Onan 5500 Generator Fuel Pump: Essential Guide to Troubleshooting, Replacement & Maintenance

A failing or faulty fuel pump is one of the most common causes of operational problems in the Onan 5500 generator series (including models like the Microlite 5500, Marquis 5500, and RV QG 5500). Understanding how the fuel pump works, recognizing the signs of failure, knowing your replacement options, and performing proper maintenance are crucial for reliable generator power.

This comprehensive guide dives deep into everything you need to know about the Onan 5500 generator fuel pump. Whether your generator won't start, runs erratically, or you want to prevent future breakdowns, this information will empower you to diagnose issues accurately and get your critical power source running smoothly again.

Understanding the Role of the Onan 5500 Fuel Pump

The fuel pump is a vital component within your Onan 5500 generator's fuel system. Its sole job is to deliver a consistent and adequate supply of gasoline from the main fuel tank to the generator's carburetor. Generators like the Onan 5500 series require fuel to be delivered at a specific pressure, typically ranging between 4 to 7 pounds per square inch (PSI). The pump ensures this pressurized fuel flow occurs reliably. Unlike fuel injection systems found in many modern vehicles, the Onan 5500 typically uses a low-pressure, electrically driven fuel pump. This pump activates when the generator cranks, powered by the starter battery. Once the generator is running, the pump continues to receive power from the generator's own charging system. Consistent fuel pressure and volume are essential for proper carburetor function and overall engine operation.

Symptoms of a Failing Onan 5500 Fuel Pump

Recognizing the early warning signs of fuel pump trouble can save you from a sudden loss of power. Here are the most frequent symptoms indicating a potential issue with your Onan 5500 generator fuel pump:

1. Failure to Start (Cranks but No Start): This is often the most obvious sign. The generator cranks normally (you hear the engine trying to turn over), but it fails to start. Since the spark plug needs fuel to ignite, a failed pump delivering no fuel is a primary suspect. Check for spark and air intake blockages first to rule those out.
2. Hard Starting: The generator struggles to start, requiring extended cranking before it finally fires up. This can indicate the pump is weak or partially clogged, still delivering fuel but not quickly or at sufficient pressure.
3. Engine Sputtering and Surging (Unstable RPM): Once running, the engine may exhibit erratic behavior. It might idle roughly, surge (rev up and down unexpectedly), or sputter as if it's being starved of fuel. This points to inconsistent fuel delivery, often caused by a failing pump that can't maintain steady pressure.
4. Loss of Power Under Load: The generator starts and may idle acceptably, but when you apply an electrical load (turning on appliances or equipment), it bogs down, stumbles, or even shuts off. This happens because the increased fuel demand under load overwhelms a weak pump. The carburetor bowl empties faster than the failing pump can refill it.
5. Stalling During Operation: The generator might run for a short period and then unexpectedly die. After stopping, it could restart after a brief cool-down period, only to stall again later. This is a classic symptom of a fuel pump failing under heat or intermittent electrical issues within the pump itself.
6. No Audible Pump "Humming": When the generator attempts to start (during cranking), you should usually hear a distinct humming or buzzing sound near the pump (usually mounted on the generator frame, along a fuel line, or near the carburetor). If you place your fingers on the pump body during cranking, you might feel a faint vibration. A complete lack of sound or vibration during cranking strongly suggests the pump isn't getting power or has failed internally. Remember to verify the pump is actually receiving power before condemning it.
7. Reduced Running Time (Potential Pump Overwork): While less direct, if you notice significantly shorter running times than expected on a full tank without any new significant loads, it could indicate a problem. A weak pump might not be delivering fuel efficiently, forcing the engine to run richer (using more fuel) or struggle to maintain power. However, always rule out other causes like fuel leaks, significant load increases, or carburetor issues first.

Testing Your Onan 5500 Fuel Pump (Basic Diagnostics)

Before replacing the pump, performing some basic checks can save time and money:

