I have gotten exhaust front-pipe (1J0254300P), and catalyst changed in my VW Golf MK4 BCB engine. The used part was an aftermarket part from BM-Catalysts.
Since then it has some vibrations between 1800-1900 rpm. It actually seem to have more vibrations in general but they peak between this range.
It is most noticable if I am going downhill on 2nd gear without pressing gas petal and car slows down with engine braking, when the RPM meter goes down slowly from 2000 to 1700...
Display shows "0 l/100km" so I know the ECU is not supplying fuel to engine. Therefore the issue can't be related to spark plugs/coils etc.
It is possible to feel it also in neutral when vehicle is not moving. If I get the engine revs around 1850RPM. So I know the issue is not related to drivetrain etc.
Before installation I inspected the catalyst and it did not have any moving parts inside. Actually it was quite cool to see catalyst element :)
I am pretty sure that the issue is related to the front pipe. Perhaps the flex piece on the front pipe has a fault or a leak somewhere. That is my guess based on some forum threads I found online with similar issues.
First, I am not sure why flex piece or a leak would cause more vibrations on certain RPM range only. Can anybody explain the mechanism behind this?
Also I am not sure exactly how this can be diagnosed with 100% certainty. Front pipe goes under the engine so it is difficult to reach and inspect when the engine is running.
Any ideas on what would be the best approach for a diagnosis?