How to Change Mercury 115HP Outboard Spark Plugs

Changing the spark plugs on a Mercury 115 outboard only takes a handful of minutes. 

Mercury recommends you change the spark plugs on a 115HP EFI motor after every 100 hours of use. Watch the video above and follow the steps below to change the spark plugs on a Mercury 115 outboard.

Tools and Parts - Mercury 115HP Outboard Spark Plug Change

  • 3/8th, 1/4th ratchets
  • 8mm wrench
  • Ratchet extensions
  • 10mm socket

Mercury outboard spark plugs

OEM Mercury outboard parts


Mercury 115HP Spark Plug Change

Step 1. Remove the top engine cowling by releasing the two latches. 

Mercury 115 outboard spark plug change

NOTE: There's one latch at the front and one at the back of the engine cowl.

Step 2. Remove the latch mechanism from the rear of the engine cover to get full access to the spark plugs. Simply remove the 8mm bolt securing the mechanism in place.

Mercury outboard spark plugs change

Step 3. Remove the spark plug cover by taking out the 10mm bolts. 

Mercury 115HP outboard spark plug replacement

NOTE: Before removing the spark plugs, notice how they're arranged vertically, and each has its own cable and boot. Usually the cables are numbered 1 through 4. Avoid getting the cables mixed up by working on one spark plug at a time. If the cables aren't numbered or have worn away, label them before removing the cables from the spark plugs.

Mercury outboard 115 spark plug change

Step 4. Grip the cable firmly and pull the boot away from the engine to remove a boot from a spark plug.

Mercury 115 outboard spark plugs replacement

Step 5. Remove the top spark plug using a wrench and a 5/8th spark plug socket. 

Merc 115 outboard engine spark plug change

NOTE: You'll need to use a wrench extension bar, as the spark plugs are set deep in the engine block.

Step 6. Remove the old spark plug and install a new one. Be careful not to cross-thread the new plug, which can damage the engine block. Install and tighten the new spark plug by hand using the plug socket and extension bar, and only use a wrench once you're sure the plug isn't cross threaded. Tighten the plug to 18 foot-pounds with a torque wrench.

Step 7. Reinstall the spark plug cable by pressing the boot over the spark plug until you hear and feel it click back into place.

Mercury outboard motor spark plug replacement

Step 8. Replace the remaining spark plugs by repeating steps 5 through 8. 

NOTE: If you remove more than one spark plug cable and boot at a time, make sure you put them back in the correct order.

Step 9. Reinstall the spark plug cover. 

Mercury 115 outboard motor spark plugs change

NOTE: Be careful not to overtighten the bolts, which can damage or break the plastic cover.

Step 10. Reinstall the latch mechanism inside the lower engine cover.

Merc 115HP outboard spark plug replacement

Step 11. Put the engine cowl back in place, and secure it with the front and rear latches.

Mercury 115 HP outboard motor

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