Fuel Delivery and Air Induction: Service and Repair

STANDARD PROCEDURE - FUEL SYSTEM PURGING

WARNING: BEFORE ANY PART OF THE COMPRESSED NATURAL GAS (CNG) FUEL SYSTEM IS OPENED FOR REPAIR, THE SYSTEM MUST BE PURGED OF ALL NATURAL GAS.

The following information contains 4 different purging procedures. They are:
- FUEL TUBE PURGING-ENGINE ABLE TO RUN
- FUEL TUBE PURGING-ENGINE NOT ABLE TO RUN
- FUEL CYLINDER PURGING-ENGINE ABLE TO RUN
- FUEL CYLINDER PURGING-ENGINE NOT ABLE TO RUN

The fuel system must first be purged of natural gas if any of the following components are to be removed or repaired:
- Any fuel tube (line) or fuel tube fitting
- The fuel injector rail assembly
- Any or all fuel injectors
- The fuel temperature sensor
- The fuel low-pressure sensor
- The fuel gauge pressure sensor
- The fuel fill receptacle
- Any fuel cylinder
- Any fuel control valve
- The manual (master) shut-off valve
- The fuel filter
- The low-pressure fuel shut-off solenoid
- The fuel pressure regulator or high-pressure fuel shut-off solenoid
- The one-way check valve

Fuel Tube Purging-engine Able To Run

NOTE: Use this procedure if NOT servicing any of fuel cylinders.

Fig.2 Fuel Control Valves (Rear Cylinders Shown):

1. Close all 4 fuel control valves clockwise (Fig. 2) on all 4 fuel cylinders.

Fig.3 Manual Shut-Off Valve (Open Position):

2. Open manual (master) fuel shut-off valve (Fig. 3). When opened, valve handle should be parallel with fuel cylinders.
3. Start and operate engine until it runs out of fuel. Attempt 3 more engine starts.If engine Will not operate, fuel tube is purged. However, always slowly loosen fuel tube fitting being serviced until all pressure is relieved.
If one-way check valve or fuel fill receptacle is to be serviced, some residual pressure will remain in fuel line. Very slowly loosen fuel tube fitting at inlet side of one-way check valve. High pressure will be observed. Leave line loosened until all pressure is relieved.This procedure must be done with vehicle outdoors.

WARNING: ALTHOUGH THE FUEL TUBES ARE NOW PURGED OF GAS, THE FUEL CYLINDERS ARE STILL UNDER HIGH-PRESSURE. DO NOT OPEN THE CLOSED VALVES UNTIL ALL HIGH-PRESSURE COMPONENTS HAVE BEEN RESEALED.

4. After repairs to fuel system have been completed, a Go No-Go gauge inspection tool must be used to check fuel tube high-pressure fittings. These high-pressure fittings are located only on high-pressure side of fuel system. Refer to Fuel Tubes and Fittings Removal/installation.

Fuel Tube Purging-Engine Not Able To Run

NOTE: Use this procedure if NOT servicing any of fuel cylinders.

1. Close control valves (clockwise) on all fuel cylinders.
2. Open manual shut-off valve (Fig. 3). When opened, valve handle should be parallel with fuel cylinders.
3. If a fuel system component is being serviced on an engine that will not start, move vehicle outdoors and remove shield under fuel cylinders mounted to left side of vehicle. Cycle ignition key to ON position a few times to activate high-pressure solenoid. Turn ignition switch OFF. Slowly loosen fuel line at lowpressure fuel shut-off solenoid. Pressure will be observed. Leave line loosened until all pressure is relieved. This part of procedure must be done with vehicle outdoors.
4. At this point, all fuel tubes are purged of natural gas between fuel cylinders and engine. It is now OK to open fuel system for repair.

WARNING: ALTHOUGH THE FUEL TUBES ARE NOW PURGED OF GAS, THE FUEL CYLINDERS ARE STILL UNDER HIGH-PRESSURE. DO NOT OPEN THE CLOSED VALVES UNTIL ALL HIGH-PRESSURE COMPONENTS HAVE BEEN RESEALED.

5. After repairs to fuel system have been completed, a Go No-Go gauge inspection tool must be used to check fuel tube high-pressure fittings. These high-pressure fittings are located only on high-pressure side of fuel system. Refer to Fuel Tubes and Fittings Removal/installation.

Fuel Cylinder Purging-Engine Able To Run

NOTE: Use this procedure if servicing any of fuel cylinders.

1. Open manual shut-off valve (Fig. 3). When opened, valve handle should be parallel with fuel cylinders.
2. Open control valve (counterclockwise) on fuel cylinder(s) to be serviced.
3. Close all other control valves on cylinder(s) not being serviced.
4. Start and operate engine until it runs out of fuel. Attempt 3 more engine starts.
5. Close fuel cylinder control valve(s).
6. At this point, it is now OK to remove fuel cylinder for repair.

Fuel Cylinder Purging-Engine Not Able To Run
1. Move vehicle outdoors.
2. Close control valves on cylinder(s) not being serviced.
3. Make sure fuel shut-off valves and fuel lines are grounded.
4. Open control valve on cylinder(s) being serviced.
5. Very slowly loosen fuel tube fitting at fuel control valve(s) of fuel cylinder(s) being serviced. High- pressure will be observed. Leave line loosened until all pressure is relieved.
6. At this point, it is now OK to remove fuel cylinder(s) for repair.
7. After repairs to fuel system have been completed, a Go No-Go gauge inspection tool must be used to check fuel tube high-pressure fittings. These high-pressure fittings are located only on high-pressure side of fuel system. Refer to Fuel Tubes and Fittings Removal/installation.