How to Replace Mercury 20HP Outboard Spark Plugs

Mercury recommends checking and/or replacing a 20HP outboard's spark plugs after every 500 hours of use, although some outboard owners tend to do it more frequently. 

Inspecting a Mercury outboard's spark plugs is a good way to gauge the engine's condition. Rather than reinstalling the old spark plugs, some people prefer to change them upon inspection because they're so inexpensive to replace. Watch the video above and follow the steps below to change the spark plugs on a Mercury 20HP outboard.

Tools and Parts - Mercury 20 Outboard Spark Plug Replacement

• 3/8th ratchet
• Ratchet extension
• Torque wrench
• 5/8th spark plug socket
• Mercury spark plugs (x4)

Mercury 20HP 4-Stroke Spark Plug Change

Step 1. Remove the engine cowling by releasing the two latches. There's one latch at the front of the cowling and one at the rear.

Step 2. Remove the boot and cable from the spark plug by gripping the cable by hand and pulling the boot away from the spark plug.

PRO TIP: There are two spark plugs on a Mercury 20HP 4-stroke outboard, and it's important that the cables are attached to the correct spark plug. Label each cable before removing them both, or remove one cable, and replace that spark plug and its cable before working on the other spark plug.

Step 3. Remove the old spark plug using a 5/8th socket, extension bar and ratchet.

Step 4. Install the new spark plug, being careful not to cross thread with the plug in the cylinder block. 

PRO TIP: It's a good idea to thread in the new spark plug and tighten it with the extension bar because it helps you feel if the plug is threaded correctly.

Step 5. Tighten the spark plug to 15 foot-pounds with a torque wrench.

Step 6. Replace the spark plug boot and cable, then press down on the top of the boot until you hear and feel the boot click back into place over the spark plug.

(Repeat steps 3-6 if you're replacing the spark plugs one at a time.) 

Step 7. Replace the engine cowling and you're done.

Got a higher horsepower Mercury outboard engine? Watch the video below to see how to change the spark plugs on a Mercury 115HP outboard.