How to Troubleshoot a Mercruiser Engine

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It's boating time, and the family can't wait to get on the water. Problems with your boat can cause heartbreak and tears, mainly for you. There is a wide variety of Mercruiser engines and models, so this is a broad guide to troubleshooting most gasoline-powered models. The key to diagnosis is narrowing down the problem. Basically, an engine requires fuel, spark and compression to run.

Items you will need

  • Basic hand tools, including wrenches, screwdrivers and pliers

  • Ratchet wrench and spark plug socket

  • Compression test gauge

  • 12-volt test light

  • Remote starter switch

Full-throttle engine rpm (revolutions per minute) running higher or lower than required by the engine nameplate may indicate an improperly sized propeller. Too high an rpm setting could also be a damaged prop or indicate that the propeller hub is slipping. A dirty or damaged boat bottom can cause low rpm at full throttle.

Remove the spark arrestor and look down the carburetor. Move the throttle lever back and forth--if there is a stream of gasoline spraying in the carb, you can likely eliminate fuel as the problem.

Remove the coil wire from the center terminal of the distributor cap. Hold the wire with spark plug pliers or other insulated means, close to a ground point. Look for spark as you turn the starter briefly. Repeat this with some or all of the spark plug wires. If you have spark during these tests, remove and inspect the spark plugs for fouling and cracks. Try starting the engine in the dark--it may reveal bad spark plug wires or other shorts.

Try a compression test. Disconnect the main wire from the coil. Remove a spark plug from the engine. Thread on the compression gauge, hose and proper spark plug hole adapter. Crank the engine for a few seconds. You should see 130 to 160 psi (pounds per square inch) per cylinder, depending on the engine model. Replace the spark plug, and repeat on each of the remaining cylinders.

Ethanol mixed in gasoline, now required in many areas, can bond to water and be a cause of bad gas. If the boat has been in storage, try a different source of fuel than the existing tank. An old gear oil quart bottle filled with gas and attached to the fuel pump with temporary hoses would suffice.

When the engine fails to turn over at all, electrical connections are usually at fault. Clean your battery terminals, put the gearshift in neutral and try bypassing the key switch--if room permits to jump the terminals at the starter solenoid. If the starter merely grunts, remove a few spark plugs from the engine and turn the starter key. If it turns over now, you may have ingested water due to a faulty exhaust water shutter.

Engine noises may indicate serious problems caused by such things as loss of oil pressure, loose or broken timing chains or bearings. Rough running can be due to ignition or carburetor problems.


  • Never smoke or use open flames around carburetors and gasoline. Fire, injury and death are possible results. Always heed the warnings of your boat operation manuals.


  • Talk to fellow boat owners in your area. There may be specific problems plaguing boats in your region, such as weeds clogging water intakes or a shipment of bad gas. Many specifications, including spark plug gap and full-throttle rpms can be found on the spark arrestor (air intake) cover. The Mercruiser Service Manual specific to your engine can be a valuable reference.