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—is through a leakdown test. A leakdown 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 leakdown 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 leakdown test on a 1-cylinder, 2.5-hp Yahama outboard. 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:
First, the engine should be warmed to operating temperature (providing it still runs).
- The internal fuel tank and cover plate are removed to expose t he flywheel.
- The spark plug is removed.
- The leakdown gauge is hooked up to the compressed air and calibrated to the zero setting.
- The adapter hose is connected to the spark plug socket.
- The cylinder to top dead center to seal off the valves.
- The gauge is examined to determine the air pressure in the cylinder. The acceptable level depends on the type and size of engine, as indicated by the manufacturer.