Spring Prep: New Boater’s Guide to “Waking Up” an Inboard/Outboard Engine

By Brian Gordon, Last updated: 12/29/2020

Mercruiser outdrive

MerCruiser inboard/outboard outdrive.

The snow has melted, new green grass is reaching for the sun and guess what? It’s time to get your inboard/outboard ready for the season ahead!

In this article we assume that your engine and outdrive were winterized with antifreeze and that the engine was treated with fogging oil prior to winter layup. While this article does discuss changing your inboard/outboard's engine oil and gear lube oil, these tasks ideally should be part of winterization. Why? Because dirty engine oil can form acids that can damage internal engine parts. And if your gear lube happens to contaminated with water (a common problem), the drive can possibly suffer damage by the time you get around to changing the gear oil in the spring. We hope (knock on wood!) that your manifolds and risers are sound (see “About Manifolds and Risers” at the end of this article) and that outdrive components such as the bellows and the gimbal bearing do not need to be replaced. This article is general in nature and should not replace the recommendations found in your inboard/outboard engine’s manual.

What you will need:

What you need to do:

1. Inspect the engine belts and hoses.

Give the engine a “once over”. First, examine the engine belts for signs of excessive wear. If in doubt, replace the belt. Second, take a look at the cooling water and exhaust hoses. Look for signs of abrasion or for cracks which may not be easy to see. Hoses should be pliable and not hard to the touch. Squeezing or bending the hoses may reveal cracks that are not visible to the naked eye. Suspect hoses should be replaced. Hoses should be securely clamped with marine-grade stainless steel hose clamps. Third, inspect the fuel lines and hoses. Again, look for cracking, or spots where the hose may be worn due to vibration-related abrasion. Fuel hoses should be rated for marine use. As with exhaust and water hoses, suspect hoses should be replaced.

2. Reconnect the battery to the battery cables.


After charging the battery, check its voltage with a multimeter.

If your battery was disconnected for winter storage (which is recommended), you now need to reconnect the cables and any other wires that were connected to the battery. Clean any corrosion from the terminals. Apply dielectric grease or dielectric spray after installation to prevent corrosion. If your battery terminals have wing nuts, replace the wing nuts with hex nuts and lock washers to ensure that the connections remain tight. Next, check the battery’s voltage with a multimeter. If it needs to be charged, charge it. After charging your battery, check its state of charge again. Optimum voltage varies with battery type. Flooded batteries should register 13.1–13.4 volts. Gel batteries 13.1–13.3 volts. AGM batteries 13.2–13.8 volts.

3. Change the spark plugs.

This is a pretty simple procedure, the only limitations being a possible lack of access on some engines, which can add to the time required to complete the job. Consult your owner’s manual for the best approach to take. Most spark plugs come pre-gapped, but we suggest you double check the new plugs against the specs in your owner’s manual.

4. Inspect the outdrive.

Start by checking the fluid level in the trim/tilt reservoir. Then test the trim/tilt and inspect the trim cylinders for leaks. Next, examine the drive shaft/U-joint bellows, which is an accordion-like rubber boot that surrounds the engine’s output shaft. Make sure there are no splits or cracks. Following this, inspect the exhaust bellows (if present) and the shift cable bellows. If any of the bellows needs replacement, replace all of them. FYI: A compromised bellows can allow water to enter your boat and potentially sink it! Note: Bellows replacement falls outside of the maintenance that many owners would consider performing themselves. So for bellows replacement (and other outdrive parts), depending on your level of expertise, you may need to contact a professional.

5. Inspect the propeller.

Stainless steel propeller

West Marine offers a complete selection of propellers. To explore how your boat's propeller affects its performance, see Selecting a Propeller

Verify that the propeller is not chipped or bent and make sure that it rotates smoothly. If it checks out ok, remove the propeller, lubricate the shaft with a quality, water-resistant marine grease and reinstall the prop. Question: Are you satisfied with your propeller's performance? The way to determine if you are getting all you can out of your prop is to check your owner’s manual for the optimum rpms that your engine should be producing at WOT or Wide Open Throttle. If it’s revving too high or too low, you may need a propeller with a lower or higher pitch. If we have piqued your interest here, check out Selecting a Propeller.

