Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Fuel System Diagnosis (CNG): Description
The Powertrain Control Module (PCM) controls the fuel system while the vehicle is operating on CNG. The PCM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. The fuel is stored in a cylindrical fuel tank. A full tank can contain fuel at 24,820 kPa (3600 psi). A High Pressure Lock-Off (HPL) solenoid in the tank and a Low Pressure Lock-Off (LPL) solenoid in the fuel line prevent fuel flow. The PCM commands ONLY the HPL open for one second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed to charge the fuel line and also allows the Fuel Tank Pressure (FTP) sensor to monitor the amount of fuel pressure in the tank. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates the engine is cranking or running on CNG.
This fuel delivery system utilizes two different pressure stages to lower the fuel pressure from tank pressure down to an injector delivery pressure.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.