Onan generators: what fails first, what it costs, and how to keep one ready for storm season

Onan is what is mounted in most motorhomes and a good many fifth wheels, and in Highlands County it fails in one dominant way: it sits. The use pattern here, parked on a resort site or in a storage lot from April to November with fuel in the system, through the hottest and wettest months of the year, is close to the worst thing you can do to a small gasoline engine. Almost every November generator call we take traces back to it, and so does every “it would not start the night the power went out” call after a storm.

The fuel problem, in detail

Modern pump gasoline is not a storage fluid. In a carburetor bowl at the temperature of a closed generator compartment in July, the volatile fractions evaporate and leave behind a sticky varnish that sets in the jets and the small passages. Those passages are measured in thousandths of an inch. Ethanol content makes it worse in a second way, and worse here than in a dry climate: it draws moisture out of 79-percent-humidity air, and that water corrodes the aluminum carburetor body from the inside.

The result is a generator that cranks fine and will not fire, or fires and dies, or runs and surges. All three are the same underlying fault and all three are fixed the same way: the carburetor comes off, gets cleaned or rebuilt, and the fuel system gets checked back to the tank.

What things cost

Same approach as everything on our pricing page: diagnose, quote, get your approval, then work.

The failure order on these units

1. Fuel system, by a wide margin, for the reasons above.
2. Plugs and air filters, neglected because the generator is out of sight, and an air filter in this humidity holds moisture.
3. Oil, which on a generator is measured in running hours and is routinely never changed at all. Generator hour meters exist for a reason.
4. No AC output with the engine running happily. This is the electrical side: brushes, capacitor, voltage regulator, or a tripped breaker on the unit itself. The good news is that it means the engine is fine.
5. Control boards and regulators after lightning. Florida leads the country in lightning density, and the generator’s control board is on the rig’s 12V system, downstream of the converter. A pedestal surge that kills the converter can reach it.
6. Starting and control faults, including the remote start panel and its wiring, and mud daubers in the compartment.
7. Exhaust, which is a safety item rather than a performance one and is covered below.

Storm season is why this matters here

Highlands County has lost power almost countywide twice in recent memory. Hurricane Irma in September 2017 left 90 percent of the county without power at its peak, and the county was the second-hardest-hit in Florida. Hurricane Ian on the evening of September 28, 2022 brought gusts of 66 to 72 mph across the county and a near-total outage for at least 24 hours, with most customers back by the following Sunday. Both times, the generator that had been sitting since April with old fuel in the bowl was the generator that did not start.

If you are in the rig through hurricane season, the generator is not a convenience. A full service in May or June, with fresh fuel run through it and a real load test, is the difference between a cool rig with a working fridge for three days and a very long weekend. The generator repair service page covers the full scope.

The habit that prevents nearly all of it

Run it for two hours a month under load. Not at idle, not for five minutes. Under load, meaning with an air conditioner running, long enough to get everything hot.

Owners who do this essentially never call us about a generator that will not start. It keeps fresh fuel in the bowl, keeps the carburetor wet, circulates oil, dries the windings, and exercises the start circuit. It is free, it takes no skill, and it is the most effective maintenance advice on this entire site.

If the rig genuinely has to sit through the summer, the next best options are treating the fuel with a stabilizer and running it through, or shutting off the fuel and running the bowl dry where the unit allows it.

Heat, humidity, and honest expectations

Generators are rated under standard conditions, and output falls as temperature and humidity rise. On a 92-degree, saturated Highlands County afternoon your unit is not making its nameplate number. If it has always struggled to carry two rooftop air conditioners in July, that may be physics rather than a fault, and we will say so instead of selling you a rebuild.

The practical fixes for that are cheap: stagger the AC starts rather than letting both compressors hit at once, and consider soft-start devices on the air conditioners, which dramatically cut inrush current. Many “my generator cannot handle my rig” complaints are solved for a fraction of any repair.

The safety paragraph

Two things we will always raise:

Exhaust integrity. A leaking generator exhaust on a motorhome is a carbon monoxide risk, and these systems live in a harsh, wet environment underneath a vehicle where steel rusts fast. If we find one, we will tell you and we will recommend not running the unit until it is repaired.

Your CO alarm. Check that it works and check its expiry date, because they do expire. That is free advice and it is the most important sentence on this page.

We service generators on site across Sebring, Avon Park, Lake Placid, the Lorida fish camps, Frostproof, Wauchula, and Okeechobee.