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How to Winterize an Inboard Boat Motor? – 7 Steps to Do

Fact checked by William Hart

how to winterize an inboard boat motor

Freezing winters are treacherous to stowed vessels. When the temperature drops below zero, any remaining water in your boat engines, tubes, and hoses will expand and crack the compartments. The damage can cost up to a thousand to repair.

Thus, you must winterize a boat before storing it in the off-season. This article will walk you through how to winterize an inboard boat motor. The aim is to remove all water to prevent clogs and accumulations during the long months of storage.

Read on to check out our detailed seven-step guide below!

Table of Contents

How to Winterize an Inboard Motor


Winterizing inboard motor requires the following tools and materials:

  • A fuel stabilizer
  • Propylene glycol antifreeze
  • A fogging oil
  • Engine oil and oil filter
  • Motor flusher and a garden hose
  • A marine oil change pump
  • A container
  • Clean cloth

Step 1. Add a fuel stabilizer


When fuel sits for a prolonged period, some of its ingredients evaporate, and some accumulate and sink to the bottom. This is why you often find it difficult to start the engine after storage time.

If you don’t let enough fuel stabilizer circulate in the tank, the sediments can evade and clog the fuel filter, fuel injectors, spark plugs, fuel lines, and other boat’s engine parts.

The best way to protect your fuel system is to add a stabilizer to the tank before stowing the vessel. You should:

  • Switch the key to the ON position without starting the inboard boat’s engine and check the fuel gauge.
  • Calculate the amount of gasoline stabilizer you need (read the product instructions for the information).
  • Pour the correct amount of stabilizer into the fuel compartment.
  • Note: You only need a stabilizer if your inboard boat runs on ethanol gasoline. In addition, you must fill the gas tank to its full capacity before using the additive. It is to limit the amount of air in the tank since the moisture it carries can freeze and drop to the fuel.

Step 2. Heat the engine

Now, you need to warm up the engine. But don’t start your engine on-land, as it can lead to severe damage to the piston, bearings, and crankshaft.

If it’s not possible to start your engine while the boat is in the water, hook your vessel to a motor flusher for a constant water supply.

  • Connect one side of a garden hose to a faucet and the other to the flusher.
  • Connect the flusher to the vessel’s water intake.
  • Turn on the water, start your boat, and let the engine run for five minutes to circulate antifreeze.

Step 3. Drain all the water from the vessel

Before adding propylene glycol antifreeze and fogging oil, you need to drain out any remaining water in the engine and cooling system.

  • Use a wrench to remove the cap of your boat’s exhaust outlets (exhaust manifold and riser) in the engine block.
  • On inboard boats, you will find two caps on each side. If you have left the engine running for a while, be aware that the cap and water will be quite hot. I suggest putting on heat-resistant gloves.
  • If you have live wells and other boat’s plumbing systems, such as the kitchen and bathroom, you’d want to drain water from the wells and pumps.
  • Besides, use boat soap and clean the watercraft thoroughly to remove the fishy smell. You don’t want the odor to linger and develop on your stowed vessel.
  • Note: Similarly, you want to drain water from inboard/outboard engine cooling systems before winterizing it.

Step 4. Add antifreeze and fogging oil

The 3rd step might remove most water from your cooling system but not all of it. Thus, you need antifreeze to prevent water and air from freezing and keep boat engine warm in winter.

  • Firstly, you should turn off the engine.
  • Then, detach the raw water intake hose and place it in a five-gallon bucket.
  • Fill the container with undiluted non-toxic antifreeze. The amount of substance you need depends on the type of antifreeze and engine; you should read the product instructions for the information. Usually, it’ll take at least two gallons.
  • Start the vessel to circulate the antifreeze through the entire engine.
  • When the antifreeze bucket is almost empty, turn off the engine and spray fogging oil into the carburetor. If you winterize inboard boat motor Mercruiser, you will find the carburetor underneath the flame arrestor.
  • Finally, remove the water intake hose from the bucket and reconnect it to the intake seacock.

