My old TQS did something similar. Initially it was gradual, just a flatness after 5k and a sort of missfirey feeling if you tried to rev it out. Logging showed nothing unusual bar a bit of timing pull. I did the usual plugs/coils/MAF etc. but it just seemed to get worse and worse over time, then very quickly deteriorated to the point where it couldnt make more than 5psi even with the actuator line completely disconnected fom the turbo, timing pull was off the scale and i was properly scratching my head.
In the end i popped the lamdba probe out the exhaust, and boost went from 5psi up to about 8. The exhaust was so blocked that the lambda probe hole massively increased the available exhaust flow.
Pulled the cat out and the monolith had disintegrated and was jamming the outlet. Another telltale that i missed was the inside of exhaust tailpipes had gone a sort of white color, rather than their usual sooty-black. Replaced the cat and boom, power returned.
However, my track car is exhibiting similar symptoms, goes properly flat at 5k and feels like its lost about 100hp. Change gear and power returns instantly. It has a decat though, so its not a cat issue on that one. My current thoughts on that is that its the intercooler heatsoaking and ECU going into protection mode, so i've got a bigger cooler for it and will see how that goes.
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