CNG Pressure Regulation and Fuel Delivery
What you must know
Fuel moves from storage at up to ~3,600 psi through a high-pressure line to a regulator that drops pressure in stages - often a high-pressure regulator reducing to an intermediate pressure, then a low-pressure (or zero-pressure) regulator feeding the injectors or mixer. Because expansion cools the gas (Joule-Thomson effect), regulators and lines can frost; coolant is often routed through the regulator to prevent ice formation and seal damage. Know the function of the high-pressure lockoff/solenoid, the low-pressure lockoff, and the fuel pressure/temperature sensor that reports to the ECM.
Traps
A classic distractor is "low power because the tank is empty" when the real cause is a restricted high-pressure filter or a failing regulator that can't maintain pressure under load. Another trap: diagnosing a frozen regulator as an electrical fault. Also remember that as cylinder pressure drops, the regulator must still deliver stable low pressure - a sagging fuel pressure under load points to supply, filtration, or regulation, not the injectors first.
Rule of thumb
Check fuel pressure at the test port under load, not at idle. Idle-only checks hide restrictions that only appear at high demand.
Worked example
A bus loses power climbing a grade; idle is fine. Fuel pressure at the low-pressure port drops from spec 90 psi to 55 psi under load. Diagnosis: supply-side restriction or weak regulator - inspect/replace the high-pressure filter and test the regulator before touching injectors.