Step 5. Change the engine oil and oil filter


Engine oil and fuel filters should be changed after 100 hours of use or every year. So, it’d be ideal to carry out this step in your winterizing checklist. Fresh oil and filter will minimize clogs and accumulations in the upcoming storage months.

Here’s how to change the oil and filter:

  • Place a pad or cloth underneath the oil compartment to absorb any spills.
  • Remove the oil fill cap.
  • Insert the tube of the marine oil change pump into the fuel tank, making sure it touches the bottom.
  • Start pumping to suction the oil out of the compartment.
  • When you finish draining the oil, twist the old filter counterclockwise to remove it.
  • Dab some oil on the O-ring of the new filter and secure it in place.
  • Add new oil to the oil compartment.

When you are done, use a dipstick to check the oil level. Ensure that you have added a sufficient amount. To learn the ideal amount of oil needed, check the owner’s manual.

Step 6. Check the exhaust system and hoses

Besides the inboard engine, you want the exhaust system and hoses to be free of water and buildup. You should refer to the boat owner’s manual for their positions and correct disconnecting methods.

  • The boat exhaust outlet that connects to the water lift muffler should be drained out completely.
  • The raw water injection hose should be free of debris and buildup.
  • The water hoses should be in good condition without softness or cracks.
  • The hose clamps should be tightly secured.

Step 7. Finish up

To finish winterizing inboard boats, close the air intake and other openings of the engines using tape, plastic caps, or plastic containers. You don’t want damp air or moisture to seep into the air inlet and damage the engine. In addition, don’t forget to:

  • Inspect the oil compartments and fuel lines for leaks.
  • Remove control cables and lubricate them to prevent them from getting stuck the following spring.
  • Dismantle the batteries, charge them, and store them separately.
  • Protect your inboard vessel with a winter boat cover, such as a marine shrink wrap.



Cost to winterize an inboard boat

The cost of the tools and materials is around $250, but you can use the remaining products and have additional antifreeze handy for many more years.

When you hire a professional for the job, you will pay double the initial material cost for the mechanic’s labor and experience. The fee for servicing your boat starts at around $500. Large vessels with more than one engine might cost a lot more.

If you are not confident with your skills, we recommend calling a boat service. Winterization requires technical knowledge that might not be familiar to every boat owner. Remember that a misplaced cap or loose joint can cost you dearly when you de-winterize the boat.

You can expect similar costs when winterizing inboard outboard vessels since the procedure is relatively similar.

Do you leave antifreeze in boat engine?

Yes, you will leave antifreeze in the boat engine during winterization to prevent water frosts and clogs. This is a critical step in order to winterize a sterndrive boat, an outboard boat, or an inboard boat—any vehicle and system that have pipes carrying water.

Do I need to clean my boat before winterization?

Your boat should be examined and cleaned at least once a year. It’s a good idea to carry out the chore before winterizing your boat. You should:

  • Pick up the gear and items on the deck.
  • Clean the interior and exterior thoroughly with boat soap and a pressure washer.
  • Apply a boat polish to restore the shine of the fiberglass.

Do you have to winterize your inboard motors?

You must winterize your boat before the temperatures drop below 25 °F (-3 °C.) Meanwhile, you don’t have to winterize your inboard if you live in a warm climate or store it in a temperature-controlled facility.

In the UK and Scotland, boat owners need to prepare their boats for winter storage in September or October. Meanwhile, owners living in the warm states of the US, such as California and Miami, rarely have to worry about winterizing their vessels.

Once winterized, your vessel will need de-winterizing before you can land and operate it on the water. De-winterization is as time-consuming as winterization. So, consider the weather carefully before deciding to put your boat in hibernation.


How to winterize an inboard boat motor? You will add a stabilizing additive to the fuel, run antifreeze through the engine, spray the fogging oil to the carburetor, change the oil and oil filter, and inspect the exhaust system and hoses.

Winterization is mandatory before storing your vessel in the freezing winters. Since it must be carried out every year, we encourage you to learn the steps and do it yourself. Once you get used to the procedure, you will get the job done quickly and precisely.

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