Tag: QG 5500 RV (EVAP)

Understanding the Problem

The Cummins Onan QG 5500 RV (EVAP) is a solid workhorse for RV and mobile applications, but it wasn’t designed to automatically adjust itself for every elevation change. When you climb above 5,000 feet, the air becomes thinner—there’s less oxygen per cubic inch. Your generator’s carburetor and ignition system were tuned at sea level, so at altitude they deliver too much fuel relative to available air. The result: rough idle, hesitation under load, black smoke, fouled spark plugs, and wasted fuel.

The good news is that altitude-related rough running is almost always fixable with basic adjustments and maintenance. You don’t need a dealer visit in most cases.

At-a-Glance: Most Likely Causes

Cause	Likelihood	Typical Cost to Fix
Carburetor not adjusted for elevation	Very Common	$0–$50 (DIY adjustment)
Air filter too restrictive or clogged	Very Common	$15–$40 (replacement)
Oxygen sensor reading incorrect	Common	$150–$300 (sensor + labor)
Fuel mixture too rich above 5,000 feet	Very Common	$0–$80 (carburetor rebuild kit)
Ignition timing needs altitude compensation	Occasional	$50–$150 (timing adjustment)
Spark plug fouled or worn	Common	$10–$25 (spark plug)

Diagnostic Walkthrough

Work through these steps in order. Stop when you find and fix the problem.

1. Check and clean the air filter. A clogged or overly restrictive filter makes rough running worse at altitude. Remove the air filter cover (usually two or three bolts), inspect the filter element, and either clean it with compressed air or replace it. At elevation, airflow is already compromised; a dirty filter adds insult to injury. This is the cheapest fix and often solves the problem on its own.
2. Inspect and replace the spark plug. Rich fuel mixtures foul spark plugs faster, especially at altitude. Remove the spark plug wire, unscrew the plug, and check the electrode gap and color. Black, wet deposits mean the mixture is too rich. Replace the spark plug with the correct OEM part for the QG 5500 RV (consult your manual for the exact type). A fresh plug often improves running quality immediately.
3. Verify fuel quality and tank condition. Old or contaminated fuel gums up carburetor jets and makes altitude adjustment harder. If your generator has been sitting for weeks, drain the old fuel and refill with fresh gasoline. If you suspect water or debris in the tank, add a fuel stabilizer and run the engine for 10 minutes to circulate it through the system.
4. Locate the carburetor adjustment screws. On the QG 5500 RV, the main fuel mixture screw and idle speed screw are on the side of the carburetor. Consult your owner’s manual or the factory service manual at cumminsonan.com/support/ for the exact location and current settings. Write down the current position (number of turns out from fully seated) before you make any changes.
5. Lean out the main fuel mixture. At altitude, you need less fuel. Slowly turn the main fuel mixture screw clockwise (leaner) in quarter-turn increments. Run the engine at full load for 30 seconds after each adjustment. Stop when the engine runs smoothest and the exhaust smoke clears. Do not over-lean; the engine should not sputter or hesitate under load. This is the most critical adjustment for altitude operation.
6. Adjust the idle speed screw. After leaning the main mixture, the idle may become too slow or too fast. Turn the idle speed screw to achieve a steady, smooth idle at the RPM specified in your manual (typically 1,200–1,500 RPM for the QG 5500 RV). Use a tachometer if available, or listen for a steady, even sound.
7. Check the oxygen sensor (if equipped). The QG 5500 RV (EVAP) has an oxygen sensor that helps regulate fuel mixture. At altitude, this sensor may read incorrectly if it’s aged or contaminated. Locate the sensor (usually near the exhaust manifold), visually inspect it for carbon buildup, and check its electrical connector for corrosion. If the connector is corroded, clean it with a small brush and dielectric grease. If the sensor looks black or glazed, it may need replacement.
8. Test run and fine-tune. Start the engine and let it warm up for 5 minutes. Run it at no load, then under half load, then full load. The engine should run smoothly at all throttle positions without hesitation, surging, or black smoke. If it still runs rough, return to step 5 and make smaller adjustments. Document your final settings so you can return to them if needed.

