How to Winterize Your Boat’s Fuel Tank

When it comes to winterizing fuel tanks, some say it’s better to fill up the tank, while others say it’s better to store the boat with a fuel tank that’s nearly empty.

We’re sticking with filling the fuel tank for winterizing your boat. Here are our reasons why you should keep the gasoline tank full for winterizing and how to do it right.

Always Fill Your Boat’s Fuel Tank

When cold air meets a cold, flat surface, condensation forms. By leaving the inner walls of your boat’s fuel tank exposed to the air, you’re allowing condensation to form inside of it, and that’s not a good thing.

The ethanol in modern boat fuels attracts and absorbs water molecules, and any condensation in the tank will be absorbed into the fuel. Eventually the fuel becomes saturated, at which point phase separation occurs. Phase separation is the separating of the water/ethanol mix from the fuel, with the denser water/ethanol combination forming a layer at the bottom of the fuel tank. This corrodes the fuel tank, leading to oxidized debris that can block the fuel system. It can also get sucked into the fuel system when the motor runs again (most fuel systems take fuel from the bottom of the fuel tank), which leads to the fuel injectors or the carburetors needing to be drained, cleaned and serviced.

How to Winterize a Fuel Tank

Filling the fuel tank when winterizing your boat eliminates condensation from forming inside the tank, which contaminates the fuel and leads to damage to the boat’s fuel system. 

Before winterizing the fuel tank, take a boat trip to burn up as much of the old fuel as possible. Next, winterize your boat’s fuel tank by doing the following:

Buy Quality Boat Fuel 

After emptying the fuel tank, fill it with fresh fuel. Use E0 boat fuel (ethanol-free fuel) if possible, because it’s less likely to experience phase separation. If you can’t fill it with E0 fuel, use a quality brand name E10 fuel from a vendor you trust. The better the quality of the fuel, the less chance it’ll break down over the storage period.

Fill the Tank to 90 Percent

Leave space within the tank for the fuel to expand as the temperatures fluctuate while the boat is in storage. Don’t overfill the tank, as this can lead to expansion damage or fuel exiting through vent, which is extremely hazardous.

Use Fuel Stabilizer

Add a marine-specific fuel stabilizer while filling the tank so that it thoroughly mixes with the fuel. Untreated fuel can break down within weeks, and marine-specific fuel stabilizer prevents this from happening while keeping the fuel fresh for months. Always follow the instructions on the fuel stabilizer’s label.

Run the Motor

Fresh stabilized fuel needs to get into the entire fuel system, not just the fuel tank. Old untreated fuel left in fuel lines, carburetors and fuel rails/injectors will destabilize, leaving behind varnish-like deposits that can easily block the fuel system. By running the motor for a few minutes, the new stabilized fuel will be pumped though the fuel system, protecting it throughout winter storage.

Change the Fuel Filters 

Replace the in-line fuel filter(s) and the filter in the fuel/water separator to remove any last traces of water from the fuel system before winterizing your boat for storage. Doing this also gives you one less maintenance task to worry about when de-winterizing your engine for when the next boating season comes around.

Winterizing your boat’s fuel system is just a part of preparing the engine for an extended period of storage. Watch the videos below to see how to winterize a 2-stroke outboard motor and a 4-stroke outboard.