A client called me on a Tuesday morning with that tone. You know the one. He’d found a two-year-old 400 kVA set listed at barely half of new price, and he wanted me to bless the deal before lunch.
By 3 p.m. we were standing in a rented warehouse yard, rain coming down sideways, and the seller already had the unit warmed up when we walked in.
I walked away from it. And the client still thanks me.
Here’s the thing — it might have been a fine machine. Somebody somewhere may have gotten a bargain on it. But a used diesel generator is the one purchase where the sticker price tells you the least and the machine’s history tells you the most. New price? You can look that up in an afternoon. (I wrote a whole pricing guide — rough price brackets for 100, 500 and 1,000 kVA sets.) What you can’t Google is whether that particular engine spent its life lightly loaded and lovingly maintained, or abused at 15% load by a crew that never changed the filters.
Over the years I’ve inspected maybe thirty used and in-stock units for clients. In-stock — meaning brand-new sets sitting in a dealer’s yard, never commissioned — can be an even better deal than used, by the way. Same warranty conversation, lower price. But both need the same inspection. Here’s my ritual, in the order I run it.
A cold engine confesses. A warm one doesn’t.
So the first thing I tell the seller, always: “I’ll be there at 8. Don’t touch it.” If the unit is purring nicely when I arrive, that’s not a good sign — that’s a curtain raised before the show.
What I’m watching for on a true cold start: how many cranks does it take? Does it hunt for the first thirty seconds? What color is the exhaust? A puff of white that clears in a minute is normal. Blue smoke that hangs around means oil is getting past the rings or valve guides — and that problem only gets expensive.
One unit I checked in a port yard took nine long cranks on a mild morning. The seller laughed it off: “Oh, the batteries are just tired.” Maybe. But tired batteries on a unit that’s supposedly two years old with 600 hours? I don’t laugh those things off anymore.
This is the one most buyers skip. And it’s the one that matters most.
A diesel generator that’s been idling at 20% load its whole life is often worse off than one that’s worked hard. It’s called wet stacking — unburned fuel and carbon glaze the cylinders because the engine never gets hot enough to burn clean. I covered the rating side of this in the two ratings that decide how long your generator lives, but in the yard, the test is simpler: put 60 to 80% load on it and hold it for at least an hour.
Rent a load bank if you have to. It costs a fraction of a bad purchase. Watch the exhaust stack for wet black slobber — that’s the actual term, no joke — listen for a miss under load, and watch the temperature and oil pressure gauges like they’re telling you a secret. They are.
I once saw a unit hold 75% load beautifully for forty minutes, then start dropping cylinders one by one. The seller said it just needed “a good run.” No. It needed an overhaul it had never received.
Pop the dipstick. Smell it. Feel it between your fingers.
Oil that smells strongly of diesel means fuel dilution — injectors leaking into the sump, which washes the bearings. That’s a walk-away finding, full stop. Oil the color of chocolate milk means coolant is mixing in. Also full stop.
Now the radiator cap. Do this cold, obviously. Look for oil film or the milky residue that says a head gasket has been whispering to you. I learned this the hard way on my own money — bought a small unit for my workshop years ago, skipped the cap check, and spent the next month topping up coolant I couldn’t find leaking anywhere. It was going out the exhaust.
Hour meters can be replaced in twenty minutes with a screwdriver. Engines can’t.
So I ignore the meter for a second and read the machine instead. Paint worn off the door handles and control panel edges says thousands of hours, whatever the digits claim. Belts glazed shiny, hoses rock-hard, mounting bolts with the paint cracked around the hex flats from being re-torqued — these are hour hands on a different clock.
Then I circle back to the meter and see if the two clocks agree. A controller’s event log is harder to fake than a meter — on most modern sets the controller logs starts, run hours, and fault history. Scroll through it. I’ve found “low oil pressure shutdown” events the seller never mentioned, logged sixty hours before my visit.
Here’s a check people genuinely forget: the automatic transfer switch.
A backup generator is two machines — the genset and the ATS that hands the load over. If the deal includes the switch, test the whole sequence. Kill the utility feed (with the seller’s electrician, please, not with a crowbar), and watch. Does it transfer? How many seconds? Does the genset pick up the actual load without tripping?
I know a plant that bought a gorgeous low-hour unit and left it bolted down for eight months. First outage, the ATS chattered, welded itself halfway, and they ran the whole night on a rental anyway. Test it in daylight, when the embarrassment is cheap.
Ask the seller if they’ve megger-tested the alternator windings. You’ll get a blank stare about half the time. That’s your answer.
Moisture is the slow killer of alternators, especially in units stored in damp warehouses or near the coast. A quick insulation resistance test on the windings takes minutes with a megger and tells you whether the insulation is healthy or rotting from the inside. Anything below a megohm on a 400-volt machine and I’d want a serious conversation — and a serious discount.
The automatic voltage regulator gets a look too. Check the set runs stable voltage across the load range. If the voltage wanders while you hold load steady, the AVR might be on its way out. Cheap part, terrible symptom.
Last step, not first. Paperwork is easy to fake and boring to read, which is exactly why it goes last — after the machine has already told you its story.
Match the serial numbers: engine plate, alternator plate, controller, and the nameplate on the canopy. They should tell the same story. If the engine plate says 2019 and the alternator says 2012, congratulations, you’ve found a marriage — fine if the price reflects it, not fine if it doesn’t.
Service records matter more than age. I’d rather buy a five-year-old unit with a folder of oil analysis reports than a two-year-old unit with nothing. Ask who maintained it. If the answer is a shrug, price that shrug.
None of these checks save you if the set is the wrong size. Condition and capacity are the two halves of the deal — here’s how contractors actually size generator sets so you don’t buy the right machine at the wrong power.
So — used or in-stock, worth it?
Honestly? A properly inspected used diesel generator is the best value in this business. New money buys you certainty; inspection buys you almost the same certainty for thirty to fifty percent less. And in-stock units — new sets sitting uncommissioned in a yard — can beat both on price while keeping most of the new warranty conversation intact.
The mistakes cost nothing to avoid. Every one of the seven checks above is something you can do in a single visit with a load bank, a megger, and a willingness to walk away. The walk-away is the whole skill. If the seller won’t let you test, won’t let you arrive cold, won’t show the controller logs — you already have your answer.
That rainy Tuesday? My client bought a different unit six weeks later. Cost him twenty percent more than the bargain. Five years on, it starts every single time.
Some bargains aren’t.
Email：[email protected]
Work：Monday to Friday,9:30-18:30
