It is the sense of the Senate that the Environmental Protection Agency should streamline the process for certification of natural gas vehicle retrofit kits to promote energy security while still fulfilling the mission of the Clean Air Act
I see that this topic has resurfaced and, having read through all 131 postings so far, I think there are few things that need some clarification. Commercial CNG injection conversions have already been available for modern, fuel injected vehicles for several years now. The better aftermarket systems (Impco/BRC, Prins, Technocarb, etc) do NOT fool the vehicle's PCM and do not interfere with the vehicle's other emission control systems either. However, fixes (like fuel rail pressure simulators for returnless fuel systems) are sometimes required for some vehicles. Although certified CNG conversions can be expensive because the conversion system manufacturer may become the vehicle's manufacturer of record if it changes the vehicle's PCM programming and/or emission controls, the components themselves are not that expensive. An economy 8-cylinder CNG injection parts package currently costs $1663 for all of the underhood components. Although inexpensive, parts packages are not recommended for the DIYer as no tech support is included in the price and the typical DIYer generally doesn't have the training to ensure that the system complies with NFPA 52.
CNG injection conversions work by intercepting the signals sent by the PCM, modifying them, and then rerouting them to the CNG injectors. Other than adding CNG injector nozzles to the intake manifold and interfacing with the engine's wiring harness, no changes are made to the engine so all existing systems remain intact and fully functional. The CNG system's ECU has built-in injection map that, when correctly set up, ensures that the fuel trims on CNG are exactly the same as they would be on gasoline. Although CNG should have it's own ignition timing strategy, the CNG system's ECU makes no changes to ignition timing as this would constitute tampering. When operating on CNG, more advance is allowed (as programmed by the vehicle OEM) because CNG's high octane number prevents the engine's knock sensors from retarding the timing.
Any EPA or CARB-certified CNG conversion starts off as a universal CNG conversion. After vehicle-specific programming (fuel maps, etc) and mounting brackets are created and the system is lab-tested, it can become a certified system. The problem is the that the lab-testing is extremely expensive and the certificates have to renewed annually. Potentially, if a gasoline-only vehicle won't pass the EPA's alternative fuel testing requirements (like the SHED test), the conversion system manufacturer would have to also provide fixes to the gasoline system to ensure that the converted vehicle meets those EPA requirements. Because of the low demand for certified systems, the expense of obtaining and maintaining certification prevents aftermarket conversion system manufacturers from offering certified conversion kits for all but the highest demand fleet applications. I know that people on this forum and other CNG forums are clamoring for CARB or EPA-certified systems but, if they actually put their money where their mouth is, there would be no shortage of certified systems.
As for older carbureted and pre-OBDII vehicles, CNG conversions are extremely straight-forward. Typically, these have done as bi-fuel systems because of the limited range of the CNG cylinders but straight (mono-fuel) conversions are doable. The engine-specific components (mixer & regulator) are very similar to propane conversions but are more challenging to do nowadays because the necessary adapters and electronic fixes are becoming obsolete.
For a discussion about tampering and the EPA, see Unraveling the EPA Code, follow the money...