Marine Generator Won't Start or Keeps Shutting Down

Turn off the generator at the remote panel and at the set if it has a local switch. Make sure your boat's CO detectors are working before you test-run anything (see the CO detector guide). BoatUS's CO advice applies every time the set runs: keep people off the swim platform and out of the water near the generator exhaust outlet, keep exhaust outlets clear of debris, and stay well away (BoatUS suggests at least 20 feet) from other boats running engines or generators.

Many no-starts are a switch in the wrong position. Check:

• The battery switch that feeds the generator (some boats have a separate generator battery or switch).
• The local/remote switch or stop button on the generator itself.
• Any emergency stop or fuel shutoff switch.
• The generator's DC breaker or fuse, usually on the set or near its start battery.

Yachting magazine lists a dead battery, dirty or loose connections and a breaker in the wrong position as the most common reasons a genset won't start.

With everything off, measure the battery that starts the generator with a multimeter. A healthy, fully charged 12-volt battery typically reads around 12.6 volts at rest (confirm for your battery type). If it reads low, or the starter clicks or cranks slowly, charge it and try again. Check that cables are tight and clean at the battery, the starter and the ground connection on the engine block. See the marine battery guide for testing and cleaning.

BoatUS recommends that a generator have its own dedicated start battery, charged by the generator when it runs and by the shore charger when it doesn't, so it can start even when the house and engine batteries are low. If yours shares a battery with the house bank, a night at anchor with lights and electronics on can leave it too weak to crank. Generator start batteries are often tucked out of sight and forgotten; look for corroded terminals while you're there.

A gasoline generator in an enclosed engine room is a vapor risk like any gas engine. Run the bilge blower for at least four minutes and sniff the compartment before cranking, every time. If you smell fuel, stop: find the leak or call a mechanic before any cranking.

Starvation is one of the most common reasons a genset won't start or sputters to a stop, according to Yachting.

• Fuel level: on many boats the generator's pickup in the tank is shorter than the main engines', so the generator runs dry first. Check the actual tank level, not just the gauge.
• Fuel valve: make sure the generator's supply valve is open; it is easy to close one by mistake when servicing filters.
• Filters: gensets often sit unused, and old fuel clogs filters. Yachting recommends always carrying a spare generator fuel filter. Check the filter bowl (if it has one) for water or debris and change the filter as your manual describes.
• Diesel that ran out of fuel usually needs the fuel system bled of air before it will run. Your manual explains how for your model; many have an electric lift pump that helps.
• Gasoline sets suffer from stale or ethanol-damaged fuel and gummed carburetors. Yachting suggests treating the fuel and not letting it sit for long periods.

Diesel gensets use glow plugs to start cold. Most need you to hold a PREHEAT button for several seconds before and during cranking; check your manual for the timing. If preheat doesn't seem to work (no voltage drop on the battery, no change when you press it), the preheat circuit may have a fault worth a technician's look.

This is the step that saves engines. A generator's raw-water pump turns every time you crank, pushing cooling water toward the wet exhaust. Boating Magazine explains that a cranking engine that won't start can't build enough exhaust pressure to push that water out, so it collects and can flow back into a cylinder when you stop. Water in a cylinder (hydrolock) can bend or break engine parts.

• Crank in short bursts only, as your manual specifies, and let the starter rest between attempts.
• If it hasn't started after a few tries, stop and find the cause (fuel, preheat, battery).
• If someone has already cranked it for a long time, don't keep trying: have a mechanic check for water in the cylinders first.

Water can also get in while the set just sits. Many generators are mounted near or below the waterline and have an anti-siphon valve (vented loop) in the raw-water line to stop sea water siphoning into the wet exhaust. Boating Magazine says these valves need taking apart and cleaning at least once a year, because salt and gunk make them stick. If a generator that ran fine last time now won't turn over, or stops with a thump, suspect water in a cylinder: stop cranking and call a technician.

Westerbeke notes that its gensets can shut down if you release the ON switch (gas) or PREHEAT switch (diesel) too soon; holding it a few seconds after the engine starts lets oil pressure build so the safety circuit doesn't trip. Many other brands work the same way. If it still dies after you hold the switch, a safety shutdown is the likely cause: go on to steps 9 to 11. Repeated restarts without finding the cause can damage the engine.

With the generator off and level, pull the dipstick.

• Low oil: top up with the oil your manual specifies, to the correct mark, and look for leaks in the drip pan under the set.
• Oil level correct but it keeps shutting down for oil pressure: stop restarting it. That points to a sensor, wiring or a real oil pressure problem, and running it again risks the engine. Call a technician.
• Oil level too high, milky or smelling of fuel: don't run it; see the oil-level-rising guide and call a pro.

Loss of cooling water trips the high exhaust temperature or high coolant temperature shutdown, often a minute or two after starting.

• Seacock: make sure the generator's raw-water intake valve is open. On a ball-valve seacock, the handle lines up with the valve and hose when open and sits across it when closed.

• Strainer: close the seacock, open the strainer, clean the basket, reassemble with a good seal and reopen the seacock.
• Intake blocked from outside: Yachting notes that a bag or weed can be sucked against the through-hull while the boat sits still. Shutting the generator down and moving the boat can clear it.
• Water at the exhaust outlet: when it runs, there should be steady water mixed with the exhaust at the generator's hull outlet. Westerbeke cautions that it is hard to judge flow by eye; a weak-looking stream can still be too little.
• Impeller: a failed raw-water pump impeller is a classic cause. Replacing it is a routine job on many sets (see the diesel raw-water impeller guide). Westerbeke points out that missing impeller pieces often lodge in the heat exchanger and must be removed.

Many generators are freshwater-cooled with a heat exchanger. Only with the engine cold, check the coolant level at the expansion tank or fill cap; never open a hot cap. Low coolant, a sticking thermostat or a heat exchanger clogged with scale or impeller bits can all trip the high-temperature shutdown. Westerbeke lists a clogged sea water strainer, a dirty heat exchanger or a clogged water-injection fitting on the wet exhaust elbow as the most common causes of overheating.

Power & Motoryacht passes on a technician's advice: turn off the AC breakers before switching from shore power to generator power, start the generator, then turn loads on one at a time. Never shut it down with loads still on. Starting several big loads at once (air conditioning, water heater, electric stove) can overload the set, which may bog it down or trip its breaker. If it only shuts down when a particular appliance comes on, the problem may be that appliance or simply too much load.

• Generator output breaker: most sets have a main breaker on the generator itself. Check whether it has tripped.
• Source selector: make sure the AC panel is switched to the generator (not shore power) and the main breaker is on.
• No output at all after long storage: Yachting explains that a generator can lose the residual magnetism it needs to produce power after sitting unused for a long time or running for long periods without load. Restoring it ("flashing the field") is a job for a technician who knows your model.

Do not open the generator's AC junction box or measure AC voltage yourself unless you are trained to; stop here and call a marine electrician.

If your panel has an AC voltmeter or a frequency (hertz) meter, glance at it once the set is warm. BoatUS explains that the output frequency depends on the engine holding a fixed speed, and wrong voltage or hertz can damage electronics; readings that wander or sit off the normal US values (120/240 volts, 60 Hz) point to a governor, voltage regulator or brush problem for a technician. BoatUS suggests checking voltage and hertz before each cruise, or at least every three months.

Westerbeke warns that a diesel left unused for a year or more may not start because stale, varnished fuel can stick the injection pump and injectors, and the raw-water impeller may have taken a set or gone brittle. That calls for a proper recommissioning: fuel system check, new impeller and filters, and turning the engine over carefully. A technician should handle anything beyond the impeller and filters.