Official Mercury Winterization Process for 240EFI Engine

(Source: http://articles.richardhartman.net/jetboat/mercury240efiwinterization.htm)

In a small outboard fuel tank, mix fresh non-alcohol/ethanol gasoline with the recommended amount of fuel stabilizer and Mercury Quickleen. Add twice the normal amount of Mercury Premium Plus 2-stroke oil. (RLH note: Mercury calls this “witch’s brew”.)

Disconnect the main fuel tank hose from the engine’s inline fuel filter and connect the outboard fuel tank hose in its place. Connect water line to flush port and turn on water. Start and idle engine.

Run until smell of exhaust changes, indicating that “witch’s brew” has made it through the entire engine. Stop engine. Disconnect temp fuel tank and drain any remaining contents into main fuel tank.

Place a catch container below vapor separator drain plug, remove drain plug, drain vapor separator, and reinstall plug. Direct quote from factory tech: “You do not want fuel pooled in the vapor separator all winter long.” (RLH note: This is the first reference I’ve seen anywhere relating to winterizing the vapor separator.)

Restart engine with no fuel tank connected and wait until engine stalls. Remove key from ignition and disconnect battery. Do NOT turn key to “ON” or “START” positions again, as this will enable fuel pump.

Shut off water, disconnect from flush port, and wait for water to finish draining.

Reconnect main fuel tank hose.

Drain main fuel tank and leave empty. Use fuel stabilizer (Sta-Bil or Mercury Fuel Additive) if tank cannot be drained.

My own additions:

• Replace fuel/water separating filter with new, empty filter. Mercury part number 35-18458Q4, ~$22.
• Replace inline fuel filter with new, empty filter. Mercury part number 35-889615, ~$8.50.
• In Spring, replace spark plugs after first full tank of fuel has been consumed. NGK part number BPZ8HS-10, stock number 3133, ~$6 each, do not substitute.

Note that these instructions do not include changing the front and rear jetdrive lube, which is a vital part of winterization that should be performed at the same time.