Why RV Generators Fail and How to Prevent It
Most generator failures are not mechanical failures. They are storage failures, and one habit prevents nearly all of them.
By Marcus Delgado, Shop Foreman and Lead Estimator
About 10 min left
Every spring our shop fills with generators that will not start, and nearly all of them failed the same two ways. Neither is a mechanical failure in any meaningful sense. Both are storage failures.
That is genuinely good news, because it means the majority of generator repair bills are preventable by one habit that takes two hours a month.
Failure One: Fuel Varnish
Gasoline degrades in storage. The light fractions evaporate, and what remains behind is a varnish that blocks the small passages inside a carburetor.
The main jet and the idle circuit are small enough that a modest amount of varnish blocks them entirely. The result is a generator that cranks strongly, may fire briefly if you prime it, and will not run.
Owners frequently interpret this as a dead generator. It is a generator that was left with fuel in it for eight months.
Cleaning is often successful. On units that have sat for years the gaskets and diaphragms have hardened alongside the varnish, and carburetor replacement becomes the better answer, because a cleaned carburetor with brittle diaphragms fails again quickly.
Diesel units suffer differently. Their problems are injector and lift pump related, plus algae growth in tanks that have sat with fuel in them, which is more common in a humid coastal environment than owners expect.
Failure Two: Wet Stacking
This one is less understood and does more permanent damage.
Generators are designed to work. Running one under little or no load means the engine never reaches proper operating temperature and combustion stays incomplete. Unburned fuel accumulates in the exhaust system and glazes the cylinder walls.
The exhaust residue is the visible symptom, which is where the name comes from. The cylinder glazing is the expensive part, because a glazed bore no longer seals properly against the rings and compression falls.
Owners produce this by starting the generator monthly with nothing switched on, which is exactly the advice they were given and exactly the wrong version of it.
The One Habit That Prevents Both
Run the generator under a real load for two hours monthly.
Real load means switching things on. Both air conditioners is ideal. An air conditioner plus the microwave plus a space heater works. What does not work is starting it, letting it idle at no load for ten minutes, and shutting it off, which is worse than not running it at all because it adds to the wet stacking without addressing the fuel.
For storage longer than a couple of months, either run the carburetor dry or treat the fuel with a stabilizer. Both work. Doing neither is what fills our shop every spring.
A Generator That Starts Is Not Necessarily Healthy
This matters more than most owners realize.
Many faults only appear under load: weak output, voltage sag, frequency drift, overheating shutdown. A generator that starts fine in your driveway can fault the moment two air conditioners come on at a campsite in July, which is precisely when you need it.
We load test rather than confirming it starts. Running a unit to rated output while monitoring voltage and frequency is what catches the problems that a start test misses entirely.
If a shop tells you your generator is fine because it started, that was not a diagnosis.
The Compartment Nobody Opens
Generator compartments accumulate debris, and rodent nesting in a generator enclosure is extremely common on stored coaches.
Both restrict cooling airflow, and the resulting overheating shutdown gets misdiagnosed as an electrical fault because the unit stops without warning and then works again once it cools.
While the compartment is open, the exhaust system needs inspection. Exhaust corrodes, and an exhaust leak inside an enclosed compartment on a vehicle people sleep in is a carbon monoxide hazard rather than a maintenance item. We inspect the full exhaust path on every generator service, whether or not that was the complaint.
Diagnostic Order
| Step | What It Rules Out |
|---|---|
| Fuel level and shutoff valve position | The embarrassing causes |
| Battery voltage and start circuit | Cranking problems mistaken for running problems |
| Compression test | Wet stacking damage and worn rings |
| Fuel system inspection | Varnish, filters, tank condition |
| Load test at rated output | Faults invisible at no load |
| Cooling airflow and exhaust | Overheat shutdowns and CO hazard |
| Control board and harness | Electrical faults, checked last |
Compression sits early in that list deliberately. It determines whether the rest of the diagnosis is worth performing, because there is no point cleaning a carburetor on a unit whose bores are glazed past recovery.
Overhaul Versus Replacement
On a high hour unit with low compression, overhaul and replacement land close enough in cost that it becomes a judgment call about the rest of the coach.
We measure compression and evaluate the control board and harness condition before recommending either. Pouring an overhaul into a unit with a failing board and corroded wiring is how owners end up paying for the job twice.
On older Onan units the other consideration is parts. Some components have been discontinued and require sourcing from rebuilders rather than the manufacturer, which affects both cost and timeline, and we check that during the estimate rather than discovering it mid job.
Cost Expectations
Fuel system service with carburetor cleaning runs $400 to $900.
Component replacement, meaning a control board, starter, fuel pump or ignition component, runs $900 to $2,500 depending on parts availability.
A full overhaul runs $3,500 to $8,500.
Replacement with a new unit runs $5,500 to $12,000 installed depending on size and whether the compartment needs modification.
Timeline
A fuel system service is one to two days. Component replacement is two to four days depending on parts. A full overhaul is one to three weeks.
The most common delay is parts on older units, which is worth knowing if you are planning around a trip.
What to Do This Month
Go start your generator and put a real load on it for two hours. If it starts and carries the load, you have just prevented the two most common failures for another month.
If it does not start, or starts and will not carry load, you have found out now rather than at a campsite three hundred miles from anywhere.
That is the entire maintenance program. It is not complicated and almost nobody does it.
Written by Marcus Delgado, Shop Foreman and Lead Estimator at OCRV Center, 23281 La Palma Ave, Yorba Linda, CA 92887. This is general information rather than a diagnosis of your specific vehicle. Call (949) 799-3387 to discuss yours.
Get Your Vehicle Looked At Properly
Call the shop, describe what happened, and we will tell you what the realistic scope and timeline look like. All work is performed in shop at our 35,000 square foot facility.