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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?

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It appears the issue was the products of BM Catalysts. They were made badly and did not fit properly. I got the front pipe and catalytic converter now changed with VW genuine items and everything is working properly.

When I compared the genuine parts with BM Catalysts parts. The difference in workmanship was huge. It looked like BM Catalysts took many shortcuts trying to build super cheap part. It was apparently simply a case of got what I paid for.

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