T-Roc Running Rich: Rough Idle and Poor Performance Fix

I dealt with very similar symptoms on my 2020 VW Golf GTI. With some experience working on VAG engines, I can share my recent repair story. The symptoms matched exactly, poor fuel economy, rough idle, and the check engine light indicated a rich mixture condition. My air-fuel ratio was way off, causing the engine to run poorly, especially during cold starts. After diagnosis, the problem turned out to be faulty fuel injectors. One injector was stuck partially open, flooding that cylinder with excess fuel. The repair involved: 1. Full diagnostic scan 2. Fuel injector flow testing 3. Replacement of 2 defective injectors 4. ECU adaptation procedure Total cost was 515€ for parts and labor. After the fix, fuel consumption returned to normal and the engine ran smoothly again. Given your symptoms, I would strongly suspect injector issues, but several other components could cause similar problems. To help diagnose more accurately, could you share: Any specific error codes from the check engine light? Does the problem get better as the engine warms up? Have you noticed any unusual exhaust smoke? When was your last spark plug change?

Last service was at 45000km and showed all clear. Since then, I have noticed a slightly sweet smell from the exhaust and black smoke occasionally. The engine runs better when warm, but the check engine light remains on constantly. The catalytic converter temperature sensor readings look normal according to my OBD scanner. The fuel trim values suggest the system is adding more fuel than normal, but I am not seeing any obvious vacuum leaks around the intake manifold that could explain the rich mixture condition. Getting around 2.5L/100km worse fuel economy than before these issues started.

Based on your latest details, especially the black smoke and sweet smell, this sounds very similar to what I experienced with my Golf GTI. These additional symptoms strongly suggest unburned fuel, a classic sign of a rich mixture condition. The fact your fuel trim values are off while the catalytic converter temperature readings are normal narrows things down. In my case, before replacing the injectors, we also checked the oxygen sensor since it directly affects fuel mixture. A faulty O2 sensor can cause the ECU to misjudge the air-fuel ratio, leading to similar symptoms. My mechanic followed this diagnostic path: 1. Tested oxygen sensor performance 2. Checked spark plugs condition 3. Performed fuel injector pulse testing 4. Verified fuel pressure readings The spark plugs showed heavy carbon fouling from the rich mixture, so we replaced those too. This often happens when excess fuel prevents proper combustion. Since your symptoms improve with engine warmth like mine did, and given the similar fuel economy impact, I still suspect either faulty injectors or a failing oxygen sensor. Getting the specific error codes would help confirm if you are seeing the same fault patterns I had. Have you noticed if the black smoke is more prevalent during acceleration or at idle?

The black smoke is definitely more noticeable during acceleration, and thanks for the detailed diagnostics path. I ran another scan and got error codes P0172 (System Too Rich Bank 1) and P0171 (System Too Rich Bank 2), which seems to match what others are experiencing. The fuel trim readings are at +25% on both banks, suggesting the ECU is really struggling to maintain the correct air-fuel ratio. Looking at the maintenance schedule, my spark plugs are actually due for replacement soon anyway, so I will start there since its a cheaper fix. If that doesnt solve it, I will have the oxygen sensor and fuel injectors tested in that order. The carbon fouling you mentioned makes sense given the black smoke, and could explain why performance improves once the engine warms up and burns off some deposits. Just to make sure I understand the testing process, they should check the oxygen sensor signals first, then test injector spray patterns and flow rates? And I assume they will need to clear the check engine codes and verify the fix with another scan after repairs?