Mercury 4-Stroke Outboard Spring Maintenance Checklist

Springtime marks a new boating season, but don’t rush your boat back into the water just yet. There’s a spring maintenance checklist to run through first. 

If you winterized your Mercury outboard before storing it last year, much of the work is already done. However, going through this Mercury 4-stroke outboard spring maintenance checklist helps ensure your motor is at its peak when it returns to the water this spring. 

Mercury 4-Stroke Outboard External Spring Maintenance

1. Give your outboard an external inspection, looking for scratches you may have missed when winterizing the unit last year, particularly on the lower unit, which is prone to damage from underwater strikes. Clean, prime and repaint any scrapes that might otherwise lead to corrosion.

2. Wash the engine cowling and lower unit, and apply a couple of coats of marine wax to protect the paintwork and the fiberglass from UV rays.

Mercury 4-Stroke Powerhead Spring Maintenance

3. Remove the top cowling and look for wildlife (birds, rodents, bugs, etc.) that may have nested within the powerhead during storage, and remove them and anything they left behind.

4. Check the hoses for signs of cracking or bulging, or that are leaking or worn at the ends. Replace any damaged or leaking hoses, and inspect and replace any loose or corroded hose clamps.

5. Inspect electrical wires for rodent damage and cracked coatings. Check that all the connectors are secure and free of corrosion, and tighten or replace any as needed.

6. Remove the spark plugs, clean them and check the gap between the electrodes. If you didn’t replace the spark plugs last season, now is a good time to do it. 

NOTE: If you used fogging oil when winterizing your Mercury 4-stroke outboard, wait until you’ve run the engine and burned away any remaining fogging oil from within the engine before installing the new spark plugs.

7. Check the engine oil level. Even if you changed the oil when you winterized the outboard, still check that the level is correct and that no water has mixed in with the oil. If water has mixed in, you’ll need to drain and replace the engine oil. However, if you didn’t change the oil during winterization, do it now and replace the oil filter too.

8. Consult your Mercury 4-stroke outboard’s user’s manual to see how often the fuel/water separator filter and any other fuel filters should be replaced. If you haven’t replaced the fuel filters since the previous spring, do so now.

Mercury 4-Stroke Lower Unit External Spring Maintenance

9. Inspect the water intakes in the lower unit, and make sure they’re clear of any debris or marine growth buildup from the previous year’s boating season.

10. Inspect the prop exhaust for debris or marine growth buildup, or for anything left behind by nesting wildlife. Clean out the exhaust if necessary.

11. Check the zincs or anodes, and replace any that have corroded or shrunk beyond 50% of their original size and mass.

12. Check the propeller for pitting and corrosion, and distorted or damaged blades, and replace it if needed. Whether you’re replacing the prop or not, make sure the prop nut is tight, and the nut’s cotter pin is in place.

13. Check the lower unit lube level in case it has settled after months of inactivity. You should’ve changed the lower unit lube as part of the winterization process, so replace it now if you haven’t already.

Mercury 4-Stroke Lower Unit Internal Spring Maintenance

14. Impellers can last a couple of years, but consult the owner’s manual to see how often the water pump impeller needs replacing and change it out if necessary. 

NOTE: A failed impeller can result in costly engine damage, which is why many outboard owners choose to replace the impeller at the start of every new boating season.

15. If you’re installing a new impeller, inspect the water pump housing while you’re at it and replace any other worn parts.

16. Replace any zincs/anodes that have corroded beyond 50% of their original size and mass. 

NOTE: If your Mercury 4-stroke outboard has any zincs/anodes, your outboard’s owner’s manual will tell you how often you should inspect them, but any worn ones you find should be replaced immediately. 

Mercury 4-Stroke Electrical Power and Fuel System Spring Maintenance

17. Inspect batteries for leaks and deformities such as swollen sides, and replace them if necessary. 

NOTE: Marine batteries have an average lifespan of about three years, so be prepared to replace a battery even if it passes the visual inspection. 

18. Clean the battery’s terminals, then charge the battery and check that it’s holding its charge. Reconnect the battery wires, and cover the battery terminals in dielectric grease to protect them. Make sure the battery tray or tie-down straps are securely tightened.

19. If your outboard has an external fuel tank, check the outside of the tank for damage, cracks and leaks, and replace it if necessary.

20. Disconnect the feed line from the outboard and use the primer bulb to collect a fuel sample in a jar. Check the sample for water, debris or other contaminants, and drain and replace the fuel tank if necessary.

21. Check the fuel bulb and feed line for cracks, wear or leaks, and replace them if necessary.

22. Check the lines running between the fuel tank and the motor for deformities or damage, and replace them if needed. Make sure all the fuel line clamps and connections are tight and free of leaks, and replace any corroded or loose clamps.

Mercury 4-Stroke Spring Engine Flushing and Running

23. Use an engine flusher and let your outboard run for a few minutes. 

24. Check that all the gauges are working, and that you have a good voltage reading.

25. Check that you’re getting a good flow of water from the tell-tale.

26. Stop the engine and allow it to cool down, then re-check the engine oil and lower unit lube levels, and top them off as necessary. If you plan on installing new spark plugs for the upcoming boating season, do it now.

That wraps up our Mercury 4-stroke outboard spring maintenance checklist. Enjoy getting your boat and its outboard back on the water, and don't forget to get your OEM Mercury outboard parts from Boats.net.