Smart ForFour Engine Issues: Possible Cam Sensor Fault

As someone with decent hands-on experience, I faced almost identical symptoms with my 2003 Smart Fortwo. The engine started showing signs of misfire and poor performance, particularly during acceleration. After some initial troubleshooting, my workshop diagnosed a faulty crankshaft sensor, which was causing incorrect engine synchronization. The sensor wasnt sending proper reference values to the ECU, leading to timing issues and poor engine performance. The repair involved: Complete diagnostic scan, Removal and replacement of crankshaft sensor, Clearing fault codes, Test drive to confirm proper engine operation Total cost came to 340€, including parts and labor. After the fix, engine performance returned to normal and fuel consumption improved significantly. However, similar symptoms can have different causes depending on the specific model and engine configuration. To provide more targeted advice, could you share: Engine size and type, Current mileage, Any recent repairs or maintenance, Specific behavior of the check engine light (constant or intermittent), Whether you notice any particular patterns when the problems occur This information would help determine if your issue matches what I experienced or points to something else entirely.

Latest service was at 89000 KM, about 3 months ago. Your description actually matches my symptoms perfectly. The engine synchronization issues started gradually, especially noticeable during highway merging where the power loss became obvious. The check engine light remains constantly on, not flashing. The workshop did a basic check during the last service but found no immediate issues. Since then, the engine performance has gotten worse and fuel consumption increased by about 20%. Your insight about the crankshaft sensor makes sense, especially since the misfire seems more pronounced during acceleration. The camshaft timing also feels off as the engine occasionally runs rough at idle. Would you recommend trying to clear the fault codes first, or should I go straight for the sensor replacement given the similarities to your case?

Given how closely your symptoms match what I experienced, I would not recommend just clearing the fault codes as they will likely return. The consistent engine light and performance issues, especially during acceleration, strongly point to timing-related problems. In my case, attempting to clear codes without fixing the underlying sensor issue was ineffective. The engine light came back within a few miles since the faulty crankshaft sensor continued sending incorrect reference values to the ECU. The gradual onset of issues matches my experience exactly. It started with subtle performance loss and eventually developed into significant power reduction and increased fuel usage. The timing belt was still in good condition, but the sensor failure caused the engine synchronization problems. Since youre at 89000 KM, its right around when these sensors often start failing. The cost I mentioned earlier (340€) might vary slightly now, but its a straightforward repair that resolves the symptoms when properly diagnosed. Based on how similar your situation is, I suggest getting the crankshaft sensor checked and replaced if faulty. The rough idle you mentioned further supports this diagnosis, as incorrect timing signals can cause unstable engine operation at all speeds. Just ensure the workshop performs a complete diagnostic scan first to confirm the exact fault codes before replacing parts.

I really appreciate your detailed response and recommendations. Based on all the information, I agree that clearing codes alone would be a waste of time. My symptoms align too closely with your experience to ignore, especially the timing-related issues and misfire patterns during acceleration. The fact that these sensors commonly fail around my mileage point makes perfect sense. The consistent engine light and progressive performance decline matches the pattern you described exactly. The crankshaft sensor theory explains both the power loss and rough idle issues I am experiencing. I will schedule an appointment with my workshop and specifically request a complete diagnostic scan focusing on the engine timing and crankshaft sensor readings. The 340€ repair cost seems reasonable considering the fuel I am wasting with the current poor performance. At least now I can go to the workshop with a clear understanding of the likely issue rather than guessing. Really helpful to hear from someone who dealt with such similar symptoms and got them properly resolved. Will save this information for reference when I take it in.