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Why would a bad valve only show itself at idle? It's smooth at other times.
That is exactly my point. I have exactly the same issue with my 2010 Smart 451. Everybody says burnt exhaust value, but why does it run smoothly at higher RPM with lots of power. My tests have indicated that the rough idle begins after the Secondary Air Pump stops and closed-loop mode begins. My conclusion is that some sensor is monitored in closed-loop mode that modifies the fuel injection and timing advance. My OBD scanner shows timing advance goes wild in closed-loop, after being stable in open-loop. Thus, the exhaust valves don't seem to be involved, because they don't change when entering closed-loop mode. I have been troubleshooting this issue for about 6 months, but I have not found the root cause yet. Still working on it.
 

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No idea "why" but it can certainly be a burned valve. Our 2011 ran fine with one for several thousand miles before getting new head and valves. :)
 

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2011 with 89,000 miles idles rough and has all misfire codes P0300-P0303 stored, although the CEL isn't on. Will check/change the spark plugs first off. Then the valve adjustment. Anything else to check?
Service dude at MB told me to run 87 octane for a tankfull. Sometimes the High test doesn't get sold as much
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Garry have you ran a compression test to be sure it isn't a compression problem? You might have a vacuum leak or anyone of several things that can cause a rough idle. If you have a burned valve it will randomly give you such as P0303,P0302,P0301 etc,...When I bought my 2011 Fortwo it was the first Smart I had ever driven or even ridden in. Within a couple days of purchasing I got the check engine light and had misfire codes. I couldn't even tell it had a miss. The computer is very keen at picking up a cylinder misfire. It is amazing how a modern engine detects misfires. The ECU monitors engine RPM between cylinder firings and when there is a minuscule drop in RPM it knows what cylinder was firing at that moment. After the ECU reads that RPM change a certain number of times in a given time span the ECU determines a misfire. But my P0303 cylinder 3 misfire was happening and I didn't notice it at all. But then I never drove the car before it had this problem so I couldn't tell. The secondary air pump pumps air into the exhaust for about 30 seconds to cut down on emissions until the heaters in the 02 sensors heat enough to accurately read exhaust gases. It's sort of splitting hairs on emissions. If your scanner does live data you can check the pulse timing on the fuel injectors and also firing voltage to the spark plugs. Higher firing voltage on a cylinder is usually a lean cylinder. Try to narrow down which cylinder is misfiring and then you can switch out the injector, coil pack, or spark plug to see if the misfire changes cylinder. But at any rate start with a compression test to eliminate a burned valve. Sorry this answer is long, I tend to overexplain, but just trying to help. DCO
 
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