VW Fox Power Loss: Knocking Issues Exposed

I had a very similar issue with my VW Polo (2011, gasoline). Having some experience with engine issues, the symptoms you describe immediately remind me of what I went through. The engine knock and power loss turned out to be caused by carbon deposits in the combustion chamber. The knock sensor was constantly triggering, causing the ECU to retard ignition timing as a protective measure. This explained the power loss and rough running. My workshop did a thorough diagnosis and found: Heavy carbon buildup on valves and pistons, Knock sensor readings were off the charts, Pre-ignition issues due to hot spots from carbon deposits The fix involved: Chemical cleaning of the combustion chamber, Adjusting ignition timing, Testing knock sensor operation, Full diagnostic system reset Total cost was 425€ and took about a day to complete. The difference was immediate, smooth acceleration, no more engine knock, and proper power delivery. To better assist with your case, could you share: Current mileage? Type of fuel you typically use? Service history, especially regarding spark plugs? Any recent changes in driving patterns?

Thanks for sharing your experience. I just hit 21812km on the odometer. The symptoms are strikingly similar to what you described. Good to know about the carbon deposits being a potential culprit, especially since pre-ignition and engine knock can seriously damage the engine if left unchecked. After reading your reply, I checked my maintenance records. The knock sensor has never been replaced, and I have recently switched to lower octane fuel to save money. I realize now this might be contributing to the engine knock issues you mentioned. I will book a diagnostic session at my workshop and specifically ask them to check for carbon buildup and test the knock sensor functionality. The repair cost you mentioned seems reasonable if it solves these issues completely.

Thanks for the update. Your mileage is actually quite low for a 2011 Polo, which makes me think the fuel quality might indeed be the main culprit here. After my repair, I learned a lot about how fuel octane affects engine performance and knocking. Using lower octane fuel in our engines can definitely trigger detonation issues, especially if there are already carbon deposits in the combustion chamber. The cylinder heads are particularly sensitive to this combination. I switched back to higher octane fuel after my repair, and it made a noticeable difference in preventing knock sensor triggers. A quick tip from my experience: before going for the full repair, try running 2-3 tanks of high-octane fuel and a good fuel system cleaner. This helped reduce my symptoms temporarily, though I still needed the proper repair to fix the underlying issues. The workshop should definitely check: Carbon deposits on cylinder heads, Knock sensor readings, Fuel pressure, Ignition timing advance Let us know what the workshop finds. Based on similar VW engines, proper engine tuning might be needed after cleaning to optimize performance with your regular fuel type.

Your experience with the lower octane fuel really opened my eyes. I took my Fox to the workshop today, and their initial diagnosis confirms many of the points mentioned. Piston slap was ruled out, but they found significant carbon buildup in the combustion chamber, just as you suggested. The knock sensor readings were indeed abnormal, and the workshop confirmed that improper ignition timing was causing the detonation issues. They recommended a similar treatment plan to yours, including a thorough cleaning and engine tuning. I have already switched back to higher octane fuel and scheduled the repair work for next week. The estimated cost is 460€, slightly higher than what you paid, but includes a full ignition system check and engine tune-up. The technician explained how pre-ignition can occur with lower octane fuel, especially when carbon deposits create hot spots in the combustion chamber. This matches exactly what you described in your experience. Thanks for helping me understand these issues better. I will update the forum once the repairs are complete to help others who might face similar problems.