How to Troubleshoot Boat Engine Problems

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Marine engines can wheeze and die at the most inopportune times, and it always seems to happen when you are far out in the deep blue ocean or in the middle of that lake or reservoir. Panic sets in and the temptation to get on the ship-to-shore radio and holler for help overwhelms the senses. Turning the ignition key or pulling the rope starter with no effect before the boat even launches can cause major frustration. The time for engine diagnosis has arrived. A step-by-step process of elimination can solve the problem and get you on your way again.

Anchor the boat and remove the engine cowl (cover) if you have experienced engine problems while out on the water. Do the same if the boat has been trailered or sits dockside. Hook a voltmeter up to the battery, or batteries if it has a dual battery setup. Have your boat owner's manual on board so you can refer to the troubleshooting guide.

Connect the red (positive) lead of a voltmeter to the positive post on the battery. Connect the black (negative) lead of the voltmeter to the negative post on the battery. Check for at least 12.5 volts. Any reading less than 12.5 volts will indicate a partial or fully discharged battery. This can cause slow engine cranking or a no-start condition.

Use a pair of pliers to loosen and remove the fuel separator bowl fastener, attached to the carburetor. Empty the bowl of any water or contaminants and clean it out with a rag. Water or contaminated fuel will cause a no-start condition. Use a screwdriver to loosen the hose clamps attached to the in-line fuel filter. Remove the filter and shake it vigorously. Blow through both ends of the fuel filter to check for blockage. Clear any obstruction or replace the fuel filter.

Use a screwdriver to loosen the hose clamp on the fuel inlet line to the carburetor. Direct the end of the hose inside a can or bucket. Have an assistant turn the engine over (key to start position) and watch for gas discharge from the hose into the container. If no gas appears after cranking the engine for 30 seconds, the problem lies with a bad fuel pump or empty gas tank. To confirm a no-gas delivery problem, remove the air cleaner and shoot a stream of ether into the carburetor throat. If the engine starts, the problem lies with the fuel pump or an empty tank.

Use a socket and wrench to remove the air filter lid and take out the air cleaner element. Hold it up to a light source to see if light appears through the element fibers. A clogged or dirty air filter element will starve the engine for air, causing a no-start condition or a rough running engine.

Remove a spark plug wire from the spark plug tip with a pair of insulated plug pliers. Have an assistant turn the engine over while you hold the end of the plug wire against a metal ground source. You should see a blue-white spark emit from the wire end. No spark indicates a problem with the coil or points and condenser in the distributor.

Use a spark plug socket and wrench to loosen and remove a spark plug from the head. Hold the plug against a metal ground source and have an assistant turn the engine over. Look for a blue-white spark at the plug electrode tip. If no spark appears, the plug or wire has a short. Check all the plugs in the same manner. Any plug that does not show spark at the electrode has shorted and must be replaced.

Use a slot screwdriver to remove the distributor cap, then check the inside of the cap for carbon cracks across the poles. Check the rotor for cracks or a broken contact electrode. Replace if necessary. With the ignition key in the "on" position, gently spread the point contacts apart with a screwdriver. You should see a blue-white spark upon opening and closing the point contacts. If not, the points and condenser must be replaced. Make sure the distributor has not come loose at its mounting bolt, which will throw the timing off.

Check the coil wire attached to the distributor top pole and coil. Make sure both contacts have clean surfaces and that both connections fit firmly into their slot holes. Hook the positive lead of a voltmeter to the positive post on the coil. Connect the negative voltmeter lead to any metal ground source. Turn the ignition key to the "on" position. You should read 12 volts on the voltmeter. If not, the problem lies with the coil, a loose wire or a problem with the ignition switch.

Remove the cover to the main fuse block by unclasping it by hand or using a socket and wrench. Check the condition of all the fuses and relays. Look for blown electrical filaments in the tube or spade-type fuses, particularly the main ignition fuse, charging (alternator) fuse and main battery fuse. Swamp the fuel pump relay with another similar relay that has the same connector and amperage features. A defective fuse or relay can cause a no-start condition.

Inspect the fresh water intake port for obstructions, like plastic bags, algae, seaweed or any trash item. If the inlet port has become blocked, it will shut off the passage to the wet manifold and overheat the engine, destroying the nylon impeller in the water pump. Make sure the port has been cleaned of all obstructions, and that the wet manifold has good fresh water circulating through it.


  • Don't smoke when checking the fuel system of a marine engine. Always purge the engine compartment with a ventilator before removing the cover or cowl.


  • Always carry a tool chest filled with adequate tools for performing an engine diagnosis in case you experience engine problems offshore.
  • Carry extra hose clamps and duct tape on board your boat, to aid in performing any emergency fuel leak repairs.


About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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