VW Passat Rich Fuel Mix Causing Power & Economy Loss

I had a very similar issue with my 2018 VW Golf with the 2.0 TSI engine that turned out to be a faulty fuel injector. The symptoms matched yours exactly, rough running, power loss, and poor fuel economy. The check engine light prompted me to get it checked. The workshop diagnosed excessive fuel delivery from a defective injector, which was causing the rich mixture. The oxygen sensor readings confirmed the rich condition. They found one injector was stuck partially open, flooding that cylinder. The repair involved: Diagnostic testing of all injectors, Replacing the faulty injector, Cleaning the intake manifold as it had carbon buildup, Replacing spark plugs that were fouled from excess fuel Total cost was 515€ for parts and labor. Since the fix, fuel economy returned to normal and the engine runs smoothly again. Before suggesting specific fixes for your Passat, could you share: Engine size/type, Current mileage, Are there any other warning lights? Have you noticed any unusual exhaust smoke? Has any recent work been done on the fuel system? This information would help determine if you are facing a similar injector issue or if other components like the catalytic converter might be involved.

Since my service at 65000km they checked all sensors and replaced the spark plugs as part of maintenance. In my case, the issue started gradually over 2-3 weeks. The exhaust has a slightly darker smoke and smells strongly of fuel. Looking at the diagnostic report, I see P0172 code which confirms the rich mixture reading. The engine temperature sensor and oxygen sensor values look normal though. Could the fuel pressure regulator be the culprit here? The air-fuel ratio seems way off despite having new spark plugs and clean sensors. My fuel economy dropped from 7.2L/100km to almost 9L/100km. It seems to get worse when the engine is warm, especially during highway driving.

Thanks for the additional details. Based on your P0172 code and symptoms, this does sound different from my injector issue, especially since it developed gradually rather than suddenly. The fact that it worsens with a warm engine and highway driving strongly points to the fuel pressure regulator. When I encountered a failed regulator on my Golf, it showed very similar behavior, the pressure would creep up as the engine got warm, leading to an increasingly rich mixture. The normal oxygen sensor readings suggest the sensor itself is working correctly and detecting the rich condition. Since your intake manifold and air filter are likely still clean from the recent service, and the spark plugs are new, we can probably rule those out. What worked for my car: Replaced the fuel pressure regulator (180€), Cleaned the mass airflow sensor while it was apart, Had them verify proper fuel rail pressure After fixing the regulator, my fuel consumption returned to normal and the air-fuel ratio balanced out perfectly. The catalytic converter thankfully hadnt been damaged by the rich condition. Before you take it in, you might want to check if your fuel pressure regulator vacuum line is properly connected, sometimes a loose or cracked line can cause similar symptoms and its a much cheaper fix.

Thanks for the helpful details about the pressure regulator. My mechanic confirmed your diagnosis was spot-on, the fuel pressure regulator was the root cause, just as you suspected. The vacuum line had deteriorated and the regulator itself was failing. After replacing both components, the rich mixture issue cleared up completely. The P0172 code is gone, fuel economy is back to normal at 7.2L/100km, and the engine runs smoothly again. Total repair cost was 210€ including labor. The mechanic also checked the fuel injectors and oxygen sensor while they had it apart, both tested fine. Really glad I didn't need to replace those more expensive components. The catalytic converter also escaped any damage from the temporary rich condition. Appreciate you sharing your experience as it helped me better understand the issue before taking it to the shop. The symptoms matching your previous regulator problem made it easier to diagnose correctly the first time.