh fuel.

“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 said. He recommends treating fuel for storage with Mercury Quickstor, one of the products in the Mercury Fuel Care System, whether your engine was built by Mercury or another manufacturer. These products are engineered to work together to optimize fuel, remove any leftover deposits from the engine and protect the fuel system over the winter months.

It is important to get that treated fuel into the entire fuel system by running the boat 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 motor’s fuel system will be ready to go next season.

Change the Oil

If you have a four-stroke outboard, the engine oil and filter 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. The line of Mercury Precision Lubricants includes four-stroke engine oil for any outboard brand, specifically formulated for the unique needs of the marine environment. Never use automotive oil in a marine engine! Check your owner’s manual or ask your Mercury dealer for the recommended lubricant products for your marine engine. The dealer will also stock the Mercury Oil Change Kit for many Mercury outboards. Each kit includes oil, an oil filter and a drain plug seal in a convenient box. 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,” suggests 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 Engine

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 models, instead of using fogging oil, squirt one 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.

Change the Gear Lube

The gear lube in the outboard lower unit should also be changed every 100 hours or once a season – check your owner’s manual for the specific service interval. Any water in the gear lube can freeze and expand during storage, potentially cracking the gearcase. As you drain the old lube, inspect it carefully. If gear lube appears white or creamy, it may indicate the presence of water in the lubricant. This is an indication that the propeller shaft seal may be compromised and is allowing water to contaminate the lubricant, which could cause severe damage to the gears. If water is suspected to be present in the gearcase, a pressure check of the gearcase should be completed by an authorized dealer. 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.

Check the Prop Shaft 

With some gearcases, you will have to remove the propeller in order to change the lube; but even if your engine doesn’t require that, it’s a good idea to pull off the prop. If you propeller is nicked or bent, the off-season is a good time to send it out for repair at a propeller shop so it will be ready to go in the spring. A Mercury dealer can often make arrangements for prop tuning or repair.

After removing the prop, check the prop shaft for fishing line. Remove the large thrust washer that is located behind the prop on some hub styles. The fishing line is often coated with grease and hard to see. Use a sharp pick or a small screwdriver to pull at the area around the propshaft seal to loosen any line that may be present. Over time, the forward thrust of the propeller presses the line into the rubber propshaft seal. If the seal is damaged, water can enter the gearcase and cause significant damage.

If the propeller is in good condition, either reinstall it or store it separately from the engine to discourage thieves. Always coat the propshaft 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.

Watch “Prop Bite: Propeller Installation”

Check the Power Trim Fluid

Check your owner’s manual for specific instructions for your outboard model. You will probably need to tilt the engine all the way up to get to the pump. The fluid inside should be bright red; if so, check the level and top it off if it is low. Trim fluid that is pink or milky indicates the presence of water, which means there may be a leak somewhere in the system, and the trim pump should be inspected by a qualified dealer.

Watch "Quick Tip: How to Check Your Outboard Power Trim Fluid"

Check Sacrificial Anodes

Sacrificial anodes (or zincs, as they are often called) are designed to protect the motor’s other submerged metals from galvanic corrosion. Most outboard motors have several sacrificial anodes, and they can be located by consulting the owner’s manual. Sacrificial anodes should be replaced when they are 50 percent deteriorated.

Watch “Quick Tip: How to Inspect and Replace Engine Anodes”

Lubricate Any Grease Points 

Most outboards have some grease zerks or other lubrication points that should be serviced. Consult your owner’s manual to see where your outboard needs grease.

Coat the Powerhead

As a last step, Hurney recommends coating the powerhead with a rust inhibitor product such as Mercury Corrosion Guard. “This is especially effective for motors used in salt water,” 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 salt water 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.

Touch up Any Nicks or Scratches

Inspect your engine’s lower unit and repaint any wear-and-tear marks. Mercury dealers carry a complete line of paint products – primer, color and clear coat – that are an exact match to the factory finish.

Check and Maintain the Battery 

If it’s a lead-acid battery, inspect the fluid level and add distilled water, if needed. Be sure the battery is fully charged, remove it from the boat, and store it in a cool, dry place. Storing the battery on a maintenance charger will keep it charged and fresh until spring.

Store the Engine Upright

Whether the boat is stored on a trailer or a rack, the motor should be trimmed down to its vertical position so that any water remaining in the engine can self-drain. If water trapped in the engine freezes, it could cause serious damage. In some situations, mice can be a problem – they may be able to get under the motor cowl and chew on wiring. Some dealers swear by this tactic: Fabric softener sheets, or drier sheets, are believed to repel mice. Just don’t forget to remove the sheet before you hit the water next season.

With the off-season storage procedure complete, your outboard will be protected for months. Come spring, getting back on the water will be a snap. Just reinstall the boat battery if it was removed, reinstall the prop, and turn the key. You should be ready for another season of fun and adventure on the water.

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