Boat Generator Runs Fine, Then Shuts Down Under Load: Step-by-Step Troubleshooting

Most gensets store or display a fault code or light up a shutdown lamp after a safety shutdown. Look at the panel at the generator and the remote panel before you reset anything, and look the code up in your owner's manual. A code that says high exhaust temperature, high coolant temperature, low oil pressure or overload points you straight to one of the steps below. In a BoatUS story, a cruising couple's generator kept shutting down after eight seconds with the same high-temperature code; their unit had two high-temperature sensors, and the failed one was the second sensor they hadn't noticed.

• Shuts down after several minutes, sooner with more load: usually heat, from weak raw-water flow or a cooling problem (steps 3 to 7).
• Shuts down the moment a big appliance starts: usually overload (step 9).
• Stumbles, surges or loses power before quitting: usually fuel (step 10).
• Shuts down right away no matter what: check oil (step 8) and the fault code, and see the "won't start" guide.

With the generator running and no load, look at the generator's exhaust outlet on the hull. You should see a steady flow of water with the exhaust. Westerbeke warns it is very hard to judge flow by eye: what looks like enough water may not be. If you have little or no water, shut down and work through the next steps. Steam or white "smoke" at the outlet can also mean too little seawater, per Westerbeke.

Make sure the generator's seacock is fully open. Yachting suggests that when a boat is sitting still, something like a plastic bag can get sucked against the intake through-hull and held there; if you're anchored, try running the boat a bit or check the intake (only a trained swimmer, in calm water, with all engines off). Then close the seacock, open the raw-water strainer, clean the basket, check the lid seal, reassemble and reopen the seacock (see the raw-water strainer guide). A leaking strainer lid can let the pump suck air instead of water.

If the strainer is clean but you still suspect the intake, a mechanic quoted in PassageMaker checks it this way, with the generator off: close the through-hull, pull the hose off after it, then open the valve briefly and see how strongly water comes in. This lets seawater into the boat, so leave it to the mechanic unless the valve works smoothly and the hose is short and easy to refit; have a bucket ready, close the valve again within a second or two, and refit and clamp the hose before you start the generator. If the valve won't close, plug the opening, call TowBoatUS or Sea Tow, and use VHF 16 if water is gaining.

Remove the pump cover and look at the impeller. Missing, cracked or curled vanes, or vanes that have taken a permanent bend, can't move enough water under load. Replace it with the one your manual specifies (see the impeller guide). Westerbeke points out that pieces of a broken impeller often lodge in the end of the heat exchanger or in the fittings after the pump; if vanes are missing, find them before you run the engine again.

Westerbeke says the most common restrictions are a clogged strainer, a dirty heat exchanger, or a clogged water injection fitting on the wet exhaust elbow. The heat exchanger has removable end caps for cleaning out scale and impeller bits (see the heat exchanger guide). The exhaust elbow, where water is sprayed into the exhaust, can clog with carbon and scale; Northern Lights says light loads plug the elbow with carbon and recommends inspecting it once a year and replacing it if you see rust, leaks or cracks (see the exhaust elbow guide). A blocked elbow cuts water to the exhaust and trips the high-exhaust-temperature switch.

On closed-cooled (freshwater-cooled) generators, check the coolant with the engine cold. Northern Lights says to check it at the pressure cap, not just the overflow bottle. Low coolant, a loose belt to the coolant pump, or a thermostat that sticks can all cause a high-temperature shutdown under load. Westerbeke notes a sticking thermostat is easy to overlook.

Low oil pressure is one of the standard shutdowns. Check the dipstick with the generator off and level, and top up with the oil your manual names. If the level is fine but it still shuts down on oil pressure, or the oil is milky, overfull or smells of fuel, stop and call a technician (see the water in engine oil guide).

Add up what was running when it quit. Boating Magazine notes you can't expect to run all your appliances at once on most boats. Air conditioners, water heaters, electric cooktops, battery chargers and watermakers are the big ones, and many motors draw a brief surge when they start, so an AC compressor kicking on can be the last straw. Westerbeke links black exhaust smoke with overload. Try this: turn every AC breaker off, start the generator, let it warm up for a minute as Northern Lights advises, then add loads one at a time, biggest first, waiting for each to settle (see the load management guide). If it only quits when a certain combination is on, you've found an overload, not a mechanical fault.

Yachting describes a generator starving for fuel: it idles roughly, has trouble picking up, hesitates or sputters to a stop. Gensets often sit unused, so their fuel filter is a common culprit; change it as your manual describes and carry a spare. Also check:

• The tank level. On many boats the generator's fuel pickup sits higher in the tank than the main engines', so the generator can run dry first.
• The fuel valve for the generator is fully open.
• Air in a diesel's fuel line after a filter change; bleed it as your manual says (see the diesel fuel bleeding guide).
• Gasoline gensets with ethanol fuel: Yachting notes they are especially prone to clogged filters and carburetors.

A generator needs enough air for combustion and cooling. Boating Magazine notes installations need enough ventilation airflow. Make sure vents and the sound shield's air openings aren't blocked by gear, and that the engine room isn't extremely hot. An infrared thermometer pointed at the generator and room after a shutdown can help you describe the problem to a tech.

If the cooling, oil, fuel and load checks are all fine, the cause may be the governor or speed control (the engine can't hold its speed under load, so frequency drops), a voltage regulator, or a faulty sensor or switch. BoatUS's surveyor writer recommends testing warning alarms and safety shutdowns during routine inspections. A technician can test each shutdown switch, check the frequency and voltage under load, and use a load bank to load the set in steps. Don't bypass or jumper a safety switch to keep it running.

Once you've fixed a cause, start with the AC breaker off, check water at the exhaust, then add load one item at a time and watch for 15 minutes or more. If it shuts down again, note the time and code and stop.

Record the date, code, what you found and what fixed it in BoatMend. Patterns over a season (for example, shutdowns only in warm water or only with both AC units running) are very useful to a technician.