CNG Maintenance and Troubleshooting 3 — Questions and Answers
Question 1: A CNG vehicle experiences hard starting in cold weather. Which maintenance item should be inspected first?
- High-pressure cylinder hydrostatic test date
- Low-pressure fuel filter and regulator heating element or coolant lines (Correct answer)
- Cylinder valve manual shutoff position
- CNG dispenser nozzle compatibility
Correct answer: Low-pressure fuel filter and regulator heating element or coolant lines
Cold-weather hard starting on CNG vehicles is commonly caused by a failed regulator heater or blocked coolant line that prevents the regulator from vaporizing fuel properly.
Question 2: What does a persistently illuminated 'Check Fuel' or 'Fuel System' warning light on a CNG vehicle most commonly indicate?
- The cylinders are above maximum fill pressure
- A fault detected by the ECM in the CNG fuel delivery circuit (Correct answer)
- Normal operation during the first 100 miles after fueling
- The PRD has activated and vented the system
Correct answer: A fault detected by the ECM in the CNG fuel delivery circuit
The ECM continuously monitors CNG system sensors; a persistent warning light indicates a stored fault code that requires diagnostic scan tool retrieval.
Question 3: How often should CNG vehicle high-pressure fuel lines be visually inspected for chafing, kinks, or abrasion?
- Only at the annual hydrostatic test interval
- At every scheduled preventive maintenance interval (Correct answer)
- Every 100,000 miles or 10 years
- Only after an accident or fuel odor complaint
Correct answer: At every scheduled preventive maintenance interval
Visual inspection of high-pressure fuel lines should occur at every scheduled PM interval to catch chafing or abrasion before a high-pressure failure develops.
Question 4: A CNG technician finds water in the fuel filter bowl. What corrective action addresses the root cause?
- Drain the filter and continue normal operation
- Notify the fleet to avoid the fueling station suspected of wet gas and replace the filter (Correct answer)
- Install a secondary filter downstream of the first
- Purge the cylinders and refill at a different station
Correct answer: Notify the fleet to avoid the fueling station suspected of wet gas and replace the filter
Water in the filter indicates the fueling station's dryer is inadequate; alerting the fleet and avoiding that station addresses the source, not just the symptom.
Question 5: Which component is inspected to verify the automatic cylinder valve (ACV) is functioning correctly?
- Fuel pressure at the high-pressure port during key-off
- Downstream fuel pressure drop rate when the ignition is turned off (Correct answer)
- Cylinder hydrostatic test tag date
- PRD activation temperature rating
Correct answer: Downstream fuel pressure drop rate when the ignition is turned off
A properly functioning ACV closes when the ignition is off; technicians verify this by monitoring whether downstream pressure drops to zero after key-off.
Question 6: What is the correct procedure when a CNG vehicle's cylinder reaches its hydrostatic retest date?
- Continue using the cylinder until it shows visible damage
- Remove the cylinder from service and send it to a certified test facility (Correct answer)
- Perform an external visual inspection and stamp a new test date
- Refill the cylinder at reduced pressure until recertified
Correct answer: Remove the cylinder from service and send it to a certified test facility
Cylinders past their retest date must be removed from service and sent to a DOT/TC-certified hydrostatic test facility — field retest is not authorized.
Question 7: A technician smells natural gas near the engine compartment after a routine maintenance. What is the immediate action?
- Start the engine to identify the leak location by sound
- Eliminate ignition sources, close all cylinder valves, and ventilate the area (Correct answer)
- Use a lighter to check around fittings for leaks
- Drive the vehicle to an open area before investigating
Correct answer: Eliminate ignition sources, close all cylinder valves, and ventilate the area
Natural gas leaks require immediate elimination of ignition sources and ventilation; manual cylinder valve closure isolates the fuel source before any investigation.
CNG Maintenance and Troubleshooting 3
A CNG vehicle experiences hard starting in cold weather.
Which maintenance item should be inspected first?