Undiagnosed Loss of Power and Strange Noise at High RPM: Audi S5 B8

[Ryreng]

Hello everyone, I need some help diagnosing a persisting issue with my Audi S5. For some context, it’s a 2010 Audi S5 B8 with the 4.2L N/A V8 FSI engine. The car has about 66,000 miles (106k km) on it. The only mods on it are a tune from JHM and a Valvetronic exhaust. I bought the car this year and as far as I know it had regular maintenance and oil changes.

For the past few months, I have been experiencing a strange issue with the vehicle. At high RPM (4-6k) and under heavy acceleration, the car emits a strange grinding noise and acceleration becomes hesitant, making it lose a significant amount of power. After driving the car hard and experiencing this strange noise, the engine (or exhaust, I still can’t find the source) emits an acrid burning smell. I’ve also been experiencing issues on start up- when the car is cold, it idles very rough and sometimes feel like a stalling manual when stopping/accelerating from a stop.

Formerly, the noise would sometimes be accompanied by a flashing check engine light (cylinder 3 misfire), but since having all ignition coils replaced the misfire has disappeared. I have also replaced the motor mounts and spark plugs recently as part of regular maintenance, but none of these repairs have fixed the issue.

The mechanic suggested that the next step would be looking at fuel injectors and performing a carbon clean, and from what I’ve read this sounds like a reasonable assessment. What should I do? Should I follow through with injectors/carbon clean? Can carbon buildup cause that weird grinding sound? I will link videos of the strange noise so you can get a better understanding of the issue. Thanks for the help!
Link to Reddit post with the videos: https://www.reddit.com/r/MechanicAdvice/comments/13ubn6v/undiagnosed_loss_of_power_and_strange_noise_at/?utm_source=share&utm_medium=ios_app&utm_name=iosc ss&utm_content=1&utm_term=1

[nhracecraft]

It's hard to tell from the video(s) exactly what the noise is (you'd definitely need to be in the car to discern that), but it's definitely NOT because it needs a Carbon Clean! That said, I wouldn't rule out you could have more than one issue due to the symptoms you're describing.

Based on what you're saying about the very rough idle/nearly stalling condition you're experiencing, I'd likely look at the HPFPs (High Pressure Fuel Pumps) and Cam Followers. I'd also run an Auto-Scan using VCDS to look for any related DTCs in the Engine Module. You can also use it to monitor certain parameters while idling OR driving to assist in diagnosing your issue, and any issues with your Injectors and/or the HPFPs would likely show up doing so. If you don't have access to VCDS, you should definitely purchase it, as it's an invaluable tool for anyone that's ever had an issue w/ an Audi, which is EVERYONE that has an Audi!

Due to the grinding noise (and the burning acrid smell), I'd also consider it could be something to do with the Exhaust and/or Driveshaft. If the Transmission Mount and/or Carrier Bearing for the Driveshaft have, or are failing, under maximum torque/heavy acceleration the Driveshaft could be making contact somewhere. It 'could' also be something to do with the 'valvetronic' actuator(s) vibrating/fluttering causing the issue. I'd get under the car and take a close look at everything.

Not sure why the 'mechanic' would be suggesting Carbon Clean (and Injectors) at 66K other than it's a great service from the billing side...Not so much for the guy paying the invoice though! Your S5 'might' benefit from a Carbon Clean, but that's not why your experiencing the grinding noise for certain!

[Benzin971]

I have this exact same issue directly after flashing the JHM tune.

You ever find out what was causing it?