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 faulty oxygen sensor providing incorrect feedback.

Answer

The spark plug gap being too wide for CNG operation.

Answer

A malfunctioning manual shut-off valve.

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 vacuum leak in the intake manifold.

Answer

Low pressure in the CNG fuel cylinders.

Answer

A restriction in the vehicle's exhaust system.

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

Observing frost formation on the regulator body.

Answer

Using an ultrasonic leak detector aimed at the regulator's weep hole.

Answer

Checking for CNG odorant smell in the engine compartment immediately after shutdown.

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

A single, small bubble that forms and pops after 30 seconds.

Answer

Very fine, foam-like bubbles that slowly accumulate around the fitting.

Answer

No bubbles are visible, but the solution appears to be slowly evaporating.

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

The CNG tanks are empty.

Answer

The manual shut-off valve at the cylinder is closed.

Answer

The high-pressure coalescing filter is completely blocked.

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?

Accepted Answer

Run the engine on CNG until it stalls to depressurize the low-pressure lines.

Answer

Disconnect the vehicle's 12-volt battery.

Answer

Purge the high-pressure lines with nitrogen.

Answer

Verify the gasoline fuel tank is less than half full.