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.

Sajeev answers:

It’s time for this Lincoln-Mercury fanboi to put aside his fears of newer BMW products and hope his Google-fu is strong.

My go to automobile specification website says your MINI Cooper has a naturally-aspirated (i.e it’s not a Cooper S) has a Prince Engine with BMW’s VANOS system for variable valve timing. The P115c code is far from a quick and easy diagnosis, so I don’t blame your mechanic for changing these parts, especially if he’s not a specialized BMW technician. Then again, replacing the MAF twice does kinda, sorta bother me. It kinda feels like he was throwing parts at the problem.

But I hate being an armchair quarterback, especially since I don’t wrench on cars for a living, so let’s get off that throne.

Since this isn’t a turbocharged model, there’s not in the way of intake plumbing to spring a leak. So VANOS system might be the problem, especially since issues happen “between 2700 and 3300 rpm” and never else. I bet the VANOS solenoids need a good clean, and perhaps a test/replacement afterwards. Accessing your average MINI Cooper VANOS solenoid doesn’t look too hard, in theory:

So my advice? Remove and clean the VANOS solenoids and see what happens.

[Image: BMW Group]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.