Ya Allah have to go all over it again. im not, just a summary. (i own one so i noe how it goes)
Mis-firing. the engine over heats.
makes the car a misery
If you never drive it on petrol another world of miseries. (yes we didnt drive it on petrol but i guess this wud hapen to all efis)
fuel tank dirty.
fuel pump dies.
fuel filter.
fuel injectors.
Premature plugs failure.
A normal car has 4 plugs wires (all dependent on there own creating equal current)
While a waste spark system as in Baleno/Liana/Swift/Santro/Laner has 2 coils and 2 wires in which the 2 coils are responsible for them selves as well as the other 2 cylinders.
On CNG the car generates about 8volts while on petrol is generates about 12volts.
As the current isnt sufficient, the coils cannot full fill the current thus its not fully transfered to the dependent cylinder. causing misfiring and pre-mature failure of coils. (each coil costs around 6-12k depending upon make and quality (chinese to genuine ones and kabuli ones).
The case gets works once your plugs become weak. Its not bad if ur plugs are new.
Experts say iridium plugs solve the problem as there designed for waste spark systems.
I havent tried em as a set cost 4.5k but its worth it as iridiums can drive upto 100,000kms.
One more thing. Washing a cars engine doesnt makes any difference. Never wash it. it would just mess up the electronics