2014 C300 with M276 engine 75,000miles. I have solved this issue and sharing in hopes that I can help someone else.

The issue started when the car was taking a bit longer than normal to start and a bit of rough idle. I checked under the hood and hear what sounded like a vacuum leak. Sprayed around the intake with some brake cleaner and got change in rpm when I sprayed near cylinder 1 intake runner under the intake manifold. I pulled the manifold and found what appeared to be a kinked cylinder 1 intake gasket. I fixed that and put it back together. Issue was resolved for only a short time and then I got a check engine light and felt a misfire. (Worse than before).

I should note that I had a shop replace all coils and spark plugs, and one bad fuel injector (cylinder 1) about 10,000 miles ago. There was also a return to that shop when a new coil failed. This same shop also did a compression check on the engine and found no issues.

I pulled the intake manifold again and pulled all the spark plugs and the coils to check for any failure signs. I noted that cylinder 6 plug looked darker than the others. At this point I decided to order a data tool (icarsoft) to help me troubleshoot.

Side note: I also need to note that in my research here and on other sites I went down the rabbit hole of oil in wiring harness. I did find oil in 3 of the 4 cam position sensors and in the ecu plug (no other connectors had oil) This added a lot of doubt for me in the process but I’ll just say now that I don’t believe this was the issue. I replaced the cam position sensors and added the pigtail connectors. I also cleaned the ecu plug and will be monitoring this every couple weeks.

When I received the data tool, I found that there was a misfire logged on cylinders 4 and 5. Cylinder 5 had more faults logged than 4. I found it odd that cylinder 6 had no faults (remember the plug was fouled before). I cleared the codes and ran the car again to see the same codes. I also found that the long term fuel trend was about -25 to -30% on bank 2 at idle. When driving normally it would climb to -10%ish so not good news. Given that there was misfire on cylinders 4 and 5, I bought some new spark plugs and changed cylinders 4, 5 and 6 (due to the fouled plug). This didn’t change the misfiring or negative LTFT. Then I went to the coils and replaced coils 4 and 5 (yes I know that I should have swapped them around first to see if the problem followed but I was pressed for time and wanted the parts on hand when I got back from a trip) Still no change. After that I ordered two new fuel injectors for 4 and 5. New seal kits for all of bank 2, seal removal/install tools. Pulled the bank 2 fuel rail and cleaned it with carb cleaner (this took a while to get all the off color varnish out) and compressed air. Installed the new injectors and reassembled everything. Started the car and the same issue returned. Getting frustrated! I had to take another trip for a couple weeks at this point so I had a lot of time to think about what might be wrong. I was thinking that maybe I had a faulty 02 sensor on bank 2 but the scan tool didn’t show it. I ordered one anyway. I also thought about checking spark plugs again because of the fouling on 6.

Well I retuned from my trip and decided to pull the coils/plugs again first because that’s way easier than replacing an 02 sensor. Cylinder 4 and 5 plugs were clean. Cylinder 6 was dirty/fouled/wet again. I bought an inspection camera and checked the cylinders and noted more fouling in cylinder 6 but no damage. This got me thinking that cylinder 6 was the issue, but how could that be if there were no codes or misfires from that cylinder. The coil, plug, on cylinder 6 were new but the injector was old. If cylinder 6 had a clogged injector that was failing to close all the way, that would explain why the computer was pulling 30% fuel from bank 2 but not the misfire on 4 and 5. However, remember that cylinder 6 is the closest to where the fuel comes into the bank 2 rail. If too much fuel is going into 6, then injectors 4,5 could be getting starved for fuel at idle and causing the misfire. That was my theory anyway. Well, I had two old injectors sitting there and I grabbed one and replaced cylinder 6 injector. The injector tip on 6 had plenty of carbon/fouling. Re-installed everything and started the car. Viola! No more misfires and my LTFTs started to return back to normal range!

I should have trusted my eyes when I saw cylinder 6 plug was fouled the first time. I hope this helps someone else out there!

 

Excellent write-up! Glad you found and corrected the problem.