CNG System Diagnostics and Repair 3 — Questions and Answers
Question 1: A CNG-powered fleet vehicle returns with a complaint of reduced power at highway speeds. Fuel pressure at idle is normal. The MOST likely cause is:
- A restricted high-pressure filter reducing flow under load (Correct answer)
- A faulty crankshaft position sensor
- Low octane rating of the CNG fuel
- A loose throttle body connection
Correct answer: A restricted high-pressure filter reducing flow under load
A partially clogged high-pressure filter may pass enough fuel at idle but restricts flow under the higher demand of highway operation.
Question 2: When inspecting CNG cylinder mounting brackets, which condition requires immediate removal from service?
- Minor surface rust on the bracket fasteners
- A crack in the mounting bracket weld (Correct answer)
- Slight paint discoloration from heat
- A missing secondary retention strap
Correct answer: A crack in the mounting bracket weld
A cracked weld on a cylinder mounting bracket is a critical structural failure that could allow the high-pressure cylinder to become a projectile.
Question 3: A technician measures 0 volts at the CNG injector signal wire with the engine running. The ECM output driver tests good. The MOST likely fault is:
- A shorted injector coil
- An open circuit between the ECM and the injector connector (Correct answer)
- A failed crankshaft sensor preventing injection pulses
- Low fuel pressure preventing injector activation
Correct answer: An open circuit between the ECM and the injector connector
If the ECM driver is functional but no voltage reaches the injector, an open circuit in the harness between the ECM and connector is the most probable cause.
Question 4: A CNG system's manual cylinder valve is difficult to open fully. The CORRECT action is to:
- Force the valve open with a wrench extension
- Tag the vehicle out of service and replace the valve (Correct answer)
- Lubricate the valve stem with petroleum-based grease
- Wire the valve in the open position
Correct answer: Tag the vehicle out of service and replace the valve
A stiff or damaged cylinder valve must be replaced — forcing it risks valve failure, and petroleum lubricants can degrade CNG valve seals.
Question 5: After replacing a CNG low-pressure regulator, the technician should verify:
- Only that the engine starts and idles smoothly
- Regulated output pressure matches specification, there are no leaks, and heated coolant connections are secure (Correct answer)
- That the ECM has been reprogrammed for the new regulator
- Only that no DTCs are set after 10 minutes of idling
Correct answer: Regulated output pressure matches specification, there are no leaks, and heated coolant connections are secure
A complete post-replacement check includes output pressure verification, leak testing all fittings, and confirming coolant heater hoses are properly connected.
Question 6: A CNG vehicle's onboard diagnostics stores a recurring P0087 (fuel system too lean / low pressure) code. After confirming low delivered pressure, the technician should check:
- The spark plug gap first
- Cylinder fuel level, filter condition, regulator output, and solenoid valve operation in sequence (Correct answer)
- Only the oxygen sensor voltage
- The throttle position sensor calibration
Correct answer: Cylinder fuel level, filter condition, regulator output, and solenoid valve operation in sequence
A systematic pressure diagnosis works from the supply (cylinder level) through the filter, regulator, and solenoid to isolate the restriction.
Question 7: Which tool is essential when torquing CNG high-pressure fittings to specification?
- A pipe wrench for maximum grip
- A calibrated torque wrench appropriate for the fitting size (Correct answer)
- An impact wrench set to low torque
- An adjustable wrench with a cheater bar
Correct answer: A calibrated torque wrench appropriate for the fitting size
A calibrated torque wrench ensures fittings are tightened to the exact manufacturer specification, preventing both leaks (under-torque) and fitting damage (over-torque).
CNG System Diagnostics and Repair 3
A CNG-powered fleet vehicle returns with a complaint of reduced power at highway speeds.
Fuel pressure at idle is normal.
The MOST likely cause is: