Same issue, full/high boost = random misfires
My engine is stock except for a COBB SF Intake.
I store my MS3 over the winter (2nd winter now) and yes, she’s fully winterized. This time around everything went south about 1/2 way into the second tank of gas when I resurrected her this spring.
Basically I started to feel a single hesitation at around 3200rpm. Over the course of a few hundred miles it became a lean backfire at high boost around 4-4500rpm. Then it started randomly throwing P2187 and a solid CEL. After three "clean" drives the CEL would go out. As soon as the CEL went out, the next drive or two would produce the problem and the cycle would start all over again. I put the stock air box back on and still had the issue though it became rather infrequent. I also cleaned the MAF sensor and the COBB air filter, and replaced the stock as well. This went on for 3 weeks, before I was finally able to get in and have her looked at by the dealership. Of course driving her like a granny and keeping off boost as much as possible during that time proved to produce zero problems.
Brown's Mazda in VA told me the P2187 was caused by a faulty purge valve. I had this replaced and drove her for a week. She still produced the same symptoms but only twice.
I put the COBB intake back on a few days ago, cleaned the MAF sensor again and she produced the symptoms more frequently, but since the purge valve replacement, no CEL, no codes. Then this happened...
The other night, after two drives that day with no issues I took her out and opened her up. This time, as soon as I got my foot in her (5Kish rpms,) she had multiple misfires (that I felt and heard,) a huge lean backfire, a blinking CEL (obviously from the misfires) that went solid and then limited boost. Limped home, pulled the codes and got P0300, P2187 and P0300.
I'm at a freaking loss and currently looking for a different dealer to take her to.
If anyone has any suggestions, give me a holler...next steps for me are new MAF and new EGR (maybe)...