Hello... i have a 2002/03 vitz running on CNG for last 1.5years... it was all doing fine but last week it started vibrating at zero speed... so i have to put it in neutral otherwise engine used to stop.... so i got it checked from Nazir autos G-8... they told me that there is some issue with tappets and as it has hydraulic tappets hence cannot be adjusted but head needs overhauling..
after 3 days i got my car back with all the valves were replaced and it costed me nearly 14K to get it fixed.. as per mechanic, its normal with a car running on CNG... if i run my car on CNG to save money and in the end i m spending 15K per annum to get it fixed, i think its better to stick with petrol... in short i'll recommend not to use CNG if there isn't much difference as in long run, u have to pay more