Parts You May Need

• Air filter (replacement element)
• Spark plug (OEM type for QG 5500 RV)
• Carburetor rebuild kit (includes gaskets, seals, and jets)
• Oxygen sensor (if equipped and faulty)
• Fuel stabilizer
• Dielectric grease (for electrical connections)
• Tachometer (optional, for precise RPM adjustment)

When to Call a Pro

Stop troubleshooting and contact a Cummins Onan dealer or certified small-engine technician if:

• The engine runs rough even after you’ve cleaned the air filter, replaced the spark plug, and adjusted the carburetor mixture.
• The oxygen sensor connector is corroded and you’re not confident cleaning it, or the sensor itself is faulty.
• You suspect internal carburetor damage (cracked body, damaged needle valve) and need a full rebuild or replacement.
• The ignition timing appears to be off (engine knocks or pings under load even after fuel mixture adjustment).
• You don’t have access to your owner’s manual or the factory service manual and can’t locate the adjustment screws safely.

Frequently Asked Questions

Why does my generator run rough only at high altitude?

At sea level, air is denser and contains more oxygen molecules per cubic inch. Your carburetor was tuned for that density. Above 5,000 feet, the air thins out, so the same amount of fuel mixes with less oxygen, creating an overly rich mixture. Rich mixtures burn poorly, causing hesitation, rough idle, and black smoke. Leaning out the fuel mixture compensates for the thinner air.

Can I use an altitude compensation kit for the QG 5500 RV?

Some aftermarket altitude compensation kits exist, but they are not widely available for the QG 5500 RV. Manual carburetor adjustment is the standard and most reliable method. If you frequently move between sea level and high altitude, document your settings for each elevation and adjust the carburetor accordingly when you relocate.

Will a richer fuel mixture help at altitude?

No—the opposite is true. A richer mixture worsens rough running at altitude. You need to lean out the mixture because the air is already thin. Adding more fuel makes the imbalance worse. Always lean the mixture at altitude, never richen it.

How often should I adjust the carburetor if I move between elevations?

Every time you change elevation by 2,000 feet or more, you should re-check and adjust the carburetor mixture. If you’re a full-time RVer who moves frequently, keep a small notebook with your settings for each location. This takes 15–30 minutes and prevents rough running, fouled plugs, and fuel waste.

Disclaimer

This article provides general troubleshooting guidance for the Cummins Onan QG 5500 RV (EVAP) generator. Always consult your owner’s manual and the factory service manual before making any adjustments or repairs. Carburetor and ignition system work requires care and attention to detail. If you are not confident in your ability to perform these adjustments safely, contact a certified Cummins Onan dealer or small-engine technician. Improper adjustments can damage the engine or void your warranty. For official support and documentation, visit cumminsonan.com/support/.

Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.

What Is the EVAP System?

The Evaporative Emission Control (EVAP) system on your Cummins Onan QG 5500 RV captures fuel vapors from the tank and fuel lines, preventing them from escaping into the atmosphere. When the engine computer detects a problem—a leak, a stuck valve, or a clogged line—it triggers the check engine light. The good news: most EVAP faults are inexpensive and straightforward to diagnose.

At-a-Glance: Most Likely Causes

Cause	Likelihood	Typical Cost to Fix
Fuel cap not sealing	Very Common	$
Fuel tank vent blocked	Very Common	$
EVAP canister saturated	Common	$$
Purge valve stuck open	Common	$$
EVAP line cracked or disconnected	Occasional	$–$$

Diagnostic Walkthrough

Work through these steps in order, starting with the cheapest and easiest checks. You’ll need basic hand tools: a screwdriver set, a flashlight, and a fuel cap wrench if your unit uses a locking cap.

