Piston Slap: Metering a Prince's Air Flow?

TTAC Commentator haroldhill writes:

Dear Sajeev,

My wife is driving (and loving) her 2009 MINI Cooper which has been a delight and remarkably trouble-free for 10 years. However, it recently developed a problem which has befuddled us and our highly reputable independent mechanic as well.

Under certain conditions the engine stumbles, feeling somewhat like an erratic misfire on one cylinder or fuel starvation; however, the stumbling vanishes when accelerator load is increased. The required conditions are: fully warmed up (10-15 minutes of driving), engine speed between 2700 and 3300 rpm, and light throttle appropriate to steady cruising. The stumbling will eventually bring on the “check engine” light and a P115C error code (Mass Air Flow).

If the engine is turned off for a few minutes (e.g. for a highway rest stop) the stumbling will disappear and won’t reappear until the car has been driven for another ten minutes, suggesting the fault is something remote enough from the engine that it can cool substantially in five minutes. After a few uneventful driving cycles, of course, the “check engine” light turns itself off. Thanks for nothing. On the other hand, this would be how I got it through emissions inspection…

Thus far the Mass Air Flow Meter has been replaced twice and the Throttle Body once. The latter seemed to help for awhile but I can’t be sure because it’s only recently that I’ve pinned down the exact conditions that will reliably bring this problem on. My wife, who does most of the driving, is a bit of a leadfoot and generally has much less trouble with this stumbling.

Our wonderful mechanic would appreciate any ideas or suggestions.

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