I have a 2005 VW Jetta 2.5L (MK5, 5-speed manual) that is having issues while accelerating.
Car suddenly began accelerating very slowly throughout all gears. Started jerking/bucking around 2000 RPM; stops jerking when I let off the throttle or upshift (then again when the RPMs raise again). Flex pipe was replaced, started running fine again for a day, then the same issue started happening again.
The car started having acceleration issues and then threw a check-engine light (code P0411) which indicated a secondary air flow issue. I checked everything I could think of (air pump works, relays, etc.) and everything checked out. I haven't checked the combination valve yet, though because I'm not sure if this system would even cause these acceleration issues or not..
Eventually I noticed the flex pipe was extremely expanded and caused the exhaust to come off of its main mount between the flex pipe and catalytic converter. I have no idea what caused this, but I took it to an exhaust shop to have the flex pipe replaced and they assumed that it was hit by something while driving. They also noted that the material inside the catalytic converter is "loose" and may need replacing soon but they didn't know exactly how bad or if it was causing any issues.
After the flex pipe was replaced, the car drove perfectly fine on the way back home and for the rest of the day (maybe 10-20 miles total driving). On the way back home at the end of the day the same issue began again. Extreme lack of power, jerking when RPMs raise, etc.
So the obvious symptoms for car jerking/bucking while accelerating seem to be things like fuel injectors, misfires, fuel filter/pump, catalytic converter blocked, etc. Would any of these things be temporarily "fixed" by replacing the flex pipe and then suddenly start having symptoms again?
I recently replaced the spark plugs, and I haven't gotten any misfire codes this whole time. The only codes it has thrown is the P0411 but since the flex pipe was replaced this code has gone away. I stopped driving it so it's possible the code would come back but I don't want to drive it unless it's to test something.
What should I start with checking? Fuel injectors? Catalytic converter?