How to Troubleshoot a Marine engine
Ten miles offshore the skies are cobalt blue, the water like glass. While trolling along a promising weed line, the outboard motor dies and stubbornly refuses to restart. What do you do?
The answer is as simple as
the sea is salt. Immediately pull in the lines because you have a bigger
problem than catching dolphin. And do not succumb to the temptation of
mindlessly cranking and cranking in hopes the dead engine will magically
come back to life. It won't. All you will do is drain the battery, at which
point you will be truly dead in the water. Instead, coolly and rationally
follow these proven troubleshooting procedures to determine exactly whatever
it is that has gone haywire. In other word, troubleshoot, diagnose and
First, the good news. An outboards behavior in the last few seconds before it conks out often provides valuable diagnostic clues. For example, if it stutters before giving up the ghost that often means a fault in fuel delivery. In contrast, ignition failures are typically immediate and final: The engine either runs, or it is dead. So an engine in its death throes tells where to at least begin troubleshooting: Either the fuel system or the ignition system.
Also helpful, ask what was the last thing you did? For example, if you topped off the tanks and shortly thereafter the engine began to run rough, suspect contaminated fuel. If the cowling was recently taken off to check the crankcase oil level and soon thereafter the outboard starts to cut out, remove the cowling once more and look for a pinched wire.
Veteran troubleshooters are fond of posing the question: Is it plugged in? Translation, Is the engine getting gas? Ignition? More particularly, does fuel flow without interruption all the way from the tank to the engine? Finding out for certain requires tracing flow every single step of the way. Is there ample gas inside the four cold walls, or is it as dry as a bone? Don't blindly trust the gauge, they are notorious for lying, saying there is plenty of fuel when in reality there is none. Next, double-check whether the fuel valve is swung wide open as it should be.
At the fuel filter/water separator, open the bowl petcock to see whether water issues forth. A big volume of water means contaminated fuel. Absorb any water in the gas with a can of fuel dryer, retrieved from your well-stocked spare parts kit. Healthy fuel, while lightly tinted for tax purposes, should be otherwise clear. Murky gas means emulsified water.
Scrutinize the filter element. Is it plugged up with debris or reduced to paper pulp due to an excess of water. When in doubt, replace it with a spare. Avoid future engine shutdowns due to a plugged fuel filter by plumbing a dedicated vacuum gauge that reveals remaining filter life with a green, yellow and red bar graph.
Also worth knowing, ethanol (alcohol distilled from field corn) has begun showing up in marine fuel supplies. What may be good for the environment is sometimes the kiss of death for marine engines. Ethanol, a solvent, has been known to corrode metal fuel tanks and turn vintage fiberglass tank lining into a gooey, black sludge. Either malady plugs up fuel filters. Symptoms of alcohol poisoning include rough running, loss of power, hard starting, stalling, clogged fuel filters and plugged carburetor jets and fuel injectors.
Another complication, alcohol is hydrophilic, sucking water out of the atmosphere and the condensation dripping down the sides of fuel tanks. There is no horsepower in water. Preventive medicine includes avoiding ethanol whenever possible. When unavoidable, treat alternative fuel with a healthy dose of fuel drier and fuel injector cleaning additives. Exactly how many ounces of additive per 100 gallons of gas is detailed on the side of the can. Follow the directions explicitly. Wear rubber gloves to prevent skin contact with the chemicals. Be sure to maintain a stock of fuel filters on board.
With twin or triple outboards, tracing fuel flow offers up a number of interesting possibilities. Are both engines dead, or just one? When both motors draw fuel from the exact same source, but only one is dead, swap fuel line connections between the good and the bad engines. If the dead engine springs to life and the formerly good engine grows cold, by simple deduction both engines are rock solid. The problem is a bottle neck in fuel flow between the Y-connector and the fuel connector that plugs directly into the dead engine. Similarly, with singles rigged with dual tanks, switch from one tank to the other. If the formerly engine comes to life, you already know what to do.
Sometimes a engine malady is not fuel related and instead, electrical gremlins are to blame. Crank the engine through a few revolutions. Even if it does not start, does it spin at normal speed, or sluggishly rotate? Slow cranking can mean a weak battery, or corroded battery connections. Cleaning terminals and posts, or switching to a different battery, may make a big difference. No cranking at all can mean either a dead battery or a dead starter. Sometimes the starter solenoid is bad, and it can be bypassed. However, doing so can be dangerous due to the potential for fire and explosion hazard.
One often overlooked culprit is the dashboard key switch, a component that fails with surprising regularity. Just because the key cranks the engine doesn't mean the internal circuit that connects the battery to the ignition is good. Sometimes all that's wrong is a loose terminal. One or two twists of the screwdriver and you are good to go. On the other hand, what looks good on visual inspection may be bad.
Checking an outboard motor for adequate ignition spark begins under the cowling. Remove the cable from a sparkplug then insert a screwdriver or other appropriately sized object into the boot. Have a trusted partner crank the engine while you position the shank of the screwdriver very near the engine block, say 1/8th to ½ of an inch. With the engine cranking you should see and hear a bright blue spark. Wear gloves to protect against shock.
No spark means an ignition problem. Likely suspects include a dead electronic control module or ignition coil. Unfortunately, without a scanner tool and replacement parts on board. You are out of luck. Typically only one ignition coil dies at a time, which results in rough running, not a dead engine.
If spark tests good , then remove each one of the sparkplugs, being careful not to drop them overboard. Wear gloves to protect your fingers from burns. Examine the firing end to make sure the gap has not spread too wide. Consult the owner's manual, strategically stowed on board in a waterproof location, to reference the spark gap specification. How wide should the gap be? About the thickness of a matchbook cover is average. Better yet, retrieve the spark plug gap tool from the onboard toolkit. Measure and tighten as necessary. If the plugs are one season old, about 100 hours running time, replace them outright with new ones.
Scrutinize the porcelain insulator deposits. Wet means gas, oil or water. Any of the three impede spark at the same time telling what might be wrong. In general, raw gas means no spark. Oil means an oil burner. Water means contaminated fuel, or worse, a leaking head gasket or cracked cylinder block.
Sparkplug insulators are white when new. Logging engine hours colors them, the colors mirror overall engine health. Exactly what each color in the palette means is illustrated in sparkplug reference books, and in some engine repair manuals. Light brown means normal combustion.. Black indicates a too rich air -to-fuel mixture, or too much raw gas. Rich running derives from a multitude of ailments including a stuck choke, a carburetor float valve stuck open and pouring raw fuel down the intake, or weak ignition voltage. Insufficient ignition translate into bad plugs or a weak ignition coil. The worst case scenario is little flecks of aluminum from a piston that's coming apart at the seams from ignition, fuel , lubrication or overheating problems. Determining center and side electrode health is easy. Edges should be sharp and square. Rounded corners don't spark as strongly.
What if the engine didn't outright die, but the engine alert warning horn goes off and engine rpm drops down to a fast idle. Immediately scan the engine temperature gage. Alternatively, you should be almost, but not quite able, to lay your hand on the cylinder head. Overheating is usually caused by sucking up waterborne debris like plastic baggies and other jetsam. Either tilt the engine out of the water, or slip over the side to look for and remove the obstruction. Needless to say, never go in the water with the engine running. If there was no obstruction, the water pump may be toast.
Ultimately you may have to face the reality that there is no Lazarus cure, the engine will not resurrect. Time for a tow back to homeport. Should that unlikely scenario ever come to pass you'll be glad you had the foresight to sign up for tow insurance.