6. Change the outdrive gear lube oil.

As mentioned at the beginning of this article, changing your outdrive's gear oil is a task best performed in the fall as part of winterization. This is because if your gear lube happens to contaminated with water (a common problem), the drive may possibly suffer damage by the time you get around to changing the gear oil in the spring.

Because outdrive designs vary, we can't describe a single gear oil change procedure that is right for all. Whereas some outdrives are relatively simple, having but a single fill/drain port at the bottom and a vent port located above it, others have three ports—a fill/drain port at the bottom, a second fill/vent port above it and a vent port at the very top. Some outdrives have a gear oil reservoir (located in the engine compartment) whereas others don't. Certain outdrives have a dipstick to check the gear oil level, while others don't. Given these variations, we suggest that you check your owner's manual for the gear oil change proceedure that applies to your drive.

Gear lube pump

Example of a gear lube pump.

In spite of the differences between outdrives, all of them require use of a gear oil pump to get the gear oil into the drive. The technique is to always pump the gear oil from the bottom towards the top. When pumping gear oil into your outdrive, you need to pump slowly, since the heavy, viscous gear oil will need time to flow into all the cavities within the drive as it rises toward the top. We also recommend that when reinstalling fill or vent port plugs that you always use new gaskets, as reusing old gaskets may result in water intrusion and damage to the drive. When you remove the fill or vent plugs, watch out for old gaskets that may be stuck on the outer surface of the holes. When this happens, they should be removed by carefully teasing them off with a screwdriver.

There are a couple of other things to watch out for. First, after removing the screw-style plug from the drain/fill hole, you will notice that it has a magnet to which fine metal filings may be stuck. Fine metal filings indicate normal gear wear and are not normally a cause for concern. However the presence of metal chips indicates the need for an inspection by a service professional. 

Going back to the beginning of this section, if you have waited until spring to change your old gear oil and the old oil appears milky, water has most likely intruded into drive. Hopefully your outdrive has not sustained any damage, but milky oil does merit inspection by a pro.

7. Change the engine oil.

Wes Marine Flat Tank Oil Changer

West Marine's Flat Tank Oil Changer includes a 3.5-gallon polyethylene tank and a 12-volt pump to make oil changes easy.

You will first need to run the engine. Before starting the engine, attach the flushing muffs to a garden hose and place them squarely over the water intake ports at the bottom of the outdrive. Turn on the water. Next, ensure that the drive gears are in neutral and give the throttle lever a couple pumps to get some gasoline into the system. Start the engine and let it run for five to 10 minutes until the oil heats up. As the engine heats up, the fogging oil from winterization will burn off as white smoke. After the engine has heated up, shut it off. You are now ready to drain the oil. The oil drain plug is at the bottom of the engine at the lowest part of the sump—where unfortunately there is often not enough clearance to accept a drain pan! Fortunately, West Marine offers many solutions to this dilemma. For small boats, we suggest an oil extraction system designed to suck the oil out via the dipstick tube. For an overview of these systems see the West Advisor,  Do-it-Yourself: Changing Engine Oil.

 After removing the old oil, it’s time to change the oil filter. Start by placing a catch pan and/or an oil absorbent sheet underneath the oil filter to catch any oil that spills out when you unscrew the filter. Next unscrew the filter. You might need an oil filter wrench to do this. After removing the old filter, lube the seal of the new filter with a small amount of oil and screw it in place. Screw it down snugly and tighten it by hand. Next fill the engine with new oil up to the “full” level as marked on the dipstick. Replace the oil fill cap, start the engine and let it run for 30 seconds. Stop the engine and check the oil level again. You will find that it has lowered slightly due to oil that has filled the new filter. You'll need to add a little more oil to bring the level up to the dip stick “full” mark again, replace the fill cap and you are done! Be sure to take the old oil, filter and soiled rags to your local oil recycling center.

