Outboard Engine Winterisation


Here I give you my top tips for getting your outboard engine ready for winter!

You should of course start by asking yourself if you really do want to winterise? There are some lovely days on the water over the winter and it is far less crowded afloat than the summer. Assuming however you will not be using your outboard for a few months over the winter read on.

While I don’t claim the following is exhaustive, here are a few things you can do to look after you petrol outboard engine if you’re laying the boat up for the winter.

Outboard Engines
Outboard Engines



Start with flushing the engine through with fresh water. On a small outboard you can do this in a large bucket, as long as the engine is secure and you do not engage gear. Larger engines will need muffs that fit on the end of a hose and clamp across the water intake at the bottom of the engine leg. Some more modern engines have a flushing plug to connect the hose to. Once you are connected up to fresh water, start the engine up and run it in idle.

Flushing an outboard engine
Flushing an outboard engine

If at any time water stops coming out of the tell tale, close the engine down. Once the cooling system is thoroughly flushed you can move onto the oil change


Keep running the engine up to operating temperature. This will make oil extraction much easier. Drain of the old oil, remove the old filter. Before fitting the new oil filter dab a little oil around the O ring. Also replace O-rings on the oil drain and re fill. Once you have changed the gearbox oil, engine oil and filters, you can think about finishing the flushing.


With fresh water still running, disconnect the fuel line and allow the engine to run until it starves itself of fuel. As the fuel begins to run out and the revs start to die, pull the choke lever out part way and the revs will pick up, keep doing this until you are at full choke and the engine dies out. You can now remove the fresh water supply.

Flushing Muffs for an outboard engine
Flushing Muffs for an outboard engine

Disconnect the hose/muffs and allow the engine to drain. The engine should be in a vertical position for flushing and draining (if your boat is on a trailer that is bow up this may require a slight trim up of the engine to bring it back to vertical). It’s important that engine is allowed to fully drain of fresh water. If you are unable to store it vertically and dry then consider filling the cooling system with anti-freeze.


There may be a small amount of fuel remaining in your carburetor. Locate the drain plug at the bottom and drain off the last remnants of fuel.


Next up, disconnect the kill cord (on Mercury/mariner engines switch off the kill cord switch) to prevent accidental starting. Disconnect the spark plug leads and remove the plugs using a spark plug socket (most will be 5/8 inches or 13/16 inches but they can vary). You now need to turn the engine over gently while spraying fogging oil in through the spark plug holes. If you have a manual start engine just pull the cord gently half way. If an electric start then short bursts of only a few second on the key should do. Remember your engine shouldn’t start as you have disconnected the kill cord, the spark plugs and their leads.

Now block the spark plug holes back up with the old spark plugs and give the whole engine a good spray with the fogging oil. You can replace the spark plug(s) with brand spankers at the start of next season.


Grease the prop shaft and the inside of the propeller hub. Place a plastic bag over the prop/shaft to avoid getting grease on anything else in storage.

Prop Shaft Greasing
Greasing an outboard engine propellor shaft

Grease all joints, moving parts, grease nipples etc.

Typical Outboard Engine Greasing Points
Typical Outboard Engine Greasing Points


Pre-mixed fuel should not be stored for long periods of time as it separates. Straight petrol was traditionally OK for a few months but the modern E10 petrol which is now standard in the UK has a much shorter life span. You can add a “stabiliser” to pre-mixed fuel or better still, aim to use it up at the end of the season and start next season with new freshly mixed fuel. If using straight petrol then to avoid condensation occurring in your tank my previous advice would have been to press the tank up to full over the winter. My distrust of E10 petrol now leads me to advise you to aim to run the tank down to empty and replace with fresh fuel at the start of the next season. Avoid storing a part full tank.


Finally store your engine vertically, ideally somewhere dry and warm until the spring.

The above tips do not replace attending our Outboard Maintenance course and reading the relevant manufacturer’s engine manual.

Outboard Engine Maintenance Course
2 students working on one of our outboards in the training room.
Outboard Owner and Powerboat Trainer