Your Onan 5500 generator sputters to life during a weekend camping trip only to stall under load, or worse—it won’t start when you need it most. Before panic sets in or you call for costly service, know this: failing spark plugs cause 60% of Onan 5500 starting issues. Replacing both plugs takes under an hour and costs less than $20, potentially saving you hundreds in avoidable repairs. This guide cuts through the confusion with precise instructions for accessing, removing, and installing plugs on your specific generator model—whether it’s mounted in an RV, trailer, or standalone unit. You’ll learn critical torque specs, gap verification tricks, and how to decode what your old plugs reveal about engine health.
Worn spark plugs disrupt combustion efficiency, leading to hard starts, rough idling, and even complete failure. The Onan 5500’s V-twin design hides one plug deep in the engine bay, creating unique access challenges most generic guides ignore. By following this how to change spark plugs on Onan 5500 generator process, you’ll restore reliable power for emergencies, tailgating, or off-grid adventures. Let’s get your generator running like new.
Safety First: Cool Down and Disconnect
Skipping this step risks severe burns or fire—never work on a hot generator. Allow 30-45 minutes for complete cooling after shutdown. While waiting, disconnect the negative battery cable using a 10mm wrench to prevent accidental starts. Next, turn off the fuel supply at the petcock valve to eliminate fire hazards near spark plug ports. Confirm you’re working on level ground in a ventilated space with safety glasses secured.
Critical safety sequence: Cool engine → Disconnect battery → Shut off fuel → Put on eye protection. Rushing this jeopardizes everything—aluminum engine components retain heat far longer than they appear. If you smell fuel or see residue around plugs, wipe it immediately with a shop rag before proceeding.
Tools and Parts Checklist
Required Equipment
- 13/16″ deep spark plug socket (3/8″ drive essential for rear plug access)
- 6-8 inch extension bar (non-negotiable for rear cylinder reach)
- Precision torque wrench (10-20 ft-lbs range—aluminum heads strip easily)
- Champion RN9YC spark plugs (exact OEM specification—never substitute)
- 0.030″ feeler gauge (gap verification is mandatory)
- Anti-seize compound (tiny dab on threads only—avoid electrode contact)
- Dielectric grease (prevents boot corrosion and eases future removal)
Optional but Helpful
A spark plug boot puller saves cracked wires during removal. A small mirror taped to a stick reveals the hidden rear plug. A universal joint attachment conquers tight angles where a straight socket won’t fit. Keep compressed air nearby to blow debris from plug wells before removal—dirt falling into cylinders causes catastrophic damage.
Locate Hidden Spark Plugs
Your generator’s installation type dictates access:
– RV-mounted units: Remove the passenger-side maintenance panel (typically 4-6 Phillips screws)
– Standalone models: Pop off the top shroud with quarter-turn fasteners
– Enclosed systems: Partially remove the lower enclosure near the engine block
The front plug (cylinder #1) sits openly near the generator’s front edge. The rear plug (cylinder #2) nests deep behind the carburetor—often requiring you to snake your socket past fuel lines. Both feature distinctive 13/16″ hex heads protruding from the cylinder heads. Shine your flashlight at a 45-degree angle to spot them; the rear plug’s position varies slightly between Onan 5500 models manufactured before or after 2015.
Remove Old Spark Plugs Safely
Access the Plugs
After panel removal, blast compressed air around each plug well for 5 seconds. This dislodges carbon grit that could fall into cylinders during removal. Identify the thick black spark plug wires capped by rubber boots—they’ll be coated in fine oil residue from normal operation.
Wire Removal Technique
Always grip the boot—not the wire—to avoid internal strand damage. Twist the boot 90 degrees clockwise, then counterclockwise to break its seal. Pull straight outward with firm, steady pressure until it releases with a pop. Inspect wires for cracks or carbon tracking (black “roadmap” lines); replace damaged wires immediately—they’re cheap but critical.
Unscrew Old Plugs
Seat your 13/16″ socket fully onto the plug hex. Attach the extension and ratchet, then turn counterclockwise with slow, even force. Initial resistance is normal—aluminum heads grip tightly. Once loose, unscrew completely by hand to avoid dropping the plug into the cylinder well. Never force a stuck plug; if it binds after 1/4 turn, reseat your socket to prevent rounding the hex. Repeat for the second plug, labeling wires as “front” and “rear” with tape.
