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Hi everyone, my car has been fitted with OPF and CAT delete downpipes, now I have 250cell CATs down the line in the exhaust, I am also running a stage 1 tune.

The car had been running great, but latéy an issue appeared:

It happens only in 4th gear around 4200rpm, under any throttle position, the engine starts to shake extremely, as if it hit some sort of limiter and will not rev past 4200rpm… if I let go of the throttle immediately, no control light… if I stay in the throttle for longer, a check engine light comes on flashing for a while, then disappears…(in the diagnostics it reads misfire of about 4 cylinders-I dont remember which ones exactly)…if i use kick-down, the car will downshift past 4200rpm and rev up to the limiter no problem… performance is there, no smoke, no vibrations, no faults…

Has anyone experienced this before? If yes what was the issue and how did you solve it?

I went to a mechanic who said it was bad fuel with sediment in the tank and clogged injectors… he sucked out the bad fuel, changed some injectors and cleaned the rest with ultrasound, but it didnt fix the issue…

Thanks in advance for any information on this?

I had a misfire on my C8 for around 18 months, it was intermittent and really manifested itself at lower revs (a little above idle) in damp conditions, initially Poole Audi and Audi UK suspected the charcoal cannister system and replaced all of this from their research, which didn't resolve the problem, they then tried various other fixes such as changing plugs etc. none of which made any difference, they finally changed most of the injectors, but after a year and a half and the car off the road for around 20% of that time, I got fed up and sold the car.
Sorry I cannot be of more use, but wish you good luck with tracking it down.