Winterize It! Outboard/IO Fuel System Prep


When it comes to storage of a gasoline engine—whether it’s an inboard, outboard or I/O—for long periods, stabilizing the fuel and protecting the engine’s internal components from corrosion caused by moisture are the main considerations.

Before you take on the pre-storage job, however, consult your engine manufacturer, dealer or owner’s manual beforehand to avoid possibly damaging the engine or voiding the warranty. The storage-prep steps described in this article and the accompanying video may not be right for your particular make, model and type of engine.

Again, the basic goals of winterizing a gas-powered engine are (1) preventing internal and external corrosion of metal parts and (2) stabilizing the gas to prevent the formation of varnish, gum and deposits in the fuel system during storage.

In the accompanying video, we’ll walk you through the steps for prepping a 2004 200-hp Mercury OptiMax DFI outboard for winter storage. The first step is to drain as much existing gas from the engine as possible before running a fuel stabilizer and conditioner mix through the fuel system to prevent the formation of gum or varnish during layup. Here’s what the engine manufacturer recommends:


  • Remove the internal fuel filter from the engine’s vapor separator tank (VST). This filter should be changed at least once a year, according to Mercury.
  • Detach the hose from the drain fitting on the right side of the VST, loosen the drain screw to the filter chamber and allow fuel to drain from the filter chamber into a container.
  • Once the fuel has drained, tighten the drain screw and reattach the hose to the right-side drain fitting.
  • Next, remove the hose from the left fitting, loosen the drain screw on the right side of the vapor separator and allow the fuel to drain.
  • When all the fuel has drained from the VST, tighten the right drain screw and reattach the hose to the fitting.
  • Pour 4 ounces of premixed fuel stabilizer and fuel conditioner directly into the filter chamber.
  • Install a fresh fuel filter and replace the engine cowling.
  • Run stabilized and treated fuel through the engine for 5 minutes (after first attaching a cooling-water source, of course).

When it comes to storing the gas in the boat’s fuel tank, the best advice is to either drain it completely before the end of the season or keep the tank topped off and treat the fuel with the appropriate amount of stabilizer and conditioner. If you go the latter route, be sure to leave a little space to account for expansion of due to changing temperatures; otherwise, you could damage the fuel lines or tank.