ASE - Automotive Service Excellence Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 1 — Questions and Answers
Question 1: Which component in a CNG fuel system is responsible for reducing high-pressure fuel from the cylinder to a usable pressure for the engine?
- Fuel injector rail
- Pressure regulator (Correct answer)
- Manual shutoff valve
- CNG filter/separator
Correct answer: Pressure regulator
The pressure regulator steps down high cylinder pressure (typically 3,000–3,600 psi) to the low operating pressure required by the fuel injectors, usually 3–10 psi depending on system design.
Question 2: A technician detects a natural gas odor around a CNG vehicle in an enclosed shop. What is the FIRST action that should be taken?
- Use a manometer to locate the leak source
- Immediately ventilate the area and move the vehicle outdoors (Correct answer)
- Apply soapy water to all fittings to find the leak
- Disconnect the high-pressure fuel line at the cylinder
Correct answer: Immediately ventilate the area and move the vehicle outdoors
Natural gas is highly flammable and lighter than air, so it accumulates near the ceiling of enclosed spaces. The priority is to ventilate the area and remove the vehicle to eliminate ignition risk before any diagnostic work begins.
Question 3: What type of valve on a CNG cylinder automatically stops fuel flow if the downstream fuel line is severed or ruptures?
- Manual shutoff valve
- Excess flow valve (Correct answer)
- Pressure relief device (PRD)
- Fuel temperature sensor
Correct answer: Excess flow valve
The excess flow valve is a safety device that automatically closes when fuel flow exceeds a predetermined rate, such as during a fuel line rupture, preventing uncontrolled fuel release.
Question 4: During inspection of a CNG vehicle, a technician finds a cylinder with visible deep gouges and abrasion damage on its surface. What is the correct procedure?
- Clean the cylinder and re-inspect it after 1,000 miles
- Fill the cylinder and perform a hydrostatic pressure test
- Remove the cylinder from service immediately (Correct answer)
- Apply epoxy filler to the damaged area and re-label the cylinder
Correct answer: Remove the cylinder from service immediately
CNG cylinders with visible physical damage such as deep gouges, cuts, or abrasions must be removed from service immediately. Structural integrity cannot be confirmed without specialized non-destructive testing, and a compromised cylinder poses a serious burst hazard.
Question 5: What is the purpose of the Pressure Relief Device (PRD) installed on CNG fuel cylinders?
- To regulate fuel delivery pressure to the injectors
- To vent cylinder pressure if the cylinder temperature rises dangerously, such as during a fire (Correct answer)
- To prevent overfilling during the fueling process
- To measure remaining fuel quantity and send a signal to the fuel gauge
Correct answer: To vent cylinder pressure if the cylinder temperature rises dangerously, such as during a fire
The PRD is a thermally activated safety device that vents cylinder contents if exposed to high temperatures, such as in a vehicle fire. This controlled release prevents a catastrophic cylinder rupture (BLEVE).
Question 6: A CNG vehicle cranks normally but will not start. The technician confirms the cylinder has adequate fuel pressure, the ignition system is functioning, and there are no communication codes. Which CNG-specific component should be suspected next?
- Oxygen sensor
- Catalytic converter
- Fuel lock-off solenoid valve (Correct answer)
- Mass airflow sensor
Correct answer: Fuel lock-off solenoid valve
The fuel lock-off solenoid valve (also called a fuel shut-off valve) controls electrical permission for fuel to flow to the engine. If it fails in the closed position or loses its control signal, fuel cannot reach the engine even though cylinder pressure is normal.
ASE - Automotive Service Excellence Practice Test (F1: Compressed Natural Gas (CNG) Vehicles) 1
Which component in a CNG fuel system is responsible for reducing high-pressure fuel from the cylinder to a usable pressure for the engine?