3.11. Fittings and Connections

3.11.4. Fitting Safety Considerations:

3.11.1. Tubing
All tubing is stainless steel manufactured according
to ASTM A-269 or ASTM A213. It is cold finished,
bright annealed, seamless tube. Tubing can be 316
SS or 304 SS, maximum hardness HRB90.

Do not bleed the CNG system by loosening or
cracking a fitting connection.

Do not disassemble new fittings prior to use.

3.11.2. Fittings
This system is plumbed with Swagelok® stainless
steel compression tube fittings designed specifically
for use in CNG applications.

Do not attempt to torque fittings while system is
pressurized.

3.11.3. Tube and Fitting Installations
Installing tube fittings correctly requires adequate
training, correct parts, and appropriate tools. Tubes
must be bent accurately to minimize system stress.
Fittings must be assembled according to
manufacturer’s specifications. For these reasons it
is mandatory that maintenance personnel receive
proper training from an authorized Swagelok®
distributor.
Do not mix tube fittings from other manufacturers;
this may result in plumbing failures causing leaks,
damage, or serious injury.

ENP-756 REV E 8/3/2022

When tightening a compression nut on a fitting,
always hold the fitting body is held securely with a
second wrench.

Figure 23. Always use two wrenches when loosening or
tightening a fitting.

(Printed and Downloaded Copies are Uncontrolled)

24 of 64

Gillig CNG Five Cylinder System Operation Manual

4. Operation and Maintenance
Routine maintenance of the compressed natural gas fuel system will help ensure the system and its components are
functioning properly.
System must not be under pressure during servicing. Doing so may cause serious injury.

This manual is intended as a supplement to training in operation and maintenance of the CNG fuel system.
Attempting to operate or maintain the CNG fuel system without proper training is dangerous.

Do not expose plastic or elastomeric materials to commercial cleaning products. Wash CNG components with a
diluted soap solution and rinse thoroughly to remove deposits.

CNG components and fittings are designed to be used exclusively in CNG applications. Parts must be ordered
directly through Hexagon® Agility® or through an approved Hexagon® Agility® OEM dealer. Any other parts may
be unsafe and will subsequently void the warranty. All replacement parts and components must receive approval
from Hexagon® Agility® prior to installation.

4.1. Inspecting Components
Daily
Walk around vehicle and check for external damage
to CNG cylinder housings and covers.
Monthly
 Verify all PRD vent lines have end caps securely
in place. A PRD vent line with a missing cap may
indicate a PRD has activated. If a PRD has
activated, contact the maintenance department or
Hexagon® Agility®. If the cap is simply missing and
the PRD has not activated, replace missing cap(s).
 Verify all system components are solidly mounted.
 Verify fuel supply tubes and flexible lines are in
good condition.
 Verify no fuel leaks are present at any fittings or
components.
 Verify there are no coolant leaks at the pressure
regulator hose connections.
 Visually inspect cylinder mounting structural
components.

Annually
 Check cylinder covers for damage.
 Verify cylinders are mounted securely. Check
mounts, brackets, and all fasteners.
 Verify cylinder labels are in place and make
sure cylinder service life has not expired.
 Inspect cylinder valves and PRDs for leaks and
damage.
 Inspect all plumbing tubes, hoses, and fuel flow
components for leaks.
 Examine all cylinders for damage using OEM
cylinder guidelines. Inspection records should
be kept with vehicle records, and the system
label should be updated to reflect the current
inspection status.
 Test supply solenoid valve.
 If equipped: Test CNG cylinder solenoid
valves.
Every year regardless of mileage
A certified (CGA C-6.4) or other qualified CNG fuel
system inspector shall perform cylinder and fuel
system inspection.

 Inspect fastened joints for loose fasteners.
 Inspect mounting bolts for damage.
ENP-756 REV E 8/3/2022

(Printed and Downloaded Copies are Uncontrolled)

25 of 64

Gillig CNG Five Cylinder System Operation Manual

4.2. Maintenance Schedule
Procedure

4.3.3. Testing for Leaks

Table 5. Maintenance schedule.

