Issued June 2020
Compressed Natural Gas (CNG) Option
Copyright 2020, Heil Environmental
Printed in U.S.A.
222
DEPRESSURIZING PROCEDURE
It is necessary to prepare the truck to be serviced. A
mechanic’s initial focus while preparing the vehicle for
service should be safety. The primary preparation involves
relieving the pressure within the system BEFORE
performing any maintenance procedures, including draining
or changing the high-pressure filter. Use the following
procedure to remove fuel pressure from the lines
connected to the high-pressure filter assembly.
WARNING
After following the Depressurization Procedure, pressure
may still remain downstream of the solenoid valve,
including inside the fuel cylinder(s). Use care when
loosening fittings for the first time. DO NOT open any
cylinder Manual Shut-Off Valves after any CNG fitting,
connection, or component is loosened or disassembled
WARNING
Never weld on a compressed natural gas vehicle unless
the compressed natural gas fuel system has been purged
with inert gas.
1. Make sure that the ignition is turned OFF.
DEPRESSURIZING PROCEDURE
(CONTINUED)
2. Close ALL cylinder Manual Shut-Off Valves (one on
each cylinder) by turning the valve clockwise to the
OFF position. See the image below.
Figure 62. Cylinder
Manual Shut-Off
Valve
3. Verify that the FMM Manual Shut-Off Valve is in the
ON position.
4. Start the vehicle and let the engine run until it stops.
5. Turn the vehicle ignition switch OFF. Follow the
manufacturer’s recommended vehicle lock-out
procedures. Remove the ignition key.
6. Make sure the high pressure gauge on the FMM
reads 0 psi.
7. Remove the FMM access panel.
8. Slowly open the bleed valve to relieve the remaining
pressure.
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