1. Inspect the fuel cap. Remove the cap from the fuel tank and examine the rubber gasket seal. Look for cracks, hardening, or visible damage. If the gasket is cracked or the cap doesn’t click firmly when reinstalled, the cap is not sealing. A loose cap is the most common EVAP fault. Try tightening it by hand; if it holds pressure, the light may clear after a few engine start cycles. If the gasket is damaged, replace the cap.
2. Check for fuel odor around the tank and filler neck. A strong fuel smell near the tank area suggests a leak in the EVAP system or a failing fuel cap seal. This is a visual and olfactory cue that vapor is escaping. Note the location of any odor for the next steps.
3. Locate and inspect the fuel tank vent line. On the QG 5500 RV, the vent line typically runs from the top of the fuel tank toward the charcoal canister. Trace it visually and feel along its length for cracks, splits, or disconnections. Look for any kinks that might restrict airflow. If you find a disconnected line, reconnect it firmly. If the line is cracked, it must be replaced.
4. Check the fuel tank vent for blockage. The vent line has a small opening that allows air to enter the tank as fuel is consumed. Debris, insect nests, or fuel residue can clog this opening. Locate the vent opening (usually a small tube on or near the fuel tank) and look inside with a flashlight. If you see debris, carefully clear it with a thin wire or compressed air. Do not use excessive force, as you can damage the vent valve.
5. Inspect EVAP hoses and connections. Follow the EVAP lines from the fuel tank to the charcoal canister and purge valve. Look for loose clamps, cracks in the hose, or disconnected fittings. Gently tug on each connection to confirm it is seated. If a hose is cracked or split, it must be replaced. Tighten any loose clamps with a screwdriver.
6. Locate the purge valve and check for fuel leakage. The purge valve is typically mounted near the engine intake manifold. It is a small cylindrical component with vacuum and fuel lines attached. Look for fuel dripping or pooling around the valve body. If fuel is leaking from the purge valve, the valve is stuck open and must be replaced. Also check that the vacuum line to the purge valve is connected and not cracked.
7. Inspect the charcoal canister for saturation. The canister is usually mounted under the vehicle or near the fuel tank. If you detect a strong fuel smell coming from the canister, or if fuel drips from the bottom, the canister is saturated and needs replacement. This typically happens after years of service or if the purge valve has been stuck open for an extended period.
8. Clear the check engine light and test. After completing repairs, use an OBD-II code reader (available at most auto parts stores for under $50) to clear the fault code. Start the engine and let it idle for 2–3 minutes, then drive the vehicle at highway speed for 10 minutes to allow the system to relearn. If the light returns within a few drive cycles, a second fault is present and you may need professional diagnosis.

Parts You May Need

• Fuel cap (OEM or equivalent)
• EVAP hose (rubber fuel vapor line, various diameters)
• Purge valve
• Charcoal canister
• Fuel tank vent valve or vent line assembly
• Hose clamps (stainless steel, various sizes)
• OBD-II code reader

When to Call a Pro

Contact a certified technician if:

• You find fuel leaking from the purge valve or canister and are uncomfortable replacing it.
• The EVAP line is cracked or disconnected in a location that requires fuel tank removal.
• The check engine light returns after you have replaced the fuel cap and cleared the code.
• You do not have access to an OBD-II code reader or are unsure how to use one.
• The charcoal canister shows signs of saturation (fuel dripping or strong odor) and needs replacement.
• You suspect a leak in the fuel tank itself rather than the EVAP system.

Frequently Asked Questions

Can I drive my QG 5500 RV with the EVAP check engine light on?

Yes, the EVAP light does not affect engine performance or safety in the short term. However, you should diagnose and repair the issue as soon as possible. A leaking EVAP system allows fuel vapors to escape, which is an environmental concern and may eventually trigger additional fault codes. In some regions, a vehicle with an active check engine light may fail emissions testing.

Why does my fuel cap keep coming loose?

A loose fuel cap is usually caused by a worn or damaged gasket seal, or by the cap’s threads becoming stripped over time. If tightening the cap by hand does not help, the gasket seal is likely cracked and the cap should be replaced. Always use the correct OEM cap or an equivalent designed for your model to ensure proper sealing pressure.

What is a saturated charcoal canister?

The charcoal canister absorbs fuel vapors from the tank. Over time, or if the purge valve is stuck open, the charcoal becomes saturated with liquid fuel and can no longer absorb vapors effectively. When saturated, the canister may leak fuel or trigger an EVAP fault code. A saturated canister must be replaced; it cannot be cleaned or refilled.

