Maintenance and Troubleshooting 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 Maintenance and Troubleshooting flashcards as text
A CNG dispenser repeatedly fails to reach fill pressure of 3,600 psi. Which vehicle-side component should be checked first?
Answer: The manual cylinder valve to ensure it is fully open
A partially closed manual cylinder valve is a common, easily corrected reason a vehicle cannot reach rated fill pressure at the dispenser.
After installing a new CNG cylinder, which regulatory step is required before placing it into service?
Answer: Confirm the cylinder has a valid DOT or TC certification label and is within its service life
Before any new CNG cylinder is placed into service, its DOT/TC certification label must be verified to confirm it meets the applicable standard and is within its service life.
What symptom would indicate a clogged CNG fuel injector on a bi-fuel vehicle?
Answer: Cylinder misfire on CNG but normal operation on gasoline
A clogged CNG injector causes a cylinder misfire only when operating on CNG; switching to gasoline bypasses the CNG injectors and normal operation confirms the diagnosis.
What is the function of the excess flow valve (EFV) in a CNG vehicle fuel system?
Answer: It automatically stops gas flow if flow rate exceeds a safe limit, indicating a line rupture
The EFV senses abnormally high flow rates caused by a ruptured line and closes automatically to prevent uncontrolled fuel release.
A CNG vehicle has low power and poor acceleration. Fuel pressure at the injector rail reads 90 psi instead of the spec 120 psi. What is the most likely cause?
Answer: A failing or improperly adjusted low-pressure regulator
Low injector rail pressure points to a low-pressure (second-stage) regulator that is failing to maintain the specified delivery pressure to the engine.
Which of the following is NOT an acceptable material for repairing a CNG high-pressure fuel line?
Answer: Standard rubber hose rated for gasoline service
Standard gasoline-rated rubber hose is not approved for CNG service because it cannot withstand CNG pressure ratings and may be permeated by methane.
During a post-maintenance pressure test, a technician pressurizes the system and leaves it for 10 minutes. Pressure drops 20 psi. What does this indicate?
Answer: A leak exists in the system that must be located and repaired before returning to service
Any pressure drop during a static pressure hold test indicates a leak; the system must not be returned to service until the leak is located and corrected.