Inspect Old Plugs for Clues
Your old plugs diagnose engine health:
– Normal wear: Uniform tan/brown electrode coating (ideal combustion)
– Rich mixture: Black, sooty deposits (indicates fuel system issues)
– Lean condition: White, blistered porcelain (overheating risk)
– Oil fouling: Wet, glossy black residue (sign of piston ring failure)
Measure the gap with your feeler gauge—0.030″ is non-negotiable for the Onan 5500. If the gap exceeds 0.035″ or electrodes appear eroded, replacement is urgent. Document findings: Two oil-fouled plugs suggest major engine work, while one fouled plug points to a single-cylinder issue.
Install New Plugs Correctly
Gap and Prep New Plugs
Verify the Champion RN9YC gap is exactly 0.030″ using your feeler gauge. Bend the ground electrode gently with pliers if needed—excessive force cracks the porcelain. Apply anti-seize compound to the first 3 threads only; excess near the electrode causes misfires.
Install with Precision
Hand-thread each plug for 2 full turns—this prevents cross-threading in soft aluminum heads. Once seated, use your torque wrench for the final turn: 15 ft-lbs maximum (aluminum heads strip above 20 ft-lbs). Over-torquing is the #1 DIY mistake—when in doubt, under-torque and recheck after 10 running hours.
Connect Wires Properly
Coat the inside of each boot with dielectric grease. Push firmly onto the new plug until you hear a distinct click. Test security with a gentle tug—boots must stay seated. Route wires away from exhaust manifolds; a single contact point can melt insulation and cause shorts.
Test and Verify Performance
Reconnect the battery and fuel supply. Start the generator and let it idle for 3 minutes while listening for smooth combustion. Critical verification steps:
– Check voltage output (should stabilize at 120V ±5%)
– Apply 50% load (e.g., space heater + microwave) for 15 minutes
– Confirm no misfires under load (uneven “puttering” sounds)
– Retest plug connections after shutdown—heat cycles can loosen boots
If performance dips under load, immediately shut down and recheck wire seating. A single loose connection causes 90% of post-replacement issues.
Troubleshoot Common Problems
Hard Starting Issues
Re-measure the gap—0.030″ is absolute for sea-level operation. Verify wires are fully seated with a click sound. If using non-OEM plugs, swap back to Champion RN9YC; heat range mismatches cause hard starts.
Rough Running or Misfires
Never swap front/rear wires—cylinders fire sequentially. Inspect wires for nicks from sharp edges during reinstallation. At altitudes above 5,000 feet, increase the gap to 0.032″ per Onan specs. Persistent misfires after correction require a compression test.
Access Challenges
For the rear plug: Attach a universal joint to your extension, then use a mirror to guide the socket onto the hex. If the plug resists removal, apply penetrating oil and wait 10 minutes—never hammer the socket. Seized plugs demand professional extraction to avoid head damage.
Maintenance Schedule and Specifications
Replace plugs every 200 hours for standard RV use, or annually if storage exceeds 6 months. In dusty environments (construction sites, desert camping), cut intervals to 75 hours. Always use Champion RN9YC or exact equivalents: NGK BP9ES, Autolite 3923, or Bosch WR9DC. Gap tolerance is critical: 0.030″ at sea level, 0.032″ above 5,000 feet—never exceed 0.035″.
Pro Tips for Long-Term Success
Before long-term storage, remove plugs and spray fogging oil into cylinders. Reinstall plugs hand-tight only; torque them properly before next use. For high-altitude operation, install one-step colder plugs (Champion RN8YC) to prevent pre-ignition. Never use iridium plugs—their fine electrodes can’t handle the Onan 5500’s combustion pressures.
Key takeaway: Mastering how to change spark plugs on an Onan 5500 generator transforms you from a stranded camper into a self-reliant troubleshooter. Mark your maintenance log for the next replacement at 200 hours, and keep Champion RN9YC plugs in your emergency kit. This simple 45-minute ritual ensures your generator delivers power when the grid fails—no service calls, no guesswork, just reliable energy on demand.
