Smart Forfour Rough Idle & Jerking: Diesel Injection Fix

I had almost the exact same issue with my 2008 Smart Fortwo diesel. As someone with moderate experience working on cars, I initially thought it might be related to the fuel injectors, but it turned out to be a leak in the exhaust system. The symptoms were really similar, rough idle, engine misfire and uneven running, especially when cold. The oxygen sensor readings were all over the place because of the leak, which triggered the check engine light. Took it to my regular workshop and they found a significant crack in the exhaust manifold. Total repair cost was 265€, including parts and labor. The fix took about 4 hours and solved the problem completely. Before we get too far into specifics though, could you share: Current mileage on your Forfour, Any recent maintenance history, Whether you notice any exhaust noise or smell, If the problem gets worse during acceleration This info would help determine if your issue is similar to what I experienced. Sometimes these symptoms can also point to ignition coil problems or timing belt issues, so its worth checking those components too. A proper diagnosis is crucial since exhaust leaks can affect engine performance and potentially damage other components if left unaddressed.

I apologize for not mentioning earlier, my current mileage is 179658 KM and last service was done 3 months ago. The symptoms definitely get worse during acceleration and there is a noticeable exhaust smell, especially when the engine is cold. Based on my mechanic checking it yesterday, he suspects an engine misfire related to either faulty fuel injectors or a failing crankshaft sensor. He mentioned testing the fuel pump and oxygen sensor as well since they could cause similar symptoms. Has anyone had their fuel injectors replaced on a similar model? Looking to get a rough idea of potential repair costs for budgeting.

The exhaust smell and symptoms getting worse during acceleration matches exactly what I experienced. While my mechanic initially suspected fuel injectors too, the crankshaft sensor and oxygen sensor readings were misleading due to that exhaust leak I mentioned. Before replacing expensive components like injectors, definitely have them pressure test the exhaust system. In my case, the manifold crack was causing false readings that mimicked injection problems. The engine misfire symptoms were actually caused by exhaust gases leaking before reaching the sensors. For reference, if it does end up being injectors: my with the same Smart model had all injectors replaced last year, running about 850€ total. But given your symptoms mirror mine so closely, especially the cold start rough idle and acceleration issues, I strongly suggest ruling out exhaust leaks first. That uneven engine running could also point to a timing belt issue, particularly at your mileage. When was the last timing belt service done? Might be worth checking as a preventive measure while diagnosing the main problem.

Based on the diagnostics my mechanic did and the feedback here, I think I need to explore both possibilities. The exhaust leak theory makes a lot of sense given the smell and cold start symptoms, but the fuel injectors could still be the culprit considering the age of my car. Going to have them check the exhaust system first since its the less expensive option. The engine misfire symptoms combined with that exhaust smell does point strongly in that direction. My timing belt was actually replaced during the service 3 months ago, so at least that is not a concern. Will schedule the exhaust inspection tomorrow and have them run another test on the fuel pump and crankshaft sensor while they are at it. Really hoping its not the injectors given that 850€ price tag mentioned for replacement. Will update once I know more. Thanks for all the detailed feedback, definitely helped narrow down the potential causes and gave me a clearer picture of what to expect cost-wise.