How long does it take for the check engine light to clear after I fix the EVAP system?

After you repair the fault and clear the code with an OBD-II reader, the light should remain off during normal operation. However, the engine computer runs a self-test cycle that may take 20–50 miles of driving (including highway speed) to complete. If the light returns after this period, a secondary fault may be present. Consult your owner’s manual or a technician for further diagnosis.

Disclaimer

This article provides general troubleshooting guidance for EVAP system faults on the Cummins Onan QG 5500 RV. Always consult your model-specific owner’s manual and service documentation before attempting repairs. Fuel system work carries inherent risks; if you are uncomfortable working with fuel components, contact a qualified technician. Improper repair or assembly can result in fuel leaks, fire hazard, or engine damage. The information provided here is for educational purposes and does not replace professional diagnosis or manufacturer guidance.

Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.

When your Cummins Onan QG 5500 RV (EVAP) generator sits idle during a power outage instead of automatically switching on, the problem almost always lies in the automatic transfer switch circuit. The ATS is the brain that tells your genset when to fire up and take over from the utility grid. If it’s not engaging, power isn’t reaching your RV’s circuits—and you’re stuck without backup power when you need it most.

This guide walks you through the most likely culprits and shows you exactly how to test each one with basic tools. Most of these checks take less than 30 minutes and require nothing more than a multimeter and a flashlight.

At-a-Glance: Most Likely Causes

Cause	Likelihood	Typical Cost to Fix
Wiring between genset and ATS damaged or loose	Very Common	$0–$50 (DIY)
ATS relay contacts welded or pitted	Common	$150–$400
Voltage sensing circuit fault	Common	$200–$500
Time delay relay malfunction	Occasional	$100–$300
ATS control board failure	Occasional	$400–$800

Diagnostic Walkthrough

Work through these steps in order. Most problems are found in the first three checks.

1. Verify the genset is actually running.
   Start your QG 5500 manually using the run switch on the generator itself. If it fires up, you know the engine and ignition are healthy. If it won’t start at all, you have a different problem—see your manual for starting troubleshooting. If it starts fine manually but the ATS never calls for it, continue to step 2.
2. Check all visible wiring connections at the ATS and genset.
   Open your RV’s electrical compartment and locate the automatic transfer switch (usually a gray or black box with terminals labeled “Line,” “Load,” “Gen,” and “Neutral”). Inspect every wire connection with a flashlight. Look for loose terminals, corrosion, or green oxidation on copper. Gently wiggle each terminal while watching for sparks or loose movement. Tighten any loose connections with the appropriate wrench or screwdriver. This single step solves roughly 40% of ATS engagement failures.
3. Test utility voltage at the ATS input terminals.
   Set your multimeter to AC voltage (200V or 250V scale). With the RV plugged into shore power, place one probe on the “Line” terminal and one on the “Neutral” terminal of the ATS. You should read 120V (or close to it). If you read 0V or a very low reading, the problem is upstream—check the pedestal connection, breaker, or shore power itself. If you read normal voltage, the ATS is receiving power correctly, and the issue is in the sensing or relay circuit.
4. Manually trigger a test by simulating a power loss.
   Some ATS units have a test button or manual override. Consult your owner’s manual for the exact location and procedure for your QG 5500 model. If your ATS has a test button, press it while the genset is running. The ATS should click audibly and transfer the load to the generator. If nothing happens—no click, no relay engagement sound—you likely have a relay or control board issue (steps 6–7). If the test button works but automatic engagement doesn’t, the voltage sensing circuit is the culprit (step 5).
5. Inspect the voltage sensing circuit connections.
   The ATS monitors utility voltage through a sensing circuit. Locate the sensing leads (usually a small gauge pair of wires connected to the “Line” and “Neutral” terminals). Check that these wires are not pinched, frayed, or disconnected. If a sensing wire is loose or broken, the ATS won’t know when utility power fails and won’t command the genset to start. Reconnect any loose sensing wires firmly.
6. Check for audible relay clicks when utility power is interrupted.
   With the genset running, have someone switch off the main breaker at your RV’s electrical panel (or unplug from shore power if safe to do so). Listen carefully at the ATS for a clicking sound. A click indicates the relay is attempting to engage. No click means the relay coil isn’t energizing, pointing to a control board or relay failure. If you hear a click but the load doesn’t transfer, the relay contacts are likely welded or pitted and need replacement.
7. Measure voltage across the relay coil terminals.
   This step requires a multimeter and some comfort with electronics. Locate the relay coil terminals inside the ATS (consult your manual for their location). Set your multimeter to DC voltage. When utility power is present, the coil should measure 0V or very low voltage. When utility power is removed (or the test button is pressed), the coil should measure 12V or 24V (depending on your model). If the coil never energizes, the control board isn’t sending the signal—the board has failed. If the coil energizes but the relay doesn’t click, the relay itself is faulty.
8. Inspect the relay contacts for welding or pitting.
   If you’ve confirmed the relay coil is energizing but the ATS still won’t transfer load, the relay contacts are likely damaged. This requires opening the ATS enclosure (which may void your warranty). If you’re comfortable doing so, locate the main relay inside and visually inspect the contact surfaces. Welded contacts appear fused together; pitted contacts look rough or corroded. Damaged contacts cannot be repaired and require relay replacement. If you’re not comfortable opening the enclosure, skip this step and contact a technician.

