Winterizing a Mercury Outboard Without Regrets

Winter sneaks up fast, and a Mercury outboard won’t forgive shortcuts. I just wrapped mine, and it was more stressful and expensive than planned—but doable.

What you’ll get:

• A clear, minimal winterization sequence for a Mercury outboard
• Practical tool notes and access warnings
• Mistakes to avoid that cost time and sanity

Stabilize the fuel and run it through

Mercury’s manual calls out fuel stabilization. Add the recommended stabilizer to the tank, then run the engine on muffs to pull treated fuel through the system. Confirm water flow at the telltale when the engine’s running.

Tips:

• Treat the fuel first, then idle the motor long enough to circulate.
• A transfer pump keeps spills down when topping the tank.

Fog the cylinders without cross-threading plugs

Fogging is simple—access isn’t. On a 4‑cyl, three plugs are easy; the fourth may be buried behind housing. Remove all four plugs, fog each cylinder for roughly three seconds, crank briefly, then reinstall carefully to avoid cross-threading.

Practical notes:

• A magnetic or tight-fitting plug socket helps retrieve and place plugs without dropping them.
• If visibility to that last plug is poor, plan extensions and a universal to line it up by feel.
• If you don’t have a torque wrench, do not over-tighten. Consider adding a 1/4’ torque wrench and adapter for future service.

Clean first, then store it right

Pressure washing heavy lake grime can balloon the time. Low PSI units barely touch buildup; stepping up helps, but expect hours if the boat sat in water. After cleaning, store the outboard vertical so any remaining water drains and can’t freeze in passages.

Cautions:

• Don’t rely on weak pressure washers for caked-on growth; it just burns daylight.
• If you can’t acid wash the toons, a thorough pressure wash is still worthwhile.

Battery and trailer: quick wins now, jobs later

Pull the battery for indoor storage and put it on a tender. Note trailer needs while you’re there—tires aging out and axle bearings due for grease are ideal early-spring tasks.

Final takeaway: Keep winterizing tight—stabilize fuel, circulate on muffs, fog the cylinders, store vertical, and bring the battery inside. Plan for the awkward plug, use the right socket setup, and you’ll save stress next time.