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2014 C350 M276 6-cyl engine w/82K miles on the odometer.

I purchased my vehicle new in early 2015. A few months ago the following issue began. I started the car in the morning, I noticed that the idle was rough and, at the same time, there was a small, but noticeable "bucking" motion. After waiting 20 seconds for the oil to circulate, I put the car in gear and drove off. After just a minute or so, while the engine was still warming up, I came to a stop-light, the rough idle disappeared and the engine continued to idle and perform smoothly for the rest of the day. But then upon cold start the following morning, the problem re-occurred. A minute of rough idle, then smooth idling/cruising for the remainder of the day.

After a week I noticed the "check engine" light was on (steady, not blinking). I took the car over to the MB dealer and they diagnosed the problem as a "misfire". The service tech asked me what kind of gas I used (91 octane), and when I last changed my spark plugs (at the 70K service with 6 new iridium plugs). Ultimately, they cleaned the fuel system (which cost me a few hundred dollars) and cleared the engine warning light.

For a few weeks, everything was normal but recently, I've noticed the return of a small rough idle which, as before, goes away after a minute of driving. Also, the check engine light is back on.

Since this engine has GDI (Gasoline Direct Injection), I know that one potential problem could be excessive carbon buildup on one or more intake valves. I haven't had this checked yet and MB made no mention of this when I took it in for service.

I'm thinking of not doing anything at this point since the car runs fine after a minute warming up and I can live with the check engine light being on since I'm having no other issues. There has been no loss of power or fuel economy and I just passed a smog check a week ago. I can't see spending another few hundered bucks to have MB go through the same procedure again with no guarantee the problem will be fixed.

Any opinions or advice appreciated.
 

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2004 SL500, 2014 C350 Sport
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Rough idle could be a vacuum leak or a marginal coil (or any number of other things). Thermal expansion of plastics or metal could close gaps or change operation of electronics, etc. Since the plugs have been changed (and they should have inspected the intake valves at that time since you have to remove the intake manifold to access the plugs), they should have advised if the valves were heavily carbon.

Not sure if a failing or weak fuel high-pressure pump would cause misfires. It would be good to know the full extent of the codes. Usually there is a general multiple misfire code (P0300) and then the specific cylinder (e.g., P0304 for cylinder 4). You may also find this article helpful: http://www.autotap.com/problem6_engine_stalls.asp
 

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2004 SL500, 2014 C350 Sport
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I always strongly recommend that anyone who owns a Mercedes and wants to do their own work MUST get a scan tool that reads Mercedes codes. The cheapest best tool for doing that is the iCarsoft MB II for only $150 on Amazon. Get that and save yourself a trip to the dealer when you don't know what's going on.
 
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