CNG - Compressed Natural Gas System Diagnostics and Repair Questions and Answers

Question 1

A bi-fuel CNG vehicle operates normally at idle and low speeds but automatically switches back to gasoline during hard acceleration or when climbing a steep grade. Which of the following is the most probable cause for this issue?

Accepted Answer

A clogged or restricted CNG fuel filter.

Answer

A clogged CNG fuel filter can allow enough fuel to pass for low-demand situations like idling, but it restricts the flow required for high-load conditions like hard acceleration. The engine control module detects the resulting pressure drop and switches back to gasoline to protect the engine and maintain performance.

Question 2

A technician connects a scan tool to a CNG vehicle and retrieves a persistent P0172 trouble code, indicating a 'System Too Rich' condition only when operating on CNG. Which of the following faults is the most likely cause?

Accepted Answer

A leaking or weeping CNG fuel injector.

Answer

A P0172 code means there is too much fuel or not enough air in the air-fuel mixture. A leaking CNG injector would continuously add extra, un-metered fuel to a cylinder, causing the oxygen sensors to detect a rich condition specifically when the CNG system is active.

Question 3

A technician is diagnosing a suspected 'creeping' primary pressure regulator, where the low-side pressure slowly increases after the engine is shut off. What is the most definitive method to confirm this fault?

Accepted Answer

Connecting a pressure gauge to the low-pressure service port and monitoring it over time with the engine off.

Answer

Regulator creep is defined as a slow rise in downstream pressure when there is no flow. The most direct and accurate way to diagnose this is to connect a pressure gauge to the low-pressure side of the system, note the initial pressure after shutdown, and observe if it rises over a period of time (e.g., 15 minutes). A stable pressure indicates a good regulator, while a rising pressure confirms creep.

Question 4

When performing a leak check on a high-pressure CNG fitting using an approved soap solution, which observation indicates a dangerous, gross leak that requires immediate attention?

Accepted Answer

Large, continuous bubbles that form and grow rapidly as soon as the solution is applied.

Answer

The size and rate of bubble formation correspond to the severity of a gas leak. Large, rapidly forming and growing bubbles indicate a significant or gross leak, which is a hazardous condition that must be depressurized and repaired immediately.

Question 5

A CNG vehicle fails to switch from gasoline to CNG operation. The technician's scan tool indicates 'No Signal' from the fuel tank pressure sensor. Which is the MOST likely root cause?

Accepted Answer

There is a damaged wire or loose connector in the sensor's circuit.

Answer

A 'No Signal' fault code specifically points to an electrical problem. The Engine Control Module (ECM) is not receiving any data from the sensor, which is different from receiving a reading of zero pressure. This is most commonly caused by an open or shorted wire, or a disconnected/corroded electrical connector for the sensor.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG - Compressed Natural Gas System Diagnostics and Repair Questions and Answers