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(Below is an slightly older text-only version of our installation manual. While it does include lots of solid information, for the full installation manual experience purchase one of the packages above.)
CNG Installs Explained
By Jim Younkin
“All cars can be converted but not all cars should be converted.”
– Motor age ASE training manual
Over the past 12 months I have been researching the CNG industry and learning how to install kits on cars and trucks. Over this period of time I have met and talked to CNG enthusiasts all over the United States and a few outside the US. They were drawn to CNG as the price of regular gasoline increased. This interest peaked a few months ago as the price of gas fell like a rock. Even though I was installing fewer kits, my interest in the CNG Industry was still strong. I have been able to find sources for kits, piping, fittings, and tanks. Others who were a little ahead of me on the learning curve helped me along the way. In this manual I will go step by step and explain my journey as I converted my first vehicles. I am writing this manual so hopefully you can avoid some of the misconceptions I had. Even if you don’t choose to install a kit yourself, you will be more knowledgeable of CNG systems and the entire install process.
Every person is responsible for what they do, if you choose to install a kit you need to familiarize yourself with the rules and regulations pertaining to the install. A standard of codes called “NFPA 52” will give you the safety regulations pertaining to the high-pressure tank, high pressure tubing, filler and PRD (Pressure Relief Device), pressure regulators, and other components. I will quote parts of the regulations but you need to get a copy of them for yourself.
If you don’t feel qualified to do an install after studying this manual by all means take your car to an expert.
Plan the Work, Work the Plan
One of the problems I had early on was trying to figure out where to mount all of the components. The major components that need to be mounted are:
High Pressure CNG Tank
High Pressure Supply Line
2 Stage Pressure regulator
CNG Controller Switch (inside cab)
CNG Mixer (Aspirated Kit) (Diesel/CNG Kit)
CNG Injectors (Sequential Kit)
CNG Filler Valve
CNG Inline Filter (if applicable)
CNG Check Valve (if applicable)
CNG Pressure gauge (for CNG fuel gauge inside car)
CNG Control Unit
Emulators (Aspirated Kits)
First I look at the engine compartment and visualize where I might like to put things; the regulator will need to be in a place where you can plumb the high-pressure line easily. Also, take into account the cooling lines to the CNG Regulator.
Next decide where to mount the CNG Filler Receptacle; I have seen it mounted inside the truck bed, in the bumper, in the grill or under the hood. The regulations for mounting the filler receptacle are here: NFPA52 6.9.1 to 6.9.4 “the fueling connection receptacle shall be mounted to withstand the breakaway force specified in 22.214.171.124…the device shall be able to withstand a 150 lb force”
Next, you need to decide where to put the tank. If you are installing the tank inside the car you will need to plan where to vent the PRD (pressure relief device) and CNG safety enclosure, this goes around all the pipe joints inside the car and vents straight down through the floor. The regulations for this are here: NFPA52 6.4.1 to 6.4.9 “CNG fittings shall be enclosed within a gastight enclosure…that is vented directly to the outside of the car.”
Before installing the high pressure line take into consideration where the exhaust pipe is, which side the regulator is on, and where to put the filter if you are using one. Regulations are here concerning clearance: NFPA52 6.5.6 “fuel should have the maximum practical clearance from the exhaust system”,
The filter needs to be mounted securely between the tank and the regulator.
The high-pressure tank needs a shut off valve and a PRD. The regulations are here: NFPA 6.6.1 “Every cylinder shall be equipped with either of the following…a manual valve or a normally closed, remotely actuated shutoff valve”.
You also need an electric shut off valve or a quarter turn valve to isolate the high-pressure tank in case of an emergency. This valve goes between the tank and the rest of the system. The regulations for this are here: NFPA52 6.6.2 “a manual shutoff (quarter turn) or a normally closed, automatically actuated shut off valve shall be installed to isolate the pressure container.”
All components need to be mounted securely, all tubing needs to be anchored, regulations are here: NFPA52 6.5.7
“Fuel lines need to be braced to minimize vibration and protected from damage”
Think Twice and Install Once
On my first few conversions I was in a hurry, I wanted it on right now and I ended up doing some of my work over and over. Look over the wiring, find common wires, and pick routing. Read through the instructions, twice. Some of the wiring you can combine like the power and ground wires in an aspirated kit, these can go to a common power and ground. Place the components in their respective places, check out where you can bolt the brackets, and after you have decided where to mount everything go to work!!
Installing the Tank and High Pressure line
As you are deciding where and which way to mount the tank take into consideration the filter, quarter turn valve and filler receptacle and how they will be plumbed. The filter needs to be between the Tank and the regulator, the quarter turn valve needs to be placed close to the high-pressure tank in order to isolate it.