Frequency

Drain LP filter

Daily*

Replace LP filter

Every 15000 miles (24000 km)*

Replace HP filter
element

Every 30000 miles (48000 km)*

Leak test fuel
system

Every engine oil change

Component
inspection

Every engine oil change

*Intervals may vary depending on fuel quality,
station condition, and vehicle use.

4.3. Leaks
This section covers leak detection,
replacement, and leak repair methods.

fitting

Adjusting fittings and tubing requires accredited
Swagelok® or Hexagon® Agility® training.

4.3.1. Tubing Bottoming Requirements
To achieve a safe leak free seal, tubing must be
flush and fully bottomed in the appropriate fitting.
Please consider the following recommendations:
1. Deburr tubing so it sits flush in the fitting.
2. Tubing must be round; elliptical or warped tubing
should not be used.

Fittings and connections should be checked
periodically for leaks. This task must be carried out
by certified personnel.

For monthly maintenance, the entire fuel system can
be leak tested with an inert gas, a leak detection
solution, and/or a methane detector. All joints and
connections should be tested.
Generally, leaks will occur at fitting connections.
When checking for leaks always use leak detecting
equipment and solutions designed for use with
stainless steel and brass such as Swagelok®
Snoop®.
To Find a Leak:
1. Apply leak detecting solution and look for
bubbling or foaming at the site of the leak.
2. Examine connections for icing or signs of
condensation around the tubing.
3. Check for signs of damaged or perforated tubing.
4.3.4. Tightening Leaking Fittings
Never tighten a fitting while the system is under
pressure. Always follow proper safety and service
procedures when fixing a leak. Failure to adhere to
safety standards and procedures may result in
system or property damage and/or personal injury.

3. Tubing must have a minimum length straight
section before a bend radius as specified in the
Swagelok® tube fitting manual.

1. If a leak is detected, depressurize the system as
described in Section 4.4, “Depressurizing
System.”

4.3.2. Tightening Fitting Nuts

2. Once the system is depressurized, tighten the
leaky fittings. Repressurize the system as
described in Section 4.5.

When pre-swaging tubing, Swagelok® recommends
fitting nuts should be tightened “1-1/4 turns beyond
snug.” Snug is defined as the point at which the tube
cannot be rotated freely in the pre-set swaging tool
or fitting body. Swagelok® fittings employ a twoferrule design; the front ferrule provides the seal, and
the rear ferrule provides a firm grip on the tubing.

ENP-756 REV E 8/3/2022

3. Once the system is repressurized, conduct a leak
test.
4. If leaks are still present, depressurize system
again and repair any leaks. Refer to Section 4.3.6.

(Printed and Downloaded Copies are Uncontrolled)

26 of 64

Gillig CNG Five Cylinder System Operation Manual

If any components are loosened or replaced, an
excessive amount of fuel is being consumed, or the
vehicle was involved in an accident, a pressure test
using nitrogen is required before re-filling with
natural gas.

1
4.3.5. Pressure Test Using an Inert Gas
(Nitrogen):
1. Before proceeding with the nitrogen pressure

test, verify the system has been depressurized.
Refer to Section 4.4.

2. Once the system is fully plumbed or repaired,

verify engine ignition is in the “OFF” position and
vehicle is at least 15 feet (9 m) away from an open
flame or other source of ignition.

3. Verify all five—one on each cylinder—manual

cylinder shutoff valve knobs (a) are in the
“CLOSED/OFF” position. Figure 24

Figure 25. FMM 1/4-turn manual shutoff valve (1) operation.
NOTE: Non-Smart Gauge equipped FMM shown; Smart
Gauge face plate similar.

5. Connect pressure testing equipment to the

system fueling receptacle using a NGV1 fueling
nozzle plumbed to a nitrogen source with a flex
hose rated to the system service pressure of 3600
psi (250 bar) and a pressure regulator. Slowly fill
the system with nitrogen to a pressure of 100 psi
(6.89 bar). While filling the system, listen for
leaking. Stop if leaks are detected. Figure 26
1

Manual cylinder valve

a

3

2
Figure 26. FMM (3), NGV1 defuel/fill receptacle (1), NGV1 defuel/fill
receptacle dust cap (2). NOTE: Smart Gauge equipped FMM face
plate shown; non-Smart Gauge face plate is similar.

