CNG Maintenance and Troubleshooting 2 — Questions and Answers
Question 1: Which solution is the approved method for field-testing CNG fuel line fittings for leaks?
- Electronic combustible gas detector only
- Soapy water or approved leak detection solution (Correct answer)
- Flame test with a lighter
- Pressurized nitrogen and visual inspection only
Correct answer: Soapy water or approved leak detection solution
Approved leak detection fluid (soapy water or commercial solution) is applied to fittings; bubbles indicate a leak without ignition risk.
Question 2: A CNG vehicle's high-pressure regulator is suspected of failing. Which symptom most directly points to a regulator malfunction?
- Excess fuel odor at the fill port
- Engine runs rich at idle but lean at high RPM
- Consistently low fuel pressure delivered to the engine (Correct answer)
- Slow cylinder fill time at the dispenser
Correct answer: Consistently low fuel pressure delivered to the engine
A failing high-pressure regulator cannot step down tank pressure correctly, resulting in consistently low delivery pressure to the engine.
Question 3: When inspecting CNG high-pressure cylinders, what condition requires the cylinder to be immediately removed from service?
- Light surface oxidation on the cylinder exterior
- Any visible crack, gouge, or deep abrasion on the cylinder (Correct answer)
- Minor paint fading or surface discoloration
- A small dent less than 1 mm deep
Correct answer: Any visible crack, gouge, or deep abrasion on the cylinder
Any crack, gouge, or deep abrasion compromises cylinder structural integrity and mandates immediate removal from service per NFPA 52 and NGV2 standards.
Question 4: After replacing a CNG fuel filter, what must be done before returning the vehicle to service?
- Purge the filter with nitrogen for 60 seconds
- Perform a full leak test on all disturbed connections (Correct answer)
- Run the engine at high idle for 10 minutes to reseat the filter
- Refill the cylinders to 3,600 psi immediately
Correct answer: Perform a full leak test on all disturbed connections
Any time a fuel system component is opened and reassembled, a leak test on all disturbed fittings and connections is mandatory before returning to service.
Question 5: A CNG technician notices the pressure relief device (PRD) has vented. What is the correct first diagnostic step?
- Replace the PRD and refuel the vehicle
- Check if the vehicle was exposed to an external heat source or fire (Correct answer)
- Assume the PRD is faulty and disregard the event
- Increase the PRD set pressure by 10%
Correct answer: Check if the vehicle was exposed to an external heat source or fire
PRD venting is a safety event; the technician must determine if excessive heat or fire caused the activation before replacing the device.
Question 6: What is the most common cause of premature fuel filter clogging in a CNG system?
- Overfilling cylinders above rated pressure
- Moisture and compressor oil carryover from low-quality fill stations (Correct answer)
- High ambient temperature during fueling
- Running the engine below operating temperature
Correct answer: Moisture and compressor oil carryover from low-quality fill stations
Poor-quality CNG from stations with inadequate drying can carry moisture and compressor oil that rapidly clog downstream fuel filters.
Question 7: Which tool is essential when torquing CNG high-pressure fittings during maintenance?
- Standard open-end wrench to 'hand tight plus one turn'
- Calibrated torque wrench set to manufacturer specifications (Correct answer)
- Impact wrench with a torque-limiting socket
- Pipe wrench for maximum grip
Correct answer: Calibrated torque wrench set to manufacturer specifications
High-pressure CNG fittings must be torqued to exact manufacturer specifications using a calibrated torque wrench to prevent leaks or thread damage.
CNG Maintenance and Troubleshooting 2
Which solution is the approved method for field-testing CNG fuel line fittings for leaks?