If your tank didn’t come with mounts you will need to make them or have them made. I have made my own from 1.5″ X 3/8″ steel strap. The regulations for tank brackets are here: NFPA52 6.3.4 “the fuel container rack shall be able to withstand the force… in 6 directions up to…. 6 times the weight of the container when full”
The PRD will need to be vented to the outside if your tank is inside your car, the regulations are here: NFPA52 6.4.2, “the venting of the PRD should be made of metallic tubing, it should be secured and vented to the outside of the car”
Also if the tank is inside the car the fittings inside the car will need to be covered and vented to the outside the regulations for this are here: NFPA52 6.4.1 to 6.4.9 “CNG fittings shall be enclosed within a gastight enclosure…that is vented directly to the outside of the car.”
All components need to be mounted securely, all tubing needs to be anchored, regulations are here: NFPA52 6.5.7 Fuel “lines need to be braced to minimize vibration and protected from damage”
I like to install the front to back high pressure line in one long piece, I push the tubing through the engine compartment and go underneath the car and snake it through where I want it. Remember to keep it away from the exhaust and cover it with rubber fuel line if it goes through the floor or over a sharp corner. Loop the tubing into a small loop when hooking to components to allow for movement.
Some kits have 200 bar (3000psi) filler nozzles; other kits have 248 bar (3600psi) fillers. If you have a 200 bar nozzle then you can use 6mm steel CNG line and fittings, if you have a 248 bar filler you will need to use line and fittings that are rated for that pressure.
Some kits have a pressure gauge sender that screws into the filler receptacle; others have the gauge in the line on a ‘T’ or in the regulator. The closer the gauge is to the CNG control unit, the less wiring you will need to do.
Installing the Regulator
Once you have decided where to install the regulator, you’ll need to make some brackets for it. Most hardware stores have steel strap material, 1″ x 1/16″ this is easy to bend and drill. The regulator needs to be rigid, since the high pressure CNG line should not move around. The bracket should have support both up and down and side to side. Next, plumb the coolant lines to it, they need to tap into the heater hoses, be sure and install one of them between the heater control valve and the engine. NAPA has hose Tees for any size coolant line if the one in your kit is the wrong size (they usually are). Another factor in mounting the regulator is the position of the low-pressure natural gas outlet; it needs to point toward the engine for ease of routing the hose.
Installing the Mixer and Stepper Motor
On aspirated kits you will need to install a mixer into the intake inlet hose. This should be installed as close to the throttle body as possible. The mixer inner circumference should be half the throttle plate circumference for optimum performance.
The formula for finding circumference is C = 3.14 x D. Some mixers bolt right onto the throttle body inlet and others fit inside the intake hose before the throttle body. Be sure and secure the mixer inside the intake hose. You can use high density foam to fill in the space between the mixer and the hose wall. Secure with automotive sealant like ‘The Right Stuff’. On aspirated kits, you will need to install the stepper motor control valve in the low-pressure natural gas line between the regulator and mixer. It needs to point motor-side up or slightly left or right, but just in a way that moisture will not accumulate in the motor itself and damage it.
Installing the CNG Nozzles and Injectors
On Sequential kits you will need to drill, tap and install nozzles into the intake manifold. These should be as close to the original gas injectors as permissible. On plastic manifolds you may be able to drill the holes without even removing the manifold. On aluminum manifolds you will want to remove the manifold to keep the metal filings out of the intake. After drilling I used a tap and then “Loctited” the fittings into place.
All the hoses from the injectors to the nozzles should be the same length, and be sure to use clamps on both ends. Mount the injectors as close to the nozzles as permissible, I was able to construct a bracket out of metal joint pieces I found at the hardware store, they are normally used for joining pieces of wood together, they have lots of holes in them. I was able to mount my CNG injectors above the valve covers using bolts that were already on the engine. Be sure and use the correct nozzle size at the injectors, some kits comes with different sizes, some you need to drill out.
Splicing into the Gasoline Injector Harness
You may want to get a wiring diagram for the vehicle you are working on. You can get this from a shop that has ALLDATA or usually the local library has manuals you can photocopy. Some kits have their own injector harness with both male and female plugs. There are different plug types so be sure and get the correct ones for your vehicle. On kits that don’t come with harnesses you will need to splice into the harness yourself. On Aspirated kits it really doesn’t matter which injector wire you splice into since they are not looking at the injector signal. On Sequential kits you will need to tap into the negative (computer side) signal wire of each injector, all injectors have a common colored wire, this will be the positive wire, the negative wires will have a different colored wire for each injector, this will be the wire to tap into. As per the instructions you will need to match up the wiring (cylinder for cylinder) the CNG injector wires with the gasoline injectors. Do this using the wiring diagram provided with the kit. If you are hard wiring the injector wires, be sure and use soldier and heat shrink tubing. You can splice in close to the vehicles ECU or at the injector harness which ever is easiest.
1) Soldier all connections.
2) Use heat shrinks tubing to cover joint.
3) Diagram all wiring before cutting and splicing.
4) Install emulators and CNG ECU’s in protected, dry places.
5) Use relays in high amperage applications.
6) Make sure all circuits are properly fused.
7) Cover open wires with loom or tape. Give it a clean, neat, and professional look.