Electric solenoid cylinder valve

6. Perform 100 psi (6.89 bar) leak test using leak

detection solution or an equivalent alternate
method.

a
Bleed nitrogen slowly to avoid possible injury.
7. If a leak is detected, disconnect nitrogen fill line

from FMM NGV1 receptacle (Figure 26), and
depressurize the system by venting nitrogen
using the bleed valve (Figures 15 & 23) and repair
it.

Figure 24. Manual cylinder valve and electric solenoid cylinder
valve manual shutoff knobs (a).

4. Turn FMM 1/4-turn manual shutoff valve (1)

clockwise to the “ON/OPEN” position. Figure 25

ENP-756 REV E 8/3/2022

8. If no leak is detected, slowly increase the

pressure to 1500 psi (103.4 bar). Increase the
pressure at no more than 500 psi (34.4 bar) per

(Printed and Downloaded Copies are Uncontrolled)

27 of 64

Gillig CNG Five Cylinder System Operation Manual

minute until a system pressure of 1500 psi (103.4
bar) is reached.
9. Perform 1500 psi (103.4 bar) leak test using leak

performed and successfully passed with no
leaks, vent nitrogen to the atmosphere using the
bleed valve on the Manifilter. Figure 27

10. If a leak is detected, disconnect the nitrogen fill

Verify all safety guidelines from Safety Section 1 are
followed prior to fueling system with CNG.

detection solution or an equivalent alternate
method.
line, depressurize the system, and fix the leak.

11. If no leak is detected, continue to slowly increase

the pressure to 3600 psi (250 bar). Increase the
pressure at no more than 500 psi (34.4 bar) per
minute.

12. Once the system is pressurized to 3600 psi (250

bar), perform another leak test.

13. If a leak is detected, disconnect the nitrogen fill

line from FMM NGV1 receptacle (Figure 26),
depressurize the system and fix the leak. Refer to
Section 4.4.

14. After all the nitrogen leak tests have been

performed, the pressure shall remain constant at
3600 psi (250 bar) for 10 minutes with no signs of
a pressure drop.
If system pressure drops locate and fix any leaks.

DO NOT TURN THE ENGINE ON to activate the
solenoid valve for the next part of the test. Use an
external 24V power supply; do not use the vehicle
onboard power supply.
15. At this point, the system has been properly

pressure tested up to the solenoid valve (5).
Figure 9.

To test the connections downstream of the
solenoid valve, apply a 24V power supply to
open the valve. This allows nitrogen to flow
through the rest of the system and up to the
secondary solenoid shutoff valve at the engine.
Perform leak test on all connections downstream
of the fuel outlet.
16. If a leak is detected, disconnect nitrogen fill line,

depressurize fuel system, and repair leak.

17. Once the 100 psi (6.89 bar), 1500 psi (103.4 bar)

and 3600 psi (250 bar) tests have been

ENP-756 REV E 8/3/2022

4.3.6. Repairing a Leak
Verify system is depressurized before attempting to
repair leak.

During the leak test or pressure test, leaks may be
caused by several issues. To repair a leak found
during either test, perform repairs in the order shown
below.
1. Once the fittings have been reinstalled perform
pressure test procedure.
2. If the leak cannot be repaired: replace the
component.
3. If replacement is necessary: follow OEM
procedures for replacing damaged parts and
ensure
correct
replacement
parts
are
requisitioned and installed.
If the repairs listed in Section 4.3.6 do not fix the
problem, please contact Hexagon® Agility® Product
Support.

