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Module 6: CNG Depressurizing and Defueling

Lesson 6.2: Depressurizing

Depressurizing ▪ Fuel Systems without Defueling Port or Bleed Valve
High-Pressure o Always follow manufacturer’s procedures
Components o Generic description of procedures
(Excluding ✓ Close cylinder valves or disconnect electric solenoid valves
Cylinders, Valves or ✓ Perform rundown procedure to depressurize
PRDs) ▪ Fuel Systems with Defueling Port and/or Bleed Valve
o Complete steps above, then use the defueling port and/or bleed valve to
depressurize the remainder of the high-pressure fuel system

Rundown Procedure 1. Shut off all cylinder valves or disconnect electric solenoid valves
2. Start the engine
3. Run until it stalls or shuts off
4. Wait 1-2 minutes; attempt to re-start engine
5. When engine will not run, the high-pressure fuel system should have low pressure
▪ Check pressure gauges to confirm
6. Open bleed valve or secondary filter drain valve

Depressurizing Low- ▪ Always follow manufacturer’s procedures
Pressure ▪ Generic description of procedure
Components o Close all cylinder valves or disconnect electric solenoid valve
o Using defueling port or bleed valve, depressurize the high-pressure fuel
system
o Use bleed valve on low-pressure fuel system (if present) to depressurize

Lesson 6.3: Reasons for Defueling CNG Vehicles

Planned Defueling ▪ Cylinder out of date
▪ Final disposal of vehicle
▪ Service related
o Component replacement (warranty, recall or failure)

Emergency ▪ Cylinder inspection reveals level 2 or 3 damage
Defueling ▪ Any damage that causes concern for failure of any
high-pressure CNG fuel system component

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