A Generator That Starts Is Not a Generator That Works

A gasoline or liquefied petroleum gas generator running with no electrical load is producing almost no mechanical work, so combustion temperatures stay low. Low combustion temperature means incomplete burn, and incomplete burn means carbon deposits on valve seats, on the piston crown and in the exhaust port. Carbon on a valve seat is what eventually produces a compression loss and a hard starting complaint.

On a diesel generator the phenomenon has a name: wet stacking. Unburned fuel and oil accumulate in the exhaust system, and in advanced cases it weeps out of the exhaust joints as a black oily residue. It fouls the turbocharger if there is one, glazes the cylinder bores, and reduces the ability of the engine to make rated power. Diesel generators specifically want to be loaded, and the manufacturer literature says so.

The other thing unloaded running fails to do is exercise the alternator and the voltage regulation. The generator end of the machine has windings, brushes on some designs, a rotor and a regulator, and none of those are meaningfully tested at no load. A regulator that cannot hold voltage under a 3,000 watt draw will look perfectly healthy at 100 watts.

Start from the nameplate. A 4,000 watt generator wants roughly 3,200 watts of load. A 5,500 watt unit wants about 4,400. A 7,000 watt unit wants 5,600. Write the target down, because the whole exercise depends on actually reaching it rather than approximately reaching it.

Then add up what you have. Two rooftop air conditioners running is typically 2,800 to 3,600 watts once compressors are up. An electric water heater element is 1,400. A microwave on high is 1,000 to 1,500. A residential refrigerator is 150 to 400 running. The converter charging a depleted battery bank can absorb 700 to 1,500. A hair dryer or a portable heater is a convenient 1,500 watt block that you can add or remove to fine tune.

Bring the load on in stages rather than all at once, and note what happens at each step. Compressor starts are the hardest moment for a generator and a unit that stumbles when the second air conditioner compressor kicks in is telling you something even if it recovers. Once you are at target, hold it for at least 30 minutes. Longer is better; the useful information about cooling and about heat related faults shows up in minutes 20 through 45.

Voltage first. Measure at an outlet or at the distribution panel, per leg on a 240 volt split phase unit. You want 118 to 124 volts per leg under load. Below 112 is a fault. A voltage that starts at 122 and sags to 108 over ten minutes as the unit heats up is a classic winding or regulator problem and it is exactly what a 30 minute test finds and a five minute test misses.

Frequency second, and this one is more diagnostic than most owners realize. Frequency is a direct readout of engine speed. Sixty hertz is 1,800 rpm on a four pole machine. Frequency that droops from 60 to 57 as load is added means the engine is not making the power to hold speed, which points at fueling, air, or mechanical condition rather than at anything electrical. A meter with a frequency function costs very little and tells you a great deal.

Third, exhaust color, watched at the moment load is applied. A puff of black smoke that clears is normal on a diesel. Continuous black means over fuelling or restricted air. Blue means oil. White that persists after warm up means unburned fuel or coolant. Fourth, sound and stability. A generator holding load should be monotonous. Surging, a rhythmic rise and fall in speed, almost always means fueling: a partially blocked jet on a gasoline unit, a regulator issue on liquefied petroleum gas, or air in the fuel on a diesel.

Runs fine unloaded, drops out when the second air conditioner starts. Usually the engine is down on power from carbon or valve condition, and the frequency reading will show it before the breaker trips. On liquefied petroleum gas units this can also be a supply problem: a regulator that cannot deliver the volume at high draw, or a tank at low level in cool weather, which reduces vapor pressure.

Holds voltage for ten minutes then sags. Heat related. Windings, regulator, or a brush and slip ring issue on machines that use them. This is the pattern that a proper 30 minute load test catches and that every short exercise cycle misses, and it is the reason I insist on the duration.

Starts hard, then runs well and carries full load. That is usually fuel system or ignition rather than a generator problem, and on a unit that has been exercised unloaded for years it is very often carbon on a valve seat producing a compression loss that matters at cranking speed and not at 1,800 rpm. Diagnostics bills at $285 per hour with a one hour minimum credited against an authorized repair, and a compression and leak down check answers this quickly.

Monthly, run it at 80 percent for 30 minutes minimum, and record the date, hours, voltage and frequency in a notebook or a phone note. Four data points across a year turn a vague sense that it seems fine into a trend line, and a trend line catches a slowly failing regulator months before it fails completely.

Annually, oil and filter, air filter, spark plug on a gasoline or liquefied petroleum gas unit, and a fuel filter on a diesel. Inspect the exhaust system for the black oily residue that indicates wet stacking, and inspect the mounting isolators, which crack with age and let vibration into the coach structure. Generator service runs $250 to $8,500, and routine annual service sits at the bottom of that band while a carbon related teardown sits well up it.

A note on where this stops being a do it yourself item. Exhaust work, fuel system work and anything involving the generator compartment sealing belongs in a shop, because an exhaust leak into an enclosed compartment on a coach with people sleeping in it is the one failure mode in this whole article that is genuinely dangerous. We are in shop only, mechanical labor is posted at $260 per hour, and an RV systems estimate is $150 credited against an authorized repair.