CNG - Compressed Natural Gas Maintenance and Troubleshooting Questions and Answers

Question 1

When troubleshooting a suspected slow gas leak at a high-pressure fitting that is not audible, what is the most reliable and recommended method for precise localization?

Accepted Answer

Applying an approved, non-corrosive leak detection solution.

Answer

Relying solely on the smell of the mercaptan odorant.

Answer

Submerging the component in a bucket of water.

Answer

Using an ultrasonic leak detector.

Question 2

A CNG vehicle that operates normally in warm weather has difficulty starting and runs rough in sub-freezing temperatures. The issue resolves after the engine warms up. Which maintenance issue is the most likely cause?

Accepted Answer

Moisture has frozen within the high-pressure regulator.

Answer

The engine's spark plugs have an incorrect heat range.

Answer

The PRDs on the cylinders have become too sensitive to cold.

Answer

The CNG's methane number is too low for winter operation.

Question 3

A CNG-powered truck exhibits poor performance and hesitation specifically under high engine load, such as climbing a grade, but it idles and cruises smoothly. What is the most probable cause of this symptom?

Accepted Answer

A failing high-pressure regulator unable to meet fuel demand.

Answer

An incorrectly gapped spark plug.

Answer

A leaking low-pressure fuel injector.

Answer

A fault in the vehicle's transmission.

Question 4

A technician checks the downstream pressure of a regulator after the vehicle has been shut off for 30 minutes and notices the pressure has slowly risen above the normal lock-up specification. This condition is best described as:

Accepted Answer

Pressure creep

Answer

Venting

Answer

Starvation

Answer

Hydraulic lock

Question 5

During a mandated 36-month/36,000-mile detailed visual inspection of a CNG cylinder, which of the following pieces of information is LEAST critical to record on the official inspection report?

Accepted Answer

The original purchase price of the cylinder.

Answer

The cylinder's manufacturing date.

Answer

The name and certification number of the inspector.

Answer

A description and location of any surface damage found.

Question 6

When diagnosing a communication failure where a diagnostic scan tool cannot connect to the dedicated CNG electronic control module (ECM), which of the following is the most essential first step?

Accepted Answer

Verifying the ECM has proper power and ground connections.

Answer

Re-flashing the CNG ECM's software.

Answer

Checking the high-pressure fuel sensor.

Answer

Measuring the resistance of the CAN bus circuit.