Boat Engine Troubleshooting


By Tom Neale
Photos by Mel Neale

What’s that funny smell? Do you hear a strange noise? Wait a second, that feels too hot! Stop and trust your senses. They’ve got some important news for you.

Using your senses of hearing, touch, smell, and sight can alert you to an impending situation with your boat that may be about to ruin your day. Here’s how to heed the warning.

Your Sense Of Hearing

Ignoring Whining Now Could Make You Cry Later

A mechanic’s stethoscope can be used to detect unusual noises in engine components. Here, Tom Neale listens to the turbo.

Some problem noises are obvious. Some are not. Get into the habit of regularly listening to critical components such as the fresh- and raw-water pumps, alternator, transmission, injector pump (if you have a diesel), and any other convenient spots. You’ll then have an idea of what’s normal. You can use a long screwdriver with plastic handle to listen. Touch the blade to the part and put your ear to the round end of the plastic handle. Far better, buy a mechanic’s stethoscope. They usually cost less than $20, and are much more sensitive and safer to use. Keep well clear of the pulleys and belts when you do this.

  • A gravelly noise from a component with bearings can indicate that the bearings are about to fail. The alternator and fresh-water recirculating pump are prime suspects when you hear this. A belt that’s too tight could hasten either of these failures.
  • Change in tilt-lift motor noise on an outboard could be a precursor to pump failure or air in the tilt motor fluid. It could also indicate drop in voltage that could indicate fault in the charging system, corroding connections, or wiring. Note: It’s normal for most tilt motors to have two different levels of sounds as the function shifts from power trim adjustment to full tilt.
  • Variation in the engine noise, called “hunting,” could indicate impurities in the fuel, an air leak in the suction line, a clogging filter, a failing fuel pump, or a failing injector pump.
  • A “thunk” when you push the starting button means problems, even if your engine then seems to start normally. The “thunk” could be caused by a hydraulic lock resulting from water standing on top of a piston. If you hear a lighter “clunk” in the starter, it may be a bad solenoid, engagement gear, or starter.
  • A squealing noise could indicate a loose V-belt, but it could also be a clue that one of the components it is turning, such as the alternator or fresh-water recirculating pump, is freezing up. Bad bearings could be causing this in both components. Overload or deteriorating internal parts could cause this in the alternator.
  • Unusual cracking or creaking sounds when hitting seas, running at speed, or otherwise stressing the hull could indicate delamination, structural bonds failing between bulkheads or supports, impending transom detachment, or other serious problems.
  • The bilge pump running more often than usual means you should start looking for a leak. Some less obvious sources of water in the bilge include the propeller shaft seal, the freshwater system, the cooling system, the pop-off valve in the hot water heater, and the hoses on the engine.
  • Unusual noises in the transmission usually signal a problem developing that could require professional help very soon.

Your Sense Of Touch

Feeling Hot, Hot, Hot


Very carefully touching some engine parts can help determine if there is excessive heating in places that should be cool.

Unusual vibration, whether in the boat or a component, is a sign of something amiss. It may be difficult to isolate, but you should try if it’s safe to do so. Examples of potential problems: steering vibration may indicate something hung on the rudder or a system problem. Vibration in the hull over your shaft strut may indicate something caught on the prop or a bent prop. Vibration of the shaft may indicate the same or it could be a misalignment.

Feel for temperatures on all equipment regularly. A good time to do this may be during noise checks. Some parts you can touch with your hand, but if you don’t know them, this could result in a serious burn. An infrared-temperature gun with laser is invaluable. With it, you can accurately determine normal operating temperatures for different components and write down the values. Check these areas:

  • Heat exchanger — The area where the engine water enters should normally be hotter than the other end where it exits.
  • Alternator — It will be hotter putting out higher amps or if the belt is too loose.
  • Exhaust — Abnormal temperatures at the point where raw cooling water enters your exhaust via the injection nipple can indicate a failing raw-water impeller, clogging heat exchanger, clogging injection nipple, debris in the raw-water strainer, engine laboring too hard, and other problems.
  • Rub some transmission fluid between your fingers shortly after running (be careful, it may be hot) or when you’re changing it. If you feel grit, there may be a problem. There’s usually some grit in many transmissions, particularly new ones as they’re being broken in, but this shouldn’t be excessive.

