RS3 Timing Issues Lead to Major Performance Drop

Had nearly identical issues with my 2017 RS4. Started with poor performance and ended up with the check engine light. As someone with basic maintenance experience, I initially thought it was just a timing belt issue. After multiple error codes pointing to engine synchronization problems, took it to a specialist workshop. They diagnosed a faulty crankshaft sensor that was sending incorrect reference values to the ECU. The cam sensor readings were fine, but the crankshaft position sensor was giving erratic signals, causing the ECU to get confused about engine timing. Total repair came to 340 Euro, including diagnostics and parts. The workshop had it fixed within a day, mainly labor costs for accessing and replacing the sensor, plus testing to ensure proper engine synchronization afterward. What engine variant does your RS3 have? Also, have you noticed any specific patterns when the symptoms occur (cold starts, highway speeds, etc.)? Could help narrow down if its the same issue I faced. The symptoms you describe sound very similar, especially the increased fuel consumption. Would recommend getting the crankshaft sensor checked specifically before looking at more complex timing issues.

Last service done at 175000 KM included major maintenance items, but notably not the timing chain inspection. The shop tested both sensors as suggested after finding similar cases to yours on technical bulletins. The crankshaft position sensor tested within spec, but they found a misfire pattern that points toward a timing chain stretch issue. The timing chain tensioner also shows abnormal wear patterns. Looking at repair quotes ranging from 2800 Euro to 3200 Euro for a complete timing chain service including guides and tensioner. The engine synchronization issues seem to be getting worse, especially during cold starts when the chain tensioner is under most stress. Before proceeding with such an expensive repair, would be helpful to know if anyone had success with just replacing the tensioner alone? The shop recommends doing the complete job since they need to open up the timing cover anyway.

Thanks for the update on your RS3 situation. When my RS4 initially showed similar symptoms, I also hoped for a simpler fix like sensor replacement. However, your symptoms actually sound more severe than what I initially experienced. From what you describe about the cold start issues and deteriorating engine synchronization, this really points toward a timing chain system problem rather than just a sensor issue. The misfire pattern your shop found matches what happens when the timing chain starts to stretch, it affects the precise synchronization between the crankshaft and camshaft timing. While replacing just the tensioner might seem like a cost-saving option, I would strongly advise against it. When I faced a similar decision on my previous car, I tried the tensioner-only route and ended up paying twice, once for the tensioner and then again for the complete timing chain service just 15000 km later. The timing chain, guides, and tensioner work as a complete system. If the chain has stretched enough to cause timing issues, the guides have likely worn as well. The complete 3200 Euro service might seem steep, but its cheaper than risking engine damage from a failed timing chain. Make sure they do a thorough inspection of the cam sensor and crank sensor wiring while they have everything apart. Sometimes the timing chain issues can mask or combine with sensor problems.

Thanks for your detailed feedback. The cold start issues definitely make more sense now as a timing chain problem. After discussing it with a few other RS3 owners, I decided to go ahead with the complete timing chain service. You are right, trying to save money by only replacing the tensioner would likely lead to more expenses down the road. The shop found significant wear on the timing chain guides once they opened everything up. The chain had stretched beyond spec, which explains the engine light and performance issues. The total repair came to 3100 Euro including a new timing chain, guides, tensioner and associated gaskets. They also checked both the cam sensor and crank sensor connections while the engine was apart. All wiring was good, confirming the timing chain was indeed the root cause of my synchronization problems. The car now runs perfectly, no more check engine light, performance is back to normal and fuel consumption has returned to expected levels. Looking back, catching this before total failure likely saved me from much more expensive engine repairs.