I just recently purchased a 2007 E550 that i thought to be in excellent condition:
Mileage: 93,000
Engine code: 273962 30 059198
Have a car fax with over 40 service records, one owner, dealer serviced, was clearly someone's baby..

Additional notes: Engine light ONLY comes on during STARTUP. light has never flashed while car is driving

The problem began when I fired up the car one night and the check engine light came on as i started it. I scanned it and a sole P0303 code popped up. (misfire in cylinder 3) The code was UNABLE to be cleared at all. To start, i swapped the coil from cylinder 2 over to 3, and restarted the engine. the code was still not able to be cleared. I then purchased a new spark plug, installed it, and got the same results. I then purchased a new fuel injector, and after installing, the code WAS able to be cleared. After clearing the code, i drove for roughly 400 miles before again, on a cold start, the check engine light came back right as the car was starting. Same code, P0303. I cleared it, which I was able to do, unlike before, and drive for as much as i could. No light, but when i scanned it a few days later, the code had reappeared, but had NOT triggered the check engine light. the code was also listed as "pending" on the reader, which I found interesting, so i did not delete this code to see if it would eventually trigger the light again. What i found even more interesting was that after re-scanning the car a few days later, was that the code DISSAPEARED altogether, without me deleting it. After this indecent, i started wondering if the vehicle's electrical system was having a hiccup, but i was also relieved that at least the code went away on its own?? Since then, I have driven another roughly 200 miles if not more, and today, again, on startup, the code re-appeared again, same as before, misfire cylinder 3. I am shocked and also seriously worried, as I have checked and replaced everything in the vehicle's ignition system. I also ran some seafoam (highly rated fuel system/injector cleaner) through the vehicle in hopes to clean the system. I do not have the tools to perform a compression test, but i am having a hard time believing that such a cared for vehicle would already have catastrophic engine problems. The engine does not seem to be misfiring to me, it idles smoothly, gets the average 16-18MPG, does not hesitate while accelerating, does not shake at idle speed, does not downshift or feel like is down on power either, i am absolutely stumped...

I am aware of the balance shaft and intake flap recall issues on these engines, however those are known for throwing a variety of codes. the ONLY code i had and continue to have is just the P0303. I was brought to the awareness of potential valvetrain issues as well, however again this engine is really in great shape, i really do not want to believe that is the case but it is starting to set in. If there are any suggestions you may have, I am always free to listen, as well as provide any additional information. my email is alexvescan14@gmail.com. thank you!