CNG - Compressed Natural Gas System Diagnostics and Repair Questions and Answers 1 — 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?
- A faulty oxygen sensor providing incorrect feedback.
- The spark plug gap being too wide for CNG operation.
- A clogged or restricted CNG fuel filter. (Correct answer)
- A malfunctioning manual shut-off valve.
Correct answer: A clogged or restricted CNG fuel filter.
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?
- A vacuum leak in the intake manifold.
- A leaking or weeping CNG fuel injector. (Correct answer)
- Low pressure in the CNG fuel cylinders.
- A restriction in the vehicle's exhaust system.
Correct answer: A leaking or weeping CNG fuel injector.
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?
- Observing frost formation on the regulator body.
- Using an ultrasonic leak detector aimed at the regulator's weep hole.
- Connecting a pressure gauge to the low-pressure service port and monitoring it over time with the engine off. (Correct answer)
- Checking for CNG odorant smell in the engine compartment immediately after shutdown.
Correct answer: Connecting a pressure gauge to the low-pressure service port and monitoring it over time with the engine off.
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?
- A single, small bubble that forms and pops after 30 seconds.
- Very fine, foam-like bubbles that slowly accumulate around the fitting.
- No bubbles are visible, but the solution appears to be slowly evaporating.
- Large, continuous bubbles that form and grow rapidly as soon as the solution is applied. (Correct answer)
Correct answer: Large, continuous bubbles that form and grow rapidly as soon as the solution is applied.
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?
- The CNG tanks are empty.
- There is a damaged wire or loose connector in the sensor's circuit. (Correct answer)
- The manual shut-off valve at the cylinder is closed.
- The high-pressure coalescing filter is completely blocked.
Correct answer: There is a damaged wire or loose connector in the sensor's circuit.
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.
Question 6: Before replacing a low-pressure CNG fuel injector, after closing the manual cylinder valves, what is the next essential safety step a technician must perform?
- Disconnect the vehicle's 12-volt battery.
- Run the engine on CNG until it stalls to depressurize the low-pressure lines. (Correct answer)
- Purge the high-pressure lines with nitrogen.
- Verify the gasoline fuel tank is less than half full.
Correct answer: Run the engine on CNG until it stalls to depressurize the low-pressure lines.
Even with the main cylinder valves closed, the fuel lines between the regulator and the injectors remain pressurized (typically 100-150 psi). The safest and most effective way to relieve this pressure is to run the engine on CNG until it automatically switches to gasoline or stalls, consuming the trapped gas. This depressurizes the system, allowing for safe removal of components like injectors.
CNG - Compressed Natural Gas System Diagnostics and Repair Questions and Answers 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?