Finding the O2, Coil and TPS signal wires
O2 Sensor Signal Wire:
Some O2 sensors have 1 wire; some have 3 or maybe 4. The signal wire will have a voltage between 0.2-0.9 volts while running at operating temperature. What I do first is probe the wires with a DC voltmeter, with the key on – engine off. One wire will be 12V, two will be negative, and the last wire will be the signal wire (less than a volt). To be sure, probe it with a volt meter with the engine warm, and running, it will range up and down below a volt DC (between 0.2-0.9 volts).
Coil Signal Wire:
Whether a coil has two or four wires the signal wire (-) provides the RPM signal. If the engine has a coil for each cylinder the signal wire will be the one that has a different color than the other coils. This is where a wiring diagram and experience comes in handy. Most Ford vehicles need the RPM signal to come from the ECM of the Vehicle. You can also use the pulse signal from the injector wire; just make sure you tap into the side that goes to the ECM.
TPS Sensor Signal:
The TPS signal wire will vary in voltage as the throttle is opened and closed with the key on.
Wiring the Battery Positive, Ignition Positive and Grounds.
If you are doing a Sequential Kit you will only need a battery positive, ignition positive and ground. On an Aspirated Kit you will need these connections plus you will have to run grounds and CNG system positive wires to all the emulators. The CNG system positive is usually blue and the negative is usually black. Consult your wiring diagram for all exact positions. You may want to take the key switch positive at the TPS; this wire will be 12 volts with the key on. The Battery Positive can be taken from the battery or a fuse box in the engine compartment. The Ground should be a good, clean metal connection.
Cleaning Up Loose Ends
After all the major components are mounted, hook up all the remaining sensors and wiring. One of the last things I install is the CNG controller and fuel gauge. You will want to run the wires for the CNG Control Switch into the cab. Try to do so through an existing hole or boot in the firewall, and then find a suitable place on the dash to mount the switch. I usually choose a location that already has switches. Run the wires from the back through a hole. After mounting the switch be sure and secure all loose wires under the dash.
Filling the Tank, Checking for leaks
With all components installed and secured you are ready to fill the tank and check for leaks. Remember that you can always turn off the CNG at the tank. Fill up the tank, listen and smell for leaks. After filling the tank spray any suspected joint with soapy water, if bubbles appear you have a leak, turn off the gas at the tank, bleed off the excess gas and retighten the suspected fitting. If the leak doesn’t stop you may need to turn off the tank, bleed the system and redo the joint, remember stainless tubing needs stainless fittings.
Tuning the Kit
After filling the tank and repairing all leaks its time to tune the kit. As stated previously there are two types of gasoline/CNG conversion kits and also a CNG/Diesel aspirated kit. The tuning of each of these is different. As a general rule, after you have made sure there are no leaks and the electrical is connected correctly according to the manufacturer’s diagrams, you can then go forward with the initial tuning. The basics are as follows:
Sequential CNG Systems:
1) Initial Calibration: this may include the CNG ECU learning and looking at the gasoline tuning at idle then at a higher RPM for a predetermined period.
2) Fine Tuning Calibration: this usually involves driving the vehicle while you are connected to the CNG ECU and watching the MAP Pressure and Gas Injector Pulse and either automatically or manually matching the CNG Injector Pulse to the Gasoline Injector Pulse during differing loads and conditions.
3) Final Tuning: this includes matching the fuel gauge sender to the CNG ECU program, saving the final tune to your laptop, checking for trouble codes and modifying the CNG tune as to eliminate problems and a final leak test and road test.
Aspirated CNG Systems:
1) Initial Calibration: this may include the CNG ECU learning and looking at the gasoline tuning at idle then at a higher RPM for a predetermined period. You may be told to watch LED lights and follow a certain predetermined tuning sequence
2) Fine Tuning Calibration: this usually involves driving the vehicle, making sure there are no trouble codes and adjusting the idle. Usually there is an idle screw on the regulator. On some Aspirated systems you have the ability through a tuning program to adjust the RPM that the CNG turns on, also adjust the maximum and minimum openings of the CNG stepper motor.
3) Final Tuning: this includes checking for trouble codes and modifying the CNG tune as to eliminate problems and a final leak test and road test.
Aspirated Diesel/CNG Systems.
1) Initial Calibration: on system that are mechanical which only use a mechanical valve and a 2 stage regulator tuning is done by driving the vehicle and determining the idle setting in order to get fuel economy and additional power without overheating the engine. Those Aspirated Diesel/CNG Systems which use an adjustable stepper motor have the ability to be programmable at different RPM’s and Throttle Positions. These systems take longer to program initially but provide adaptability to differing loads and speeds.
2) Fine Tuning Calibration: this usually involves driving the vehicle, making sure there are no trouble codes and adjusting the idle. Diesel/CNG run the risk of overheating if the CNG/Diesel ratio is too great.
3) Final Tuning: this includes checking for trouble codes and modifying the CNG tune as to eliminate problems and a final leak test and road test.