Reviewers Note: Words/phrases in blue will be links to specific diagnostic procedures when this document is published in Freshdesk.

Compressed Natural Gas (CNG) Vehicle Fueling Procedure

DSM.0154 Ver. 1.0 January 11, 2023

Instructions for the safe fueling of compressed natural gas (CNG) fuel systems with an explanation of the
relationship between temperature and pressure on complete system filling.
NOTICE Verify fuel quality meets minimum CNG fuel specifications and check fuel dispenser
maintenance records.

CNG Fueling Procedure
NOTICE Always refer to fuel station directions and procedures and follow fuel dispenser manufacturer
operating instructions.
1. Note location of fuel facility / fuel dispenser emergency fuel shutdown switch.
2. Open FMM door (if applicable) and/or remove fuel receptacle cover or kill cap from desired fuel receptacle
(standard NGV1 or transit fast fill).
3. Wipe fuel receptacle and verify receptable O-ring is installed and in good condition.
4. Remove fuel hose nozzle from fuel dispenser.
NOTICE Before dispensing fuel, wipe station fill hose nozzle and check for any signs of oil or other
contaminants. An oily or dusty nozzle may be an indication of poor fuel quality or a station that is not
well-maintained; report either of these conditions to station operator.
5. Turn dispenser switch to the ON position.
6. Connect fuel hose nozzle to fuel receptacle per operating per instructions for each nozzle type.
NOTICE Fuel receptacles are designed so the nozzle will not come off under pressure.
7. Turn nozzle valve to “FILL” or “ON” position and fuel will start to flow.
WARNING If a gas odor is present when filling system, turn off dispenser and check fuel receptacle Oring. If O-ring is worn or damaged, replace O-ring. If O-ring is OK, notify station operator.
NOTICE If during rapid filling, the fill receptacle freezes, turn 1/4-turn manual shutoff valve to
“OFF/CLOSED” position until ice is no longer visible. Slowly OPEN the 1/4-turn manual shutoff valve
and gas will stop flowing from the receptacle.
8. Continue filling until fuel station pump shuts off automatically. NOTE: fuel dispenser pressure gauge should
read 3600 psi (250 bar).
WARNING Do not attempt to add additional fuel if the HP fuel gauge or OEM fuel gauge reads less than
expected. Fuel fill pressure is temperature-compensated.
9. Return fuel nozzle valve to “VENT” position and release it from fuel receptacle.
10. Return fuel hose nozzle to dispenser.
11. Once fueling is complete, replace fuel receptacle cover or cap and/or close FMM door.

CNG Fueling: Pressure and Temperature Relationship
Typically, stations fill to a service pressure of 3600 psi (250 bar).
During fast filling, gas heats as it compresses inside the cylinders, which is normal. On a hot day, pressure from
the filling station will indicate full system pressure, but the cylinders may not be filled completely. This is normal.

Reviewers Note: Words/phrases in blue will be links to specific diagnostic procedures when this document is published in Freshdesk.

NOTICE Gas pressure increases as temperature increases. When temperature decreases, pressure
decreases, but the amount of gas does not change.
An example of the effect of temperature upon fuel system pressure readings:
1. At the end of the workday a delivery truck is fast-filled at a CNG station. The ambient temperature is 120°F
(50°C), and the CNG dispenser stops at 3600 psi (250 bar) as expected.
2. The truck is parked in its designated spot. During the night, ambient temperature drops to 70°F (20°C). As the
gas cools, pressure in the cylinders decreases. The engine is not run while the vehicle is parked, so no fuel is
consumed.
3. The following morning, however, the high-pressure gauge indicates 3000 psi (207 bar) instead of 3600 psi
(250 bar).
4. Later in afternoon, when the ambient temperature rises to 120°F (50°C), the high-pressure gauge indicates
3600 psi (250 bar) once again.
NOTICE Fueling stations can partially compensate for the heat generated during fast filling, but
generally cannot achieve more than 70% to 80% full.
Slow filling (overnight) results in a nearly 100% full system, because the gas can cool during the filling process.
Slow fill stations are practical when fleet vehicles return to a home base at the end of the workday and can fuel
during the night.