Troubleshooting marine motors can be likened to diagnosing problems with an automotive engine, with some additional concerns and extra systems. The typical marine engine, whether outboard, inboard or stern drive, sustains much more stress than automobile engines. Marine engines do not have the luxury of coasting and suffer constant load and torque pressures. Becoming stranded on a boat far from shore can be a major inconvenience, as well as dangerous for the pilot and crew. Troubleshooting a marine engine requires a patient process of elimination to find a failing component or system, as well as a knowledge of how that component or system works.
Set a multimeter for DC volts on the lower scale and attach the red meter probe to the red (positive) lead on the battery terminal. Attach the negative meter probe to the negative (black) terminal on the battery. The meter should read 12.5 volts or more, which is the standing voltage. Any volt reading less indicates a battery in the state of discharge. If the engine runs, keep the meter hooked up to the terminals and bring the engine to a fast idle. Turn on all your accessories. The reading should indicate 13.5 to 14.4 charging volts. Any reading less indicates a problem with the alternator or regulator.
Unsnap the engine cowl and remove the plug wire or wires. Inspect the plug for cracks and disconnection at the fittings. Use a socket and wrench to remove the spark plug and check the electrode for a light brown or tan coloration — a normal indicator of a good plug. Black, wet or white electrodes will indicate overly rich, flooded or an overheated spark plug. Connect the plug wire to the spark plug and set it on the engine block, close to a ground source. If the engine will start, turn it over and look for a spark jump from the plug to the ground. No spark indicates a problem with the coil or ignition system.
Remove the spark plug (or plugs) with a socket and wrench. Screw in a compression gauge adapter hose into the spark plug threads. Disconnect the coil wire completely and set the transmission selector in neutral. Turn the engine over six to eight times with the starter key and read the gauge. Proper compression for a motor's cylinder should range between 115 and 125 pounds per square inch (psi) for a high compression engine. Some lower compression engines might require only 80 to 100 psi. Check your repair manual for the correct psi range. Cylinders which differ by 10 psi or more indicate a compression problem with the valves, rings or head gasket.
Use a fuel filter wrench to remove the fuel-water separator from your engine. Unscrew the canister counterclockwise and inspect the interior. There should be no water inside the canister. Pour the contents out into a clear glass jar and look for water and oil separation level. If water exists, it means water has entered the fuel system, possibly causing a vapor lock, or cylinder lock. Refill the separator canister with fresh fuel from your gas tank, via the primer bulb and hose, and then screw it back onto the engine by hand.
Clear the exhaust ports on the lower motor unit of seaweed, silt, plastic and other debris, if experiencing an overheating problem. For stern drive and outboard motors, the exhaust water should be warm to the touch and exiting the exhaust ports in steady streams or sporadic bursts. For an enclosed fresh water cooling system, make sure the radiator level is up to maximum, and that the thermostat opens properly after the motor reaches normal operating temperature. Check the wet exhaust manifolds for leaks, if so equipped, and tighten any loose bolts with a socket and wrench. No flow from the exhaust ports on the lower unit indicates a wedged or broken water pump impeller.