How do i fix cylinder 4 missfire after engine decarbonization with watter in a vw new beetle 1.6L 8-valve?

Engine decarbonization with water? Wow, there is so much to check here, loose piece of carbon stuck in a valve, clogged up fuel injector, fouled spark plug, blown gasket from the sudden change in temp. I do not recommend even introducing water into a running engine, so much can go wrong, too much and you risk bending a rod or valve.

You need to remove the spark plug on that cylinder and see if it is fouled by carbon. While you have it out run a compression test to see if carbon is preventing a valve from seating. You may have also destroyed an O2 sensor. Water decarbonization is an old school way of removing carbon but it must be done correctly or you can cause serious engine damage. This should only be done by a professional and is not a DIY job.

thanks guys for the help, i just checked the spark plug, and it was fouled with carbon.

i cleaned up and put it back. it works fine now.

if you use small amounts of watter, likes spraing with some device, u will not cause any issue in the engine. offcourse the engine has to be warm and in 2500rpm soo the watter being trasformed to stem.