CNG System Diagnostics and Repair 3

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:

Accepted Answer

A restricted high-pressure filter reducing flow under load

Answer

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?

Accepted Answer

A crack in the mounting bracket weld

Answer

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:

Accepted Answer

An open circuit between the ECM and the injector connector

Answer

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:

Accepted Answer

Tag the vehicle out of service and replace the valve

Answer

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:

Accepted Answer

Regulated output pressure matches specification, there are no leaks, and heated coolant connections are secure

Answer

A complete post-replacement check includes output pressure verification, leak testing all fittings, and confirming coolant heater hoses are properly connected.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG System Diagnostics and Repair 3