Performing an Outboard Cylinder Leakage Test

OVERHEATING AN ENGINE IS NEVER A GOOD THING, AS IT CAN CAUSE INTERNAL DAMAGE TO THE CYLINDERS, SEALS, VALVES, PISTONS AND OTHER COMPONENTS. THE ENGINE MIGHT STILL RUN, BUT ITS EFFICIENCY AND PERFORMANCE COULD BE COMPROMISED.

One way to check for this type of damage—or simply test the general condition of the engine cylinders—is through a cylinder leakage, or leakdown, test. A leakage test involves forcing air into the cylinder via the spark-plug socket. Damage is revealed either by air escaping at certain points on the engine, such as the breather tube, the intake valve, the exhaust valve or the dipstick tube, or by the leakage gauge, which shows exactly how much air is escaping from the cylinder.

In the accompanying video, outboard technician Jeff Harrison of Burr Brothers Boats in Marion, Massachusetts, performs a cylinder leakage test on a 1-cylinder, 2.5-hp Yahama outboard that had recently overheated. In the case of this engine, Harrison only has to test the lone cylinder, but larger, multi-cylinder engines require that each individual cylinder be tested.

Here’s a step-by-step breakdown of the test:

  1. Warm engine to operating temperature (providing it still runs!)
  2. Remove internal fuel tank and cover plate to expose the flywheel.
  3. Remove spark plug on one cylinder,
  4. Hook the leakage gauge to the compressed air and calibrate to the zero setting.
  5. Connect adapter hose to the spark plug socket.
  6. Position cylinder to top dead center to seal off the valves.
  7. Examine gauge to determine air pressure in the cylinder. The acceptable level depends on the type and size of engine, as indicated by the manufacturer.