8. Change the fuel filter element.

Replace the factory-installed fuel filters based on the manufacturer's recommendations. To find an OEM-equivalent filter element for your engine, take advantage of our Boat Engine Parts Finder.

9. Optional Upgrade: Install a 10 micron fuel filter.

One upgrade to consider is installation of an inline 10 micron fuel filter upstream of the “stock” filter. We feel that adding a 10-micron fuel filter/water separator upstream of the filter at the engine is good way to help reduce the contaminants that might load up the stock fuel filter—many of which filter in the two micron range. For additional commentary on why this upgrade is a good idea and how to perform the installation, see Multi-Stage Fuel Filtration.

Martyr Brand Anode Kit

Available anode kits include a complete set of OEM-equivalent anodes for your outdrive or outboard motor.

10. Inspect the sacrificial anodes and if necessary, replace them.

Sacrificial anodes prevent damage caused by galvanic corrosion. Consult your owner’s manual for the location of these anodes. West Marine offers complete anode kits for most makes and models of inboard/outboard engines. Be sure to select anodes of the correct material: Zinc for boats used in saltwater, aluminum for boats used in brackish water and magnesium for boats used in freshwater. Anodes that are more than 50% eaten away should be replaced. In addition to the anodes located on the outdrive, the engine itself will have one or more anodes. Again, consult your owner’s manual for their location. Note: If you use zinc anodes and they have been subjected to freshwater (rain or snow) over the winter period, the white buildup needs to be removed for them to function properly when the boat goes back into service.

11. Consider replacing the water pump impeller.

Most outdrive manufacturers recommend replacing an outdrive's water pump impeller annually or after no more than two seasons. Impeller replacement is a relatively simple procedure, easily mastered by most novices. Follow the specific recommendation of your outdrive's manufacturer.

About Exhaust Manifolds and Risers

Corroded, rusty manifolds and risers pose a perpetual headache to owners of boats with raw-water-cooled inboard or inboard/outboard engines. The reason for this is that manifolds and risers are made of cast iron, which is highly susceptible to rust and corrosion—particularly in saltwater environments. The purpose of manifolds is to cool the hot exhaust gasses of an engine and direct these gases to the riser where they combine with the cooling water and get pushed through the exhaust hose and expelled overboard. Manifolds consist of an outer water jacket that surrounds an inner passageway through which the hot exhaust gasses pass. Unfortunately, over time, rust and corrosion can develop and restrict the passage of water through manifolds, which can lead to overheating problems or in the worst case scenario, water entering the cylinders and ruining the engine.

Rusty manifold port

Water jacket of manifold port seriously obstructed by rust. Photo courtesy BoatUS.

According to BoatUS, in saltwater environments, the life of a manifold can be as short as three years! As mentioned, a sign that your manifolds are going bad is engine overheating. Of course, if your engine is overheating, it might also be due to a bad thermostat, blocked water intake or a faulty water pump. If the problem is due to a bad manifold, you need to correct the problem stat! Unfortunately, the outward appearance of a manifold or riser is in no way indicative of its “health”. There are a couple of ways to check the status of your manifolds. A crude way is to drip a small amount of water on to each riser. If the water evaporates at different rates, you may have a flow restriction. A more accurate way to determine the status of your manifolds is to have a marine mechanic check them with an infrared, digital thermometer. Replacement of manifolds and risers falls outside of the maintenance that many owners would consider performing themselves. If your manifolds or risers need to be replaced, depending on your level of expertise, you may need to contact a professional. Note: If your engine is freshwater cooled (runs engine coolant through a heat exchanger), manifold corrosion is eliminated, but you still can have a problem with the risers since they are always raw water cooled.