Ok guys I sadly don't have my code scanner, somehow left it in my mom's car and going to grab it tomorrow.

I am getting an intermittent misfire on my 2012 CLS550 (m278). I have an OE Tuning Stage 1 tune on it right now, but honestly I baby it 99% of the time and this issue has seemingly gotten worse. If I can't diagnose any problem I will put the stock tune back on and see if I can find a problem.

The first time it happened it was at cold start, idling. It started up just fine and then started idling really rough and I realized it was misfiring. Shut it down and restarted it and it was fine. Threw an upstream O2 sensor code

Few weeks later, happened again but this time at hot idle, at a stoplight in like 110*F heat. Same deal, pulled it over - turned the car off waited about 60-90 seconds and restarted it. Drove the rest of the way home fine and couldn't reproduce the issue. Threw a misfire code but I don't think it gave me

Starting today after the car sat for a week or so, it is now way more common, today it happened 3 times. Each time same deal, if I pull over, turn it off and wait a minute or two, then turn it back on. Then it is fine for the rest of the drive, and I drove quite a lot today with several stops.

I came home and for the last hour I have been trying to get it to misfire so I can unplug each coil and see which cylinder is misfiring. And of course now it won't misfire and seems to be running fine.

I am going to pull the bottom cover and start investigating tomorrow. Specifically I have seen posts mentioning oil in the o2 sensor wiring harness/connectors, and some other connectors but I have zero leaks under the car and I do not believe I have an oil leak. Definitely possible the o2 sensors are fouled from the Stage 1 tune.

I am ordering all 4 o2 sensors, Spark Plugs, Coil boots, and 1 coil pack. In general I was going to do the o2 sensors and spark plugs but I am kind of just hoping I can throw the kitchen sink at it and solve the problem. I have never ever had a spark plug "go bad" or cause a misfire (and I have been building cars since I was a youth, lots of wrench time but all American cars and this is my first complicated euro car).


Anyone got any advice, or know if this is a common problem? I do not think this is tune related, as the fuel map at idle and lower RPM should be pretty similar. I drive a lot so it's not bad gas related (and I filled it up at a busy Shell station today just to be sure).
Or maybe best guess? I am thinking coilpack, or o2 sensor? Could it be another sensor?

Thanks in advance for any troubleshooting advice

 

What did you find out?

 

I would say definitely put the stock tune back on just to solve the issue. I had a different encounter, ended up with a bad injector. But the check engine light for me did not come on until I switched to the OEM tune file. I did have stage 1 from OE Tuning, and i don't have anything against them. The dealer replaced under warranty, as the emission system here in Cali mandated MB to have extended warranty 80k miles / 7 (or 8) years.

Mine started similarly to yours, rough idle when cold engine, would go away. Later got worse. So stock tune + a good scanner should point to the right direction.

Regards