By on August 23, 2019

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 [email protected]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.


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18 Comments on “Piston Slap: Metering a Prince’s Air Flow?...”

  • avatar

    Was the MAF replaced with an OE part? I have had very bad luck historically with aftermarket Mass Airflow Sensors. If you’re issue returns after Sajeev’s suggestion of cleaning/replacing the VANOS solenoid, that’s where I would start if an OE MAF wasn’t previously used.

  • avatar

    Subscribed .


  • avatar

    P115C MINI – Mass or Volume Air Flow ‘A’ Air Mass Too Low Compared to Model

    Can’t really see a “VANOS” cleaning (isn’t that an oil change?) being the issue. If it was stuck/sticking there would be VANOS position codes.

    I would look at the inputs to the air flow model: throttle body (replaced), MAF (replaced) and MAP (Hmmm, no mention of that)

    Using an cheap OBD scanner and app I would graph or datalog the MAF, throttle body position (absolute), MAP and engine speed.

    Drive the car while data logging until the stumble happens, then look at the data. The MAF and MAP should be approximently opposite in value and should chagne smoothly with throttle position. If you see any sharp/quick dips, I would focus on that sensor.

    If there are no obvious dips in the data log, then I would look at VANOS position (desired vs actual), but that requires specialized software.

    • 0 avatar

      If you can datalog, that’d be my next step as well. I would focus on logs pertaining to MAF readings g/s, coolant temperature, & throttle position.

      