Audi TT 1.8T Power Loss & Smoke Points to Knock Sensor

I had a very similar issue with my 2005 Audi A4 1.8T. As someone with some mechanical experience working on turbocharged engines, I can share what worked for me. The symptoms matched exactly, power loss, jerking during acceleration, and smoke. After inspection, my workshop found significant carbon deposits in the combustion chamber affecting engine performance. The cylinder heads needed thorough cleaning, and the knock sensor was sending incorrect signals due to the deposits. The fix involved: Complete fuel system cleaning, Removal and cleaning of cylinder heads, Replacing spark plugs, Recalibration of the engine management system Total cost was 425 Euro and resolved all issues. The engine tuning returned to normal, and the knock sensor started functioning correctly again. Before suggesting specific solutions, could you share: Current mileage, Fuel type typically used, When was the last time spark plugs were changed, Any recent changes in performance or maintenance, Complete error codes from the ECU This information would help determine if your issue is similar to what I experienced with carbon deposits or if there might be other underlying problems.

Last service was at 27500 KM. Further investigation revealed detonation issues caused by engine knock, which triggered the knock sensor protecting the engine from severe damage. The piston slap sound became more pronounced when the engine was cold. Using higher octane fuel temporarily reduced the symptoms, but did not fix the root cause. The combustion chamber inspection revealed heavy carbon buildup, similar to your experience. My mechanic confirmed the deposits were causing irregular combustion and affecting engine timing. Currently scheduling the recommended cleaning and sensor calibration. Would you mind sharing the specific cleaning process used for your cylinder heads? Planning to show those details to my mechanic.

Thanks for the additional details. The cleaning process on my A4 was quite thorough and matches what you are describing with the carbon buildup issues. The workshop used a two-stage cleaning approach for the combustion chamber: 1. Chemical cleaning with specialized carbon dissolving agents while the heads were still mounted 2. Complete removal of cylinder heads for manual cleaning and inspection The piston slap sound you mentioned is concerning though. In my case, the knock sensor activation was purely due to carbon deposits, but your symptoms suggest possible additional wear. Make sure your mechanic checks the piston clearances while the heads are off. Important: Use only fuel with 98 octane or higher after the service. Lower octane grades can accelerate deposit formation, especially in turbocharged engines prone to engine knock. The total process took about 8 hours of labor. Key steps included: Fuel system flush, Removal of intake manifold, Cylinder head cleaning, New gaskets installation, Knock sensor testing and calibration, ECU adaptation reset After 15000 KM since the service, no recurring issues with engine knock or performance. The combustion chamber has stayed clean by maintaining proper fuel octane levels.

Thank you for the detailed cleaning process information. This helps a lot with my repair planning. I scheduled the service for next week and purchased some 98 octane fuel as recommended. The piston slap noise has gotten slightly worse, so the timing of this repair seems crucial. My mechanic confirmed they will follow a similar cleaning protocol for the combustion chamber and cylinder heads. They will also perform a thorough inspection of piston clearances while the engine is apart, given the concerning sounds. The ignition timing issues and knock sensor warnings make more sense now, knowing how carbon deposits can affect these systems. Really hoping this service resolves both the performance issues and that worrying piston slap sound. Will update the forum once the repair is complete and share the results of the combustion chamber cleaning. Thanks again for the detailed breakdown of your experience, it helped confirm I am on the right track with this repair approach.