Parts You May Need

• Multimeter (digital, AC/DC capable)
• ATS relay (replacement, if contacts are welded or pitted)
• Time delay relay (if the relay is faulty)
• ATS control board (if the board has failed)
• Electrical wire (14 AWG or 12 AWG, depending on circuit)
• Wire connectors and terminals (crimp-on, if rewiring is needed)
• Dielectric grease (to prevent corrosion on terminals)

When to Call a Pro

Stop diagnosing and contact a licensed technician immediately if:

• You smell burning plastic or see scorch marks inside the ATS enclosure.
• The relay coil is energizing but the ATS still won’t transfer, and you’ve confirmed all wiring is tight.
• Your multimeter shows 0V at the ATS input even though shore power is connected—this indicates a problem upstream that requires professional electrical diagnostics.
• You’re uncomfortable working with electrical circuits or opening the ATS enclosure.
• The control board appears visibly damaged (burned components, cracked solder joints, or discolored areas).
• You’ve completed all diagnostic steps and the problem persists—the control board or a complex relay circuit has likely failed and requires board-level repair or replacement.

Frequently Asked Questions

Why does my ATS work fine sometimes but not other times?

Intermittent ATS engagement usually points to a loose wiring connection or a relay with partially welded contacts. When contacts are partially stuck, they may make contact under certain conditions (vibration, temperature, or load) but fail under others. Start by re-tightening every terminal at the ATS and genset. If the problem persists, the relay contacts are likely damaged and need replacement.

Can I manually switch to generator power if the ATS won’t engage automatically?

Yes. Most ATS units have a manual selector switch or override button. Consult your owner’s manual for the exact procedure for your QG 5500 model. This is a temporary workaround but does not fix the underlying ATS fault. You should still diagnose and repair the automatic function to ensure your genset engages on its own during future outages.

What’s the difference between a relay contact failure and a control board failure?

A relay contact failure means the coil energizes (you hear a click), but the contacts don’t actually close to transfer power. A control board failure means the coil never energizes at all (no click). You can distinguish between the two by listening for the relay click when utility power is removed. If you hear a click but no power transfer, the relay is bad. If you hear nothing, the control board has failed.

How much does it cost to replace an ATS relay or control board?

A replacement relay typically costs $100–$400, depending on the relay type and whether you hire a technician for installation. A replacement control board can range from $400–$800. Labor for installation usually adds $150–$300. Costs vary by region and service provider, so get a quote from a Cummins Onan authorized service center before committing to repairs.

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Disclaimer: This article provides general troubleshooting information for the Cummins Onan QG 5500 RV (EVAP) automatic transfer switch. Always consult your model-specific owner’s manual and follow the manufacturer’s procedures before attempting repairs. Electrical work can be hazardous. If you are unsure about any step, contact a licensed technician or an authorized Cummins Onan service center. For official support, visit https://www.cumminsonan.com/support/.

Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.