Onan RV generators: what fails in Florida and what repairs cost

Onan is what is mounted in most motorhomes and a good many fifth wheels, and in Florida it fails in one dominant way: it sits. The seasonal use pattern here, run through the winter season from November to April and then parked from spring to fall in humid heat with fuel in the system, is close to the worst thing you can do to a small gasoline engine. Most November and December generator failures trace back to it.

The fuel problem, in detail

Modern pump gasoline is not a storage fluid, and ethanol gasoline in Florida is worse than most places. Ethanol is hygroscopic: it pulls water out of the air. In a carburetor bowl vented to air that sits above 90 percent relative humidity every night from May to October, the fuel takes on water until the ethanol and water drop out of the gasoline as a separate layer at the bottom of the bowl. That layer sits on the main jet. Above it, the light fractions of the gasoline evaporate and leave a varnish that sets in the passages. Those passages are measured in thousandths of an inch.

The water layer also corrodes the aluminum of the bowl and the jets from the inside, which is why a carburetor from a Florida rig often needs replacing rather than cleaning where a desert one would clean up.

So the Onan in your bay either cranks without catching, catches and quits, or keeps running while it hunts up and down. All three are the same underlying fault and all three are fixed the same way: the carburetor comes off, gets cleaned or replaced, and the fuel system gets checked back to the tank.

What things cost

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

What we find, most common first

1. Fuel system, far ahead of everything else, because of the moisture story above.
2. Corrosion on connectors and grounds. The generator’s control wiring and the remote start harness live in a compartment that is damp for half the year. Green connectors and a poor chassis ground produce start faults, fault codes, and no-output complaints that have nothing wrong with the engine or the windings.
3. Plugs and air filters, neglected because the generator is out of sight. A paper air filter that has spent a summer in humid air is soft and partly collapsed.
4. Oil, due by running hours on a generator, and on plenty of the rigs we see it has simply never been changed. Generator hour meters exist for a reason.
5. No AC output even though the engine sounds healthy. That points us at the electrical end: brushes, the capacitor, the voltage regulator, or the unit’s own breaker having tripped. The voltage regulator is a circuit board, and a nearby lightning strike can take it while the generator is sitting idle in the bay. On the bright side, the engine itself is usually sound when this is the fault.
6. Starting and control faults, the remote start panel and the wiring that feeds it among them.
7. Exhaust, which is a safety item rather than a performance one and is covered below. Exhaust systems rust faster here than anywhere dry.

The habit that prevents nearly all of it

Give it two hours under load every month. Idling it, or a five-minute run, does not count. Under load, meaning with an air conditioner running, long enough to get everything hot.

Owners who do this essentially never end up calling about a generator that will not start. It keeps fresh fuel moving through the bowl, burns off the moisture, circulates oil, dries the windings, and exercises the start circuit. It costs nothing, anyone can do it, and of everything we tell RV owners in the Zephyrhills parks, it does the most good.

If the rig genuinely has to sit from April to November while you are north, the next best options are treating the fuel with a stabilizer that is made for ethanol and running it through, or shutting off the fuel and running the bowl dry where the unit allows it. Running the bowl dry is the better answer here, because a dry bowl cannot take on water.

Heat, humidity, and honest expectations

Generators are rated under standard conditions, and output falls as temperature and humidity rise, because moist hot air carries less oxygen per cubic foot than cool dry air. On a 90-degree August afternoon in Zephyrhills with the dew point in the 70s your unit is not making its nameplate number. If it has always struggled to carry two rooftop air conditioners in summer, 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. A lot of owners who think the generator is too small for the rig fix it this way for less than any repair would cost.

The safety paragraph

Two things that should come up on any Onan call:

Exhaust integrity. A leaking generator exhaust on a motorhome is a carbon monoxide risk, and these systems rust through in a humid climate well before the engine is tired. If we find one, we tell you, and our advice is to leave the generator off until it is fixed.

Your CO alarm. Press the test button and read the expiry date on the back; these alarms age out. Of everything on this page, that free check matters most.

The generator repair service page covers the full scope of on-site generator work across Zephyrhills, Dade City, Wesley Chapel, San Antonio, Land O’ Lakes, Lutz, and Plant City. If you are arriving in November to a generator that sat since April, the arrival checklist is worth ten minutes before you press the start button.