Month: July 2016

CNG Engine Fuel Filter R&R


⇒ Spin Type oil filter wrench

⇒ CNG Fuel Filter Socket (FFW113 – Fraser Gauge)

⇒ 1/2 Ratchet

⇒ Adjustable Wrench – optional

FFW-113, CNG FIlter tools, CNG R&R
CNG Filter Tools


⇒ Ensure you are familiar with the system you are working on. Most 40′ plus foot bus manufacturers use the same standard layouts. Smaller buses and cutaways may have various layouts.

⇒ The working pressure of a CNG system is 3600psi. Depending on temperature of the day the system pressure can be as much as 4400psi.

⇒ Keep flames at least 100 feet away from any CNG bus.

⇒ Eliminate any source of static electricity.

⇒ Conduct work on ventilated area.

⇒ Wear proper personal protective equipment.


Tech Info:

The recommended PM intervals vary by manufacturer, the typical change interval is 6000 miles. The life of the filters will be affected by the oil residue from the pumping station. It is very important to drain the filters on a regular basis, to remove oil contamination.

Depending on the transit bus you are working on, one of the following will apply:

⇒The system has a high pressure a low pressure coalescing filters.

⇒The systems has a high pressure coalescing filter and a low pressure spin-on type.


CNG, Filters, High pressure, Low Pressure
CNG Low Pressure and High Pressure Fuel Filters


1. Shut-off the fuel supply to the engine by closing the ¼ turn fuel shut-off valve. This valve in most cases will be RED in color; you may encountered a few that are BLACK in color. Regardless of color the valve will be label as “Emergency Fuel Shut-Off Valve”.¼ turn ball valves are in the OPEN position when handle is in line with the fuel line and in the CLOSED position is perpendicular to the fuel line.

fuel shut-off, 1/4 turn valve, CNG
Emergency Fuel Shut-Off Valve


2. Once the valve is in the OFF position. Start the engine, let it run until it stalls. Once the engine stalls, check the pressure at the gauges. Both high and low pressure gauges should be at 0psi.

CNG Filter R&R
0 Psi. Both Gauges

3. Once the pressure has been verified to be 0psi at both the High and Low Pressure gauges, wait 10 minutes and recheck pressure ensuring that it has remained at 0psi. Waiting ensures that the fuel shut-off valve is not leaking. If it is leaking the pressure will rise within minutes. if pressure has remained at 0psi, proceed to replace filters.

Note: Some vehicles do not have a low pressure gauge and some vehicle do not have both gauges located in the fueling panel. The low pressure gauge may be located in the engine compartment.  

Warning: this point is very critical, you will encounter buses that once the emergency fuel shut-off valve is turned off part of the system remains under high pressure, to ensure your safety shut off the fuel supply at each tank; then verify 0psi of pressure.


4. Remove high pressure filter housing using designated tools, if the correct socket is not available, use an adjustable wrench. Change filter element and housing O-ring, lube O-ring to prevent tearing during installation. Torque to specified value.

HP filter, coalescing, CNG filter R&R
High Pressure Filter R&R


5. Remove low pressure filter using designated tools. If the low pressure filter is coalescing type use same procedures as for the high pressure filter. If it is a spin-on type, change filter assembly using designated tools; lube filter O-ring to prevent tearing during installation.

CNG Filter, spin-on filter, kow pressure filter
Low Pressure Filter R&R


6. Once the filters have been changed, is time to pressurize the system. Open ¼ turn fuel shut-off valve, do so in 3 stages. Allow pressure to stabilize between stages. Opening in stages prevents pressure pulsations that can damage components.

Fuel Shut-Off Valve, 1/4 turn valve, CNG valve
Fuel Shut-Off opening


7. Check for leaks once full pressure has been reached. Use a methane leak detector or a liquid type leak detector to find leaks. If leaks are found depressurize the system before making repairs. Start engine, verify proper engine operation.