CNG Fuel Injection and Engine Management
What you must know
Dedicated CNG engines use gaseous injectors (often at the intake port) driven by the ECM with pulse width based on mass air flow, manifold pressure, fuel pressure, fuel temperature, and O2 feedback. Because gas displaces air in the intake, fueling strategy differs from liquid fuel: the ECM must account for gas density changes with pressure and temperature. Know the sensors: MAP, MAF (if used), IAT, ECT, fuel pressure/temperature, throttle position, and wide-range or switching O2 sensors. Know that injector flow is rated at a specific pressure, and that a clogged injector causes a lean misfire on one cylinder.
Traps
Don't confuse a rich/lean mixture code with a failed injector - check fuel pressure and sensor rationality first. A common trap is treating CNG injectors like gasoline injectors for resistance/flow testing; use the OEM-specified test. Another is ignoring fuel composition/temperature effects on mixture. Also expect questions on the fuel shutoff during decel (DFCO) and on injector driver faults setting cylinder-specific codes.
Rule of thumb
Single-cylinder misfire with a lean code = suspect that cylinder's injector or its driver circuit. All-cylinder lean = suspect supply pressure, sensor, or a vacuum/intake leak.
Worked example
P0302 (cylinder 2 misfire) plus P0171 (lean bank). Swap the cylinder 2 injector to cylinder 3; the misfire follows to cylinder 3. Diagnosis: faulty injector, not ignition or compression.