Practice Test Geeks(CNG) Compressed Natural Gas Practice Test
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System Diagnostics and Repair Flashcards

7 cards from real CNG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 System Diagnostics and Repair flashcards as text

  1. A CNG vehicle experiences intermittent stalling. The technician confirms fuel pressure is normal. The NEXT diagnostic step should be:

    Answer: Check for intermittent electrical faults using a data logger while duplicating the stall condition

    Intermittent stalls with normal pressure suggest electrical or sensor issues that require a data logger to capture the fault while it occurs.

  2. The CNG cylinder's hydrostatic test date has expired. The technician should:

    Answer: Remove the cylinder from service until it is retested and recertified per DOT regulations

    DOT regulations require periodic hydrostatic testing; an expired cylinder must be removed from service and professionally retested before continued use.

  3. During a road test diagnosis, the CNG vehicle surges at steady throttle. Long-term fuel trims are at +18%. This indicates:

    Answer: The system is running lean; the ECM is adding fuel to compensate

    A positive long-term fuel trim of +18% means the ECM is adding significant fuel, indicating the system is lean — likely from low pressure or a flow restriction.

  4. A technician uses an infrared gas detector near a CNG pressure relief valve and gets a continuous reading. The correct response is to:

    Answer: Evacuate the area, ventilate, and replace the pressure relief valve before returning the vehicle to service

    A leaking PRV is a safety hazard requiring immediate evacuation, ventilation, and valve replacement — it must never be capped or returned to service while leaking.

  5. A CNG vehicle with a dedicated natural gas engine has excessive oil consumption. The technician should inspect:

    Answer: Valve stem seals and piston rings, as oil contamination in the combustion chamber is accelerated by dry CNG combustion

    CNG burns very dry and removes lubrication from valve stems; worn valve stem seals and rings are a common cause of high oil consumption in dedicated CNG engines.

  6. What is the primary purpose of the excess flow valve found on CNG cylinders?

    Answer: To automatically shut off gas flow if the downstream line ruptures, preventing catastrophic release

    The excess flow valve senses abnormally high flow caused by a downstream rupture and closes automatically to prevent large-scale gas release.

  7. After completing CNG system repairs and reassembly, the FINAL step before returning the vehicle to service is:

    Answer: Perform a full-system leak test at operating pressure and verify no DTCs are present

    A leak test at operating pressure combined with a DTC scan confirms the repair is complete and the system is safe before returning the vehicle to service.