Buying a used marine generator can save real money over new, but the units that turn into expensive mistakes almost always looked fine at the dock. What actually predicts whether a used generator runs another five seasons or fails within a month comes down to a short list of things you can check before you hand over a deposit, not the things that are easy to see at a glance.
Hours mean less than how they were run
A generator with 800 hours that ran hard every weekend and got serviced on schedule is often in better shape than one with 300 hours that sat unused for long stretches between short runs. Ask for service records before you ask about hours. A seller who can produce oil change intervals, impeller replacement dates, and coolant service is telling you more than the hour meter ever will. No records at all is itself useful information.
Start it cold, not warm
A generator that’s been idling for twenty minutes before you arrive to look at it will start easily whether it’s healthy or not. Ask to see it started cold, from a dead stop, ideally after it’s sat overnight. A generator that needs to be warmed up, primed, or coaxed to start cold has a problem the seller already knows about.
Listen and look under load
Idling smooth tells you almost nothing. Put a real load on it, a couple of AC units, some galley equipment, whatever you can draw at the dock, and listen for the sound to stay steady rather than labor or surge. Watch the exhaust for smoke under load specifically, since a unit that’s clean at idle and smokes when loaded is showing you a problem it hides at low output.
Check what a survey often misses
A general marine survey covers the generator, but it’s rarely a deep look at the unit itself. Raw water flow at the exhaust, the condition of the heat exchanger, coolant that looks original versus recently serviced, and the state of the mounts and exhaust hose all matter more than a surveyor with limited generator experience tends to catch. If you’re serious about a boat with a generator you haven’t run yourself, a pre-purchase look from someone who works on these units every week is worth the cost before closing, not after.
Westerbeke, Kohler, and Onan age differently
Parts availability and typical failure points aren’t the same across brands, and a fair price for a used Westerbeke isn’t a fair price for a used Kohler with the same hours. If you’re not sure what’s normal wear for the specific unit you’re looking at, that’s exactly the kind of question worth asking before you buy rather than after.
A used marine generator is a good way to add reliable power without paying new prices, as long as you check the things that actually predict failure instead of the things that are easy to glance at. We do pre-purchase generator inspections as part of our diesel and generator repair work, and it’s a lot cheaper than finding out what’s wrong after the sale closes. Call us at (609) 242-8448 before you buy.
Have any questions?
Contact Forked River Diesel and Generator for support with your marine diesel engine or generator concerns.