4.4. Starting Vehicle
Starting a CNG vehicle requires a delay between the
battery power being turned on and the starter motor
being activated.
1. Verify all five cylinder manual shutoff valves (one
on each cylinder) are “ON/OPEN.” Figures 24,
33a, and 33b
2. Verify FMM 1/4-turn manual shutoff valve (5) is
turned to the “ON/OPEN” position. Figure 27
3. Verify standard Gillig pre-vehicle
procedures are followed.

start-up

4. Start engine.
5. If vehicle is starting from cold, let engine idle for
five minutes. This allows coolant to warm the fuel

(Printed and Downloaded Copies are Uncontrolled)

28 of 64

Gillig CNG Five Cylinder System Operation Manual

and ensure low-pressure lines (down-stream of
the pressure regulator) do not freeze.

5

3

4

On extremely cold days allow vehicle to
idle for a longer period until coolant
temperature is high enough to warm fuel.

4.5. Depressurizing CNG System Tubing

2

The following procedure DOES NOT depressurize
the CNG storage cylinders.

1
This procedure depressurizes fuel system tubing
and the FMM panel ONLY and is for use with manual
cylinder valve or electric solenoid cylinder valve
systems.
This procedure does not require electrical power
except to run the engine. Electrical power to the
electric solenoid cylinder valve is not required to
conduct this procedure.

Figure 27. Bleed valve location in the FMM.
Bleed valve (2), Manifilter (1), HP gauge (3),
LP gauge (4), 1/4-turn manual shutoff valve (5).

The system is now fully depressurized up to the
supply solenoid valve.
When loosening fittings for the first time, a small
amount of gas may leak out of any fitting
downstream of the supply solenoid valve; this is
normal.

1. If equipped: Turn manual cylinder shutoff valves
(one on each cylinder) clockwise to the
“CLOSED/OFF” position. Figure 24

4.6. Re-Pressurizing System

2. Turn FMM 1/4-turn manual shutoff valve (5)
counterclockwise to “ON/OPEN” position. Figure
27

2. Verify bleed valve (2) on Manifilter (1) is closed.
Figure 27

3. Start vehicle and run engine until it stops.
4. Verify vehicle is off and that proper vehicle lockout procedures are followed.
5. Check FMM fill panel gauges to verify all the
pressure is relieved. Figure 27
Some pressure will remain in the system.

6. Relieve remaining pressure by SLOWLY opening
HP fuel filter (1) bleed valve (2) counterclockwise.
Figure 27

1. Verify vehicle power is OFF.

Torque bleed valve to 4 ft-lbs to 5 ft-lbs
(5.42 Nm to 6.78 Nm).
3. Verify Manifilter bowl (3) and drain plug (2) are
installed and tightened. Figure 28
4. If removed: Re-install fuel fill panel cover.
5. Slowly turn manual cylinder shutoff valves (one
on each cylinder) counterclockwise to the
“OPEN/ON” position. Figures 24, 32, 32a, and
32b
6. Slowly turn FMM 1/4-turn manual shutoff valve (5)
to the “ON/OPEN” position. Figure 27
7. Verify pressure test procedure was performed if
any parts were loosened or replaced. Refer to
Section 4.4.
8. Start engine.

ENP-756 REV E 8/3/2022

(Printed and Downloaded Copies are Uncontrolled)

29 of 64

Gillig CNG Five Cylinder System Operation Manual

4.7. Maintaining Manifilter HP fuel filter
Regular Manifilter high-pressure (HP) fuel filter
element replacement is important for maintaining
CNG fuel system performance. In general, HP filter
elements should be replaced every 30000 miles
(48000 km).
Element replacement interval may be more frequent
depending on fuel station cleanliness and fuel
quality.

4.7.1. Draining the Manifilter Assembly
Drain the HP fuel filter before changing the filter
element.
1. Depressurize fuel system. Refer to Section 4.5.
2. Slowly remove Manifilter drain plug (2) using an
11/16-in. wrench. Figure 28
1

5

3
2

4

Figure 28. Manifilter assembly FMM location and drain plug.
Manifilter housing (1), drain plug (2), bowl (3),
solenoid valve (4), pressure regulator (5).

3. Drain Manifilter (1) until no liquid or residue
remains. Figure 28
Collect any fluid and dispose of properly.
4. Inspect drain plug O-ring. Figure 29 Obtain a