What an Onan fuel filter does and why it matters
Your Onan generator's fuel filter traps dirt, rust, and water before they reach the engine's fuel injectors and carburetor. Over time, the filter clogs with debris, forcing your engine to work harder to pull fuel through. A clogged filter can cause hard starting, rough idling, loss of power under load, or complete failure to run. Replacing it is one of the simplest maintenance tasks you can do yourself, and it costs far less than repairing fuel system damage.
Onan makes several generator models — portable units, RV generators, standby home generators — and each uses a different filter. The filter type depends on your specific model and year, not on the generator's size or power output. Finding the right part number before you buy prevents wasted trips and ensures the replacement fits correctly.
Key Takeaways
- Your Onan model and year determine which fuel filter you need; the part number is printed on the old filter or in your owner's manual.
- Fuel filters on Onan generators are located either in the fuel line between the tank and engine, or inside the fuel tank itself, depending on the model.
- Replacing an in-line filter takes 10 to 20 minutes and requires only a wrench or hose clamp tool; tank-mounted filters require draining fuel first.
- You should replace the fuel filter every 100 to 200 operating hours, or annually if the generator sits unused for months.
Locating your Onan generator's fuel filter
Onan generators use one of two fuel filter designs. The first is an in-line filter mounted in the rubber fuel line between the tank and the engine. You will see it as a small cylindrical or cartridge-shaped component with fuel lines connected to both ends. This type is the easiest to replace because you do not have to drain the tank.
The second design is a tank-mounted filter, which sits inside or on top of the fuel tank itself. This style is common on older Onan models and some RV generators. To replace it, you must first drain or siphon fuel from the tank to prevent spilling. Check your owner's manual or look at the generator itself to determine which type yours has. If you cannot find the manual, the part number stamped or printed on the existing filter will tell you which replacement to order.
Finding the correct part number
The easiest way to get the right filter is to look at the old one. Most Onan fuel filters have the part number printed directly on the housing. Write down that number exactly as it appears, including any letters or dashes. If the number is worn away or unreadable, check your owner's manual — it lists the fuel filter part number in the maintenance or parts section.
If you do not have the manual, you can find the part number by knowing your generator's model and serial number. These are stamped on a nameplate attached to the frame or engine. Once you have the model and serial number, contact an Onan dealer or search the manufacturer's parts catalog online. Onan is now owned by Cummins, and parts information is available through Cummins Power Generation. Providing the model and serial number takes the guesswork out of ordering.
Replacing an in-line fuel filter
Before you start, turn off the generator and let it cool for at least 10 minutes. Fuel is flammable, so work outdoors or in a well-ventilated space away from ignition sources.
Locate the fuel line connections on both ends of the filter. Some filters use barbed fittings that slide into rubber hose; others use threaded connections with small nuts. For barbed fittings, loosen the hose clamps on each end with a screwdriver or hose clamp tool, then gently twist and pull the hose away from the filter. For threaded connections, use a wrench to hold the filter body steady while you unscrew the fuel line nut on each side.
Once both lines are disconnected, remove the old filter. Some fuel will drip out — place a small container or rag underneath to catch it. Install the new filter by reversing the process: push the hose onto the barbed fittings and tighten the clamps, or screw the fuel lines back onto the threaded connections and tighten firmly with a wrench. Make sure the clamps or connections are snug but not over-tightened, which can crack plastic fittings.
After installation, turn on the fuel valve (if your generator has one) and check for leaks around both connections. Run the generator for a few minutes to confirm it starts and runs normally. If you see fuel dripping, turn off the generator when ready and tighten the connection further.
Replacing a tank-mounted fuel filter
Tank-mounted filters require more preparation because you must drain fuel first. Turn off the generator and let it cool completely. Locate the fuel shutoff valve on the fuel line between the tank and engine, and turn it to the off position. If there is no shutoff valve, you will need to siphon fuel from the tank using a hand pump or gravity drain.
Once the tank is empty or nearly empty, locate the filter on or inside the tank. Some models have a filter bowl that screws onto the outside of the tank; others have a filter cartridge that sits inside. For external bowl filters, use a wrench to unscrew the bowl counterclockwise, then remove the old filter element from inside. Install the new element, explore a thin coat of fresh fuel to the rubber gasket on the new filter, and screw the bowl back on hand-tight, then snug it with a wrench.
For internal tank filters, you may need to remove the fuel cap and reach inside, or disconnect the fuel line and pull the filter assembly out. Follow the steps in your owner's manual for your specific model, as the exact procedure varies. Once the new filter is in place, refill the tank with fresh fuel, turn the shutoff valve back on, and run the generator to check for leaks and proper operation.
Fuel filter maintenance schedule and tips
Replace your Onan fuel filter every 100 to 200 operating hours of use. If you run the generator regularly, that may mean once or twice a year. If the generator sits idle for months at a time, replace the filter at least once a year before you start it up again, because fuel degrades and deposits build up in the filter even when the engine is not running.
Always use fresh fuel in your generator. Stale or contaminated fuel clogs filters faster and damages the fuel system. If your generator will sit unused for more than a month, drain the fuel tank completely or add a fuel stabilizer to prevent gum and varnish from forming. When you restart the generator after a long idle period, the fuel filter will have already absorbed some of the degraded fuel, so replacing it first prevents starting problems and rough running.
Keep a spare fuel filter on hand if you rely on your generator for backup power. A clogged filter can leave you without power at the worst time, and having a replacement ready means you can fix the problem in minutes rather than hours.
Frequently Asked Questions
How do I know if my fuel filter is clogged?
A clogged filter causes the generator to start hard, run rough at idle, lose power under load, or shut down unexpectedly. If the generator ran fine last time and now struggles to start or stalls when you try to use it, a dirty fuel filter is often the culprit. The only way to know for certain is to look at the filter itself — if it is dark or discolored inside, it needs replacement.
Can I clean and reuse my fuel filter instead of replacing it?
Most Onan fuel filters are not designed to be cleaned and reused. Attempting to rinse or blow out a filter can damage the internal media and leave particles behind. Replacement filters are inexpensive — usually between $15 and $40 — and a new filter guarantees clean fuel flow. It is worth the cost to avoid engine damage.
What type of fuel should I use in my Onan generator?
Use clean, fresh gasoline or diesel fuel depending on your generator model. Onan makes both gasoline and diesel units. Avoid fuel that has been stored for more than 30 days without a stabilizer, as it breaks down and leaves deposits that clog filters. If you are unsure which fuel type your generator uses, check the fuel cap or owner's manual.
Do I need to drain the entire fuel tank to replace an in-line filter?
No. In-line filters can be replaced without draining the tank because they are mounted in the fuel line away from the tank. You may lose a small amount of fuel when you disconnect the lines, but this is normal and manageable with a rag or small container underneath.
Where can I buy an Onan fuel filter?
Onan fuel filters are sold through Cummins Power Generation dealers, online parts retailers, and some hardware or outdoor equipment stores. Having your model number and serial number ready makes ordering faster and ensures you get the correct filter for your specific generator.