System Diagnostics and Repair Flashcards
7 cards from real CNG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 System Diagnostics and Repair flashcards as text
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:
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.
When inspecting CNG cylinder mounting brackets, which condition requires immediate removal from service?
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.
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:
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.
A CNG system's manual cylinder valve is difficult to open fully. The CORRECT action is to:
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.
After replacing a CNG low-pressure regulator, the technician should verify:
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.
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:
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.
Which tool is essential when torquing CNG high-pressure fittings to specification?
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).