How to Compression Test an Outboard Motor

  • By Allan Lloyd
  • Aug 02, 2018
How to Compression Test an Outboard Motor



             Everybody hears about performing an outboard motor compression test, and everyone knows that when you buy a boat with a used outboard on it, you should get a compression check. Yamaha outboard technician Sean Stahl from Annapolis Boat Sales says a compression test will help tell you how much life the engine still has, how much wear is on the piston and the piston rings, and how much air is being pushed through the engine. Essentially the engine is a giant air pump.

             You want to be sure the PSI reading for each cylinder is within 10 percent of the others. To do a compression test, your outboard must be hot. First remove the spark plugs, then remove the 15-amp fuel pump fuse to keep fuel from entering the cylinders. Thread in the pressure gauge hose into the first cylinder, then attach the gauge. Turn the key as if you were starting the engine and take a reading. Repeat in the other cylinders and then compare the readings. If they're all within 10 percent of each other, you're good to go.

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