CNG experts

Without proper inspection, we are able to make only an educated guess that you may have a misaligned Tappet situation. On CNG-mode, you will hear a tick-tick-tick sound from the top cover of your engine block. It is the sound of tappet knocking on the Intake/Exhaust Valve Head.

Common symptoms associated with a misaligned Tappet are:
a. Engine vibrating
b. Poor Idling
c. Tappet knocking sound, tick tick tick

If you are experiencing misaligned Tappet problem, it is appropriate that you should understand how this problem come about too. Here it is:

When you operate in petrol-mode, you should not be able to hear the tick-tick-tick sound significantly. This is because petrol is a wet fuel, and does not dry up the lubricating engine oil along the IntakeExhaust valve shalf. The engine oil (and another petrol solvent Benzene) muff the sound, making it difficult to be heard by you.

Howeverso, you can hear it loudly when you operate in CNG-mode. CNG fuel is deemed as a dry fuel, and often dries up the engine oil along the Intake Valve Rod. After awhile, the drying oil engine can result to poor lubrication and finally, tappet starts to misalign. In addition, the knocking sound is NOT muffled out without lubricant , unlike with petrol above. Thus, you hear the knocking sound.

If you have a tappet misalignment, you are experiencing POOR piston compression. High compression is needed by CNG-combustion, but the misalignment caused poor compression. In this result, your engine will vibrate due to poor compression-combustion, and consequently poor idling. For high speed, your car will be underpowered as well.

Please take a look at your Tappet Alignment to confirm our deduction here. It can also be other problems, but tick-tick-tick sound is common to Tappet Misalignment. After all you are buying a used CNG-car, it is also likely that the previous owner did not maintain his CNG-performance after he sold the car to your brother.