Your Sense Of Smell

Find The Stink Before You Sink

Your nose knows when something is amiss. Any changes in the way your boat smells — either when you open it up for the weekend or while running — could mean a problem is developing. Fuel fumes (either gasoline or diesel) must be dealt with right away. Often a leak is caused by vibration of a fuel line against another part of the engine. If this isn’t corrected, it will worsen the breach. If a high-pressure fuel line is leaking, it may be spraying a fine mist of diesel into the air and this could cause a fire.

  • Burning rubber may indicate a slipping belt that can overheat an alternator, or it could indicate freezing bearings in an alternator or water pump that will soon destroy the belt and the component.
  • Steamy water, antifreeze, or overheating paint smells in the engine space indicate an overheating engine. Antifreeze smell may also indicate a breach in the engine’s cooling system, ranging from a burst hose to a cracked head or block.
  • Hot lube oil smell indicates an oil leak, probably requiring immediate shutdown.
  • A burnt oil smell when you pull the dipstick signals a serious internal problem. Have the engine checked immediately.
  • Burning electric insulation smells should never be ignored. It could mean that a terminal is overheating and about to arc and/or short out, or that there is too much of a load somewhere in your electrical system, or that an electrical component is suffering an internal meltdown. Its source must be located immediately. Try to turn off whatever wire or component is involved, preferably at the breaker. It may be a good idea to depower the entire boat, DC and AC, and then turn circuits back on, pausing with each one to see if that’s the cause of the smell.
  • A change in bilge smell, such as a new musty smell, could indicate leaking condensate from the air conditioning or refrigeration units. A leaking shower sump causes its own bad smell.

Seeing Is Believing

A Close Look Might Reveal Trouble Beneath The Surface

Old and new pop-off valve from a water heater. Notice the green area of the old valve, an indication of leaking.

A periodic visual inspection of important components could prevent a bad surprise later, if you know what to look for. Areas of rust, or a small crack or scaling on a stainless-steel component, may be a sign of crevice corrosion, which could result in failure of the part.

  • Gelcoat cracks may be of only cosmetic significance, but they could indicate serious flex from too much stress in the area, or delamination underneath.
  • Changing exhaust color with no obvious cause may be a precursor to problems. Generally, black exhaust means fuel burning poorly; white or bluish smoke may indicate excessive oil burning; and steam can indicate overheating. But exhaust colors have many nuances. Diesels emitting black smoke could need a new air filter. Diesels billowing smoke might need new injectors. Gas engines might smoke on startup, but more than 10 seconds could indicate a problem with the fuel mixture. Steam from either engine type could mean water getting past the head gasket (and will be accompanied by the smell of steamy water or antifreeze).
  • Frayed lines may be evidence of age and too much exposure to UV, chafing, and poor fairleads. Figure out the cause and address it, and then replace the line.

Observing closely in areas where problems are likely to occur may alert you to potential issues. Cracking, rust, and pitting of the shaft, visible here at the stuffing box, are an indication that the shaft should be pulled, inspected, and likely replaced.

  • Discoloration of metal parts in contact with water could indicate corrosion or electrolysis. Unusual discoloration of heated metal parts can indicate excessive heating. Many metal parts (such as an exhaust manifold) subject to high heat will discolor to some extent, but watch for continued, more noticeable discoloration of these parts. Discolored paint can also indicate excessive heat. Bronze parts that have turned pink may indicate a stray current problem.
  • Discoloration of wire terminals or insulation could be a sign of excessive heating or corrosion underneath the insulation.

Shaft wobble in the worn cutlass bearing produces excessive vibration, compared to the new cutlass bearing.

  • A sheen on the water at your exhaust when the engine is running can indicate poor burning of the fuel. Possible causes could include bad injectors, timing, plugs, or poor compression, as from valve or ring problems. Pull the spark plugs on your gas-powered engine. Beige or brown is the right color. Grey indicates detonation/timing issues. The sheen could also indicate lube oil entering the exhaust or transmission oil escaping into the exhaust through a leaking heat exchanger

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