Diagnosis And Testing - Visual Inspection - CNG

For Compressed Natural Gas (CNG) powered vehicles:

A visual inspection for loose, disconnected, or incorrectly routed wires and fuel tubes should be made. This should be done before attempting to diagnose or service the fuel injection system. A visual check will help spot these faults and save unnecessary test and diagnostic time. A thorough visual inspection will include the following checks:

1. Be sure manual 1/4 turn gas shut-off valve is open. Valve handle should be parallel with fuel cylinders (or fuel line).
2. Be sure the fuel (manual) control valves on all 4 fuel cylinders are fully open. Valves are fully opened when turned fully counterclockwise.
3. Be sure electrical connector at low-pressure fuel shut-off solenoid is connected, clean and tight.
4. A pigtail wiring harness connects high-pressure fuel shut-off solenoid to main chassis wiring harness. The pigtail connection is made above and to rear of left-side mounted fuel cylinders near manual shut-off valve. Be sure this electrical connection is secure, clean and tight.
5. Be sure electrical connectors at fuel pressure and fuel temperature sensors are connected, clean and tight.
6. Verify that the three 32-way connectors are fully inserted into connectors of Powertrain Control Module (PCM).
7. Inspect battery cable connections. Be sure that they are clean and tight.
8. Inspect Automatic Shut-down (ASD) relay. The ASD relay is located in the Power Distribution Center (PDC). The PDC is located in engine compartment. Inspect relay connections. Inspect relay for signs of physical damage and corrosion.
9. Inspect high-pressure fuel shut-off solenoid relay. Relay is located in the Fuse Block at the left end of instrument panel. Inspect relay connections. Inspect relay for signs of physical damage and corrosion.
10. Inspect starter motor relay connections.
11. Inspect ignition coil connections. Verify that coil secondary cable is firmly connected to coil.
12. Verify that distributor cap is correctly attached to distributor. Be sure that spark plug cables are firmly connected to distributor cap and spark plugs are in their correct firing order. Be sure that coil cable is firmly connected to distributor cap and coil. Be sure that camshaft position sensor pigtail harness connector (at distributor) is firmly connected to engine harness connector. Inspect spark plug condition. Connect vehicle to an oscilloscope and inspect spark events for fouled or damaged spark plugs or cables.
13. Inspect fuel tubes (lines) for any damage or for being kinked.
14. Inspect all fuel tube (line) connections for leaks. Refer to DIAGNOSIS AND TESTING - TESTING FOR FUEL LEAK - CNG 
15. Verify that generator output wire, generator connector and ground wire are firmly connected to generator.
16. Inspect system body ground connections for being loose, dirty or corroded. Refer to appropriate Wiring Diagrams information for location of ground connections.
17. Verify positive crankcase ventilation (PCV) valve operation. Verify PCV valve hose is firmly connected to PCV valve and manifold.
18. Verify that hose connections to all ports of vacuum fittings on intake manifold are tight and not leaking.
19. Inspect accelerator cable, transmission throttle cable and cruise control cable connections, if equipped. Check their connections to throttle arm of throttle body for any binding or restrictions.
20. Verify that vacuum booster hose is firmly connected to fitting on intake manifold. Also check connection to brake vacuum booster.
21. Inspect air cleaner inlet and air filter element for dirt or restrictions.
22. Inspect radiator grille area, radiator fins and air conditioning condenser (if equipped) for restrictions.
23. Verify that intake manifold air temperature sensor wire connector is firmly connected to harness connector.
24. Verify that MAP sensor electrical connector is firmly connected to MAP sensor. Also verify that rubber nipple from MAP sensor to throttle body is firmly connected.
25. Verify that fuel injector wire harness connectors are firmly connected to injectors in the correct firing order. Each harness connector is numerically tagged (INJECTOR #1, INJECTOR #2 etc.) with its corresponding engine cylinder number.
26. Verify harness connectors are firmly connected to idle air control motor and throttle position sensor (TPS).
27. Verify that wire harness connector is firmly connected to engine coolant temperature sensor.
28. Inspect for exhaust system restrictions such as pinched exhaust pipes, collapsed muffler or plugged catalytic convertor.
29. Verify that electrical harness is firmly connected to neutral safety switch.
30. Verify electrical harness is firmly connected to rear wheel speed sensor. Verify rear wheel speed sensor is firmly attached to rear axle with proper air gap. Refer to ANTILOCK BRAKE SYSTEM (ABS) - SERVICE INFORMATION - 2500 HD & 3500 HD for information.
31. If equipped with 4-wheel antilock brake system, verify electrical harness is firmly connected to each front wheel speed sensor. Verify both front wheel speed sensors are firmly attached. Refer to ANTILOCK BRAKE SYSTEM (ABS) - SERVICE INFORMATION - 2500 HD & 3500 HD for information.
32. Inspect transmission torque convertor housing for damage to timing ring on drive plate/flywheel.
33. Verify firm connection of electrical connector at top of fuel gauge pressure sensor (transducer or sending unit). Sensor is attached to a fuel control valve located at rear of side mounted fuel cylinders.
34. Verify that battery cable and solenoid feed wire connections to starter solenoid are tight and clean. Inspect for chaffed wires or wires rubbing up against other components.