1. Check Power Supply:
   • Locate the fuel pump. Trace the wires back to the electrical connector.
   • Using a multimeter set to DC Volts, check for voltage at the pump connector while the generator is cranking (attempting to start). You should typically see battery voltage (around 12V) during cranking. If there is no voltage during cranking, check fuses, wiring, connections, and the control board output to the pump circuit. If voltage is present during cranking but the pump doesn't activate, the pump itself is likely the problem.
2. Check for Fuel Blockages:
   • Inspect the fuel lines from the tank to the pump and from the pump to the carburetor for kinks, crushing, or obvious damage.
   • Check the fuel filter (if equipped – sometimes there's an in-line filter between the tank and pump, or a strainer in the carburetor inlet). Replace a clogged filter. If your setup lacks a filter before the pump, consider adding one.
   • Verify the fuel tank has adequate fuel and the tank vent is not blocked, preventing fuel flow to the pump.
   • Temporarily disconnect the fuel line from the pump outlet (line going to the carburetor). Place the end into a suitable container. With safety precautions (avoid sparks, have fire extinguisher handy), crank the generator. A functional pump should pulse or stream fuel into the container. Measure the volume over 10-15 seconds of cranking; refer to Onan specifications for exact flow rate (usually around 1 pint or ~0.47 liters per minute at cranking speed). Weak flow confirms a pump issue.
3. Check Fuel Pressure (Advanced - Requires Gauge):
   • While the most definitive pump test, it requires a low-pressure fuel pressure gauge kit suitable for carbureted engines (0-15 PSI range).
   • Locate a point to connect the gauge – often requires installing a "T" fitting between the pump outlet and the carburetor inlet, or sometimes a Schrader valve test port kit needs to be installed.
   • Crank or run the generator (if possible) and observe the pressure reading. Compare it to Onan specifications for the 5500 model (usually 4-7 PSI). Pressure significantly below this range indicates a failing pump.

Replacing the Onan 5500 Generator Fuel Pump

If testing confirms the pump is faulty, replacement is necessary. Here's a step-by-step guide:

1. Safety First:
   • Disconnect the generator's negative (-) battery cable.
   • Ensure the generator is cool. Work in a well-ventilated area away from sparks or flames. Have a fire extinguisher readily available.
   • Relieve any residual fuel pressure by carefully loosening a fuel line connection after placing rags beneath it.
2. Locate and Access the Pump: Find the pump mounting location. On many Onan 5500 generators, the pump is mounted on the generator frame, often near the carburetor or along the fuel line route. Remove any covers or components necessary for safe access.
3. Disconnect Fuel Lines and Wiring:
   • Identify the inlet (from tank/filter) and outlet (to carburetor) lines. Carefully clamp off each line near its connection point to minimize fuel spillage if possible (small pinch-off pliers designed for fuel line work best). Use absorbent rags liberally.
   • Use appropriate wrenches or fuel line disconnect tools (if quick-connect fittings) to disconnect both fuel lines. Cap or plug the open ends of the lines to prevent debris entry and leakage.
   • Disconnect the electrical connector from the pump.
4. Remove the Old Pump: Unscrew the mounting bolts or screws securing the pump bracket or the pump itself to the mounting surface. Remove the old pump. Pay attention to how any mounting bracket is oriented.
5. Prepare the New Pump: Compare the new pump to the old one. Ensure the inlet/outlet ports are identical and facing the correct direction. Some pumps may require an installation kit with gaskets or spacers; use them if provided.
6. Install the New Pump:
   • Position the new pump (and bracket, if applicable) exactly as the old one was mounted. Ensure any rubber isolating mounts are correctly placed.
   • Reinstall the mounting bolts or screws. Tighten them securely but do not overtighten, especially if mounting through an isolating grommet/bushing.
   • Reconnect the electrical connector to the new pump.
7. Reconnect Fuel Lines:
   • Remove any clamps or caps from the fuel lines.
   • Reconnect the inlet fuel line (from tank/filter) to the designated port on the pump. Reconnect the outlet fuel line (to carburetor) to the pump. Ensure connections are tight to prevent air leaks, which can cause starting and running issues. Use new hose clamps if the old ones are compromised. Ensure the fuel flow direction (In -> Out) is correct (pump bodies usually have arrows).
8. Final Checks and Testing:
   • Double-check all connections are secure – fuel lines and wiring.
   • Reconnect the generator's negative (-) battery cable.
   • Before attempting to start, open the fuel shut-off valve if your system has one located between the tank and pump.
   • If fuel spilled during the process, ensure it's fully cleaned and evaporated before proceeding.
   • Turn on the generator's control switch to the "Run" or "Start" position and crank the engine. Listen for the new pump's humming sound. Observe for fuel leaks around connections immediately.
   • If the generator starts and runs smoothly without leaks, the repair was successful. Let it run under load to ensure stable operation.

Choosing a Replacement Onan 5500 Fuel Pump

You have several options when selecting a replacement pump:

1. Genuine Onan OEM Replacement Pump: These pumps are designed and manufactured specifically for Onan generators. They come packaged in Onan/Cummins boxes and carry the Onan part number (e.g., 167-0322, 167-0259 – verify the exact number for your specific 5500 model as design changes occur). Advantages include guaranteed fit and performance, meeting original specifications, and backed by Onan. Disadvantages are typically a higher cost.
2. High-Quality Aftermarket Pumps: Several reputable manufacturers produce fuel pumps specifically compatible with the Onan 5500 series. These pumps are designed as direct replacements, meeting the required pressure and flow specifications. Popular brands include Facet, Airtex, Carter, and others. Advantages include significantly lower cost than OEM while often offering comparable quality and reliability. It's crucial to purchase from a reputable supplier to avoid cheap knock-offs. Ensure the pump is listed for the Onan 5500.
3. Universal Electric Fuel Pumps: While technically possible to adapt a generic universal pump, this is generally not recommended for the Onan 5500 generator. Universal pumps require careful selection of pressure rating and flow rate, custom fuel line modifications, specific electrical wiring, proper mounting, and vibration isolation. The risk of incorrect pressure (too high can force fuel past the carburetor needle valve and flood the engine, too low causes starvation), improper mounting leading to premature failure, or electrical issues outweighs the minimal cost savings compared to a known direct-fit pump for most users.

When choosing, ensure the pump matches:

• Operating Voltage: 12V DC.
• Flow Rate: Typically around 1 pint (~0.47 liters) per minute at cranking speed (check spec for your model).
• Pressure: 4-7 PSI (check spec for your model).
• Connector Style: Inlet/Outlet size and type (barbed fittings for rubber hose are common, often 1/4" or 5/16").
• Mounting Configuration: Direct bolt pattern or bracket compatibility.

Maintaining Your Onan 5500 Fuel Pump for Longevity

Preventative maintenance can significantly extend the life of your fuel pump and generator:

1. Use Clean, Fresh Fuel: Stale fuel breaks down, forming varnish and gum that can clog the pump inlet screen, internal valves, and carburetor jets. Use fuel stabilized specifically for ethanol-blended gasoline, especially if the generator sits for long periods. Never store the generator with untreated fuel in the tank/carburetor. Drain fuel if storing for more than a few months.
2. Regularly Exercise the Generator: Run your Onan 5500 generator under a substantial load (around 50-75% of rated output) for at least 30 minutes every month (or according to the manufacturer's recommendation). This keeps the pump moving, prevents seal drying, and burns off fuel residue throughout the system.
3. Protect the Pump from Debris: Ensure any pre-pump fuel filter is used and maintained. Replace inline fuel filters according to the maintenance schedule (often annually or every 100-200 hours). A filter prevents contaminants in the fuel tank from entering and damaging the pump internals.
4. Inspect Fuel Lines and Connections: Periodically check fuel lines from tank to pump to carburetor for brittleness, cracking, swelling, kinking, or leaks. Replace damaged lines immediately. Ensure all hose clamps are tight and secure. Inspect the pump wiring connector for corrosion or damage.
5. Avoid Running the Tank Dry: While the fuel pump won't necessarily fail instantly if the tank runs dry, it does cause the pump to work without lubricating/cooling fuel flowing through it. This can cause unnecessary wear and heat buildup. Keep the tank reasonably full, especially during extended operation.

Cost Considerations for Replacement

Costs vary depending on the source and pump type:

• Genuine Onan OEM Pump: Typically ranges from $100to$250+.
• Quality Aftermarket Direct Replacement Pump: Usually ranges from $30to$80.
• Labor Cost: If hiring a technician, expect 0.5 to 1.5 hours of labor, depending on generator location and accessibility in your RV or installation. Rates vary ($80-$150+ per hour). DIY replacement saves this cost.

When to Seek Professional Help

While replacing a fuel pump on an Onan 5500 is generally considered a moderate DIY task for those comfortable with basic tools and following safety procedures, consider professional assistance if:

• You are uncomfortable with fuel system work or electrical troubleshooting.
• Testing suggests a problem but you cannot definitively isolate the fuel pump as the culprit (could be electrical, control board, carburetor, ignition issue).
• The pump location is extremely difficult to access in your specific installation (e.g., complex RV compartment).
• After replacement, problems persist, indicating a deeper issue.

Conclusion

A functional fuel pump is non-negotiable for the reliable operation of your Onan 5500 generator. By understanding the symptoms of failure (failure to start, sputtering, surging, power loss under load), performing basic diagnostic tests (power check, flow check), choosing the right replacement (OEM or quality aftermarket), and following the replacement procedure carefully, you can effectively resolve this common issue. Implementing preventative maintenance habits – using clean stabilized fuel, exercising the generator regularly, and replacing filters – will maximize the life of your new pump and ensure your Onan 5500 generator is ready to deliver dependable power whenever you need it most. Address fuel pump problems promptly to avoid inconvenient and potentially costly downtime or further damage to other components.