yeah man, CNG is somewhat troublesome for this new VVTI Corolla .. as one of my cousin had installed CNG in it and after 50000 km, he had to do work of valves (now i dont exactly know why or what was that) but 50000 for a Corolla engine is surely nothing and it shouldn't have given trouble at all, but my conclusion is that CNG doesn't work well with the VVTi engine, as is the case with new shape Vitz i guess :S