Hello,

I purchased a 2011 MCS Convertible last August and soon thereafter it started throwing a check engine light for misfires - I have a Bluetooth code reader and would always see P0300 plus some combination of P0301, P0302, P0303, P0304. Usually just P0302 and P0303 for cylinders 2 and 3, but sometimes all four, sometimes just one; it was kind of random. Whenever this happens while the engine is running, the CEL flashes and the car goes into a reduced power "limp mode". But if I simply restart the engine, the limp mode goes away and the engine appears to run normally, but the CEL remains. Over time I've come to notice the pattern of this limp mode + flashing CEL occurring roughly every 60-100 miles of driving. Sometimes it'll happen closer together than that but typically I get at least 60 miles between occurrences, regardless of the time span.

I have attempted to diagnose and fix this with the help of a local shop, but the behavior has not changed and I got tired of throwing money at the issue. Here is what I did and when:

• 9/1/21 @ 127,860 mi: Replaced battery (the one that came with the car was depleted as the car had barely been driven in the 1.5 years the previous owner had it)
• 9/10/21 @ 128,040 mi: Replaced all four ignition coils w/ OEM Delphi coils. I started here at the advice of the previous owner who mentioned that a shop he took the car to recommended this if the CEL returned.
• 9/16/21 @ 128,140 mi: The issue came back so I had a shop perform a diagnosis. They noticed the spark plugs were showing early signs of failure so I had them install 4 new ones.
• 10/1/21 @ 128,275 mi: Still not fixed so I went back to same shop, this time they tested a few different things and ultimately replaced the Crankshaft Position Sensor claiming it had some metal filings on it and may have gone bad.

When sporadic misfires continued after the 10/1 fix, I took the car back to the same shop once again and they said that based on all their time diagnosing this issue, the next culprit was the ECU itself. Basically, they think there's some sort of electrical fault in the ECU's onboard circuitry that is causing it to falsely detect misfires. I decided to skip on that due to the amount of money I'd already thrown at this issue with no improvement and how costly a new ECU was sure to be.

As far as I can tell the motor runs fine, at least for a 11 year old BMW powertrain. Pulls hard and turbo kicks in just like it should, and no weird sounds or vibrations when it's not in limp mode. I'm sort of at an impasse as to what I should do next. I'm moderately handy for DIY stuff but have limited tools and resources for doing complicated diagnoses or fixes. I've seen threads on NAM covering this issue, and after changing plugs and wires it seems common to recommend a carbon cleaning. I'm considering doing that, but I'd probably have a shop do it since the full walnut blasting seems involved and has the same up-front cost to do yourself as it would cost at a shop. So I'm sort of weighing whether I should ask a (different) shop to diagnose this issue and re-open the can of worms, or just try the carbon cleaning and hope that fixes the issue, or pursue some other DIY tests/remedies.

Curious if any other N18 owners have experienced this sort of problem and if so, what fixes worked for them.

I'm including the ECU data that my Torque app is able to pull which shows misfire counts - this was taken right after the flashing CEL which had the misfire codes for cylinders 2 and 3.

Thanks for reading!

 

With almost 130k miles, has the fuel filter been replaced? Why not throw a bottle of fuel system cleaner through a tank. (Maybe you have some injector fowling)

 

Honestly have no clue since I have very limited maintenance records from before my ownership. From some quick research, it looks like there will be a sticker over the fuel filter housing if the filter hasn't been changed yet, so I'll give it a look. And good idea on the fuel system cleaner, I don't think I've done that yet so I'll start there. If that doesn't fix the issue and I suspect the fuel filter hasn't been replaced, would that be a better next step vs. doing the carbon cleaning in your opinion?

 

I'm following up on this thread in case it's helpful for anyone else who runs into this issue. My problem ended up being a crack in the air mass meter boot (air duct going to the turbo). It took a while to find since it was on the underside but once that was replaced and the turbo was getting the right amount of air, the misfires went away and haven't come back.