How to Compression Test an Outboard Motor

Performing a compression test on your outboard is an important window into what's happening inside your engine. 

The specifications for a compression test vary from engine to engine, but the basics of performing one are the same. Here's how to perform a compression test on a Yamaha F225 outboard.

Tools Needed - Yamaha Outboard Compression Test

Testing Compression on an Outboard

Step 1. Remove the three latches that hold the cowling in place: one on the front of the motor; one on the left side; and one on the right side. Once all three are unlatched, you can lift off the cowling. 

Yamaha outboard compression test

Yamaha outboard engine compression test

Step 2. Remove the cover to access the spark plug wires. This cover is held in place by four 5mm bolts. Remove the bolts and lift off the cover. 

Yamaha outboard motor compression test

Outboard compression testing

Step 3. Pull the spark plug leads. On the F225, there are six of them, one for each cylinder.

PRO TIP: When you pull on the wires to remove them, pull near the end on the hood where the wire actually connects to the spark plug. This will prevent you from accidentally damaging the plug wire. 

Outboard compression test spark plug removal

Yamaha outboard compression test spark plugs

Step 4. Use your spark plug socket and a ratchet to remove the six spark plugs.  

Outboard motor compression test spark plugs

Step 5. Open the throttle and the throttle bodies.

NOTE: Our outboard was no longer attached to a motor, so we had to do a couple of things. First, we had to connect it to a battery for power, then we used zip-ties to hold the throttle bodies open. If your outboard is connected to a boat, you'll have power and you simply need to put the boat in neutral.   

Outboard engine compression testing

Step 6. Disable the fuel injection by unplugging the harness, which is found on the left-hand side, facing the motor from the rear. 

Outboard compression testing fuel injection

Step 7. Connect your compression tester by screwing the adapter into the spark plug opening, then connecting the adapter to the tester. 

NOTE: Our kit comes with several adapters to fit different spark plug opening sizes. You'll need to find the one that fits. Follow the instructions included with your test kit to complete this part of the process.

Outboard engine compression tester adapter

Step 8. Turn the engine over with the starter to test the cylinders.  

NOTE: To get consistent readings, we turned the engine over the same number of times (in this case, seven) as we tested each cylinder. 

Outboard compression test cylinders

Step 9. Repeat the process one-by-one for each cylinder.

Our six cylinders gave us the following readings: 170, 170, 140, 190, 160, and 130. With that much variance between cylinders, we can tell something is wrong, and we're going to have to dig deeper to understand why our engine is down on compression.

Watch the video above for the next diagnosis test, the Yamaha F225 leak down test.

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