Smart Diesel Problems Point to Faulty Heater Filter

I had a very similar issue with my 2006 Smart Forfour diesel about a year ago. Having some experience with diesel engines, the symptoms you describe match exactly what I encountered, power loss, rough idle, and increased fuel consumption. After noticing the engine stalling during cold starts, I took it to my regular workshop. The diagnosis revealed a faulty connection in the fuel filter heater wiring harness. The repair was straightforward and cost 95€, which included testing the fuel system and replacing the damaged connector. The issue stemmed from corrosion in the electrical connections, affecting the fuel filter heater operation. This caused poor fuel efficiency and engine performance issues, particularly during cold weather. After the repair, the engine ran smoothly again and fuel consumption returned to normal. To provide more specific advice for your case, could you share: Current mileage on your Fortwo, How long the check engine light has been on, Any diagnostic codes you might have pulled, Whether the issues get worse in cold weather, If you notice any unusual sounds from the fuel system These details would help determine if your situation matches what I experienced and could guide you toward the most effective solution.

Thanks for the detailed response. The rough idle and power issues started about 2 weeks ago at 57997 KM. The check engine light came on around the same time. I did get a diagnostic scan which showed code P0201 and P0202 related to fuel system problems. The symptoms definitely worsen in cold weather, particularly during morning starts. The fuel efficiency has dropped significantly, using about 20% more fuel than normal. I will have the fuel filter heater connection checked based on your experience. The repair cost you mentioned sounds reasonable. Did you have any issues after the repair was completed?

Thanks for the additional details. Those error codes P0201 and P0202 actually point to injector circuit issues, which could be related to the fuel filter heater problem I experienced, but might need a broader diagnosis. On my Smart Forfour, after the initial 95€ repair of the fuel filter heater connection, the workshop discovered some carbon buildup affecting the injectors. A fuel system cleaning service resolved the remaining rough idle issues completely. The total cost including both repairs came to around 280€. The cold start problems and poor fuel efficiency completely disappeared after both issues were addressed. My diesel engine returned to its normal smooth operation, especially during cold weather starts. Given your error codes and symptoms are nearly identical to what I experienced, I would suggest: 1. Having the fuel filter heater connection checked first 2. Getting the injectors tested and possibly cleaned 3. Consider a full fuel filter replacement if it has not been done recently The carbon buildup is fairly common in these diesel engines, particularly around your mileage point. The combination of a faulty fuel heater and dirty injectors can create a compound effect on engine performance.

Thanks for the thorough explanation. Your feedback has been really helpful. I went ahead and booked an appointment with a local workshop for next week. The symptoms match up perfectly with what you described, especially the connection between rough idle and cold weather performance. My biggest concern was the potential repair cost, but 280€ for both the fuel filter heater connection and injector cleaning seems reasonable considering the fuel efficiency issues I am experiencing. The diesel engine problems have been quite frustrating, especially during morning starts. I will make sure they check both the fuel line connections and injectors as you suggested. The fuel filter replacement also makes sense since I cannot remember when it was last changed. Will report back after the workshop visit to let others know what they find, in case someone else encounters similar issues with their Smart diesel. Again, really appreciate the detailed information about your experience. It helps narrow down the problem and gives me a good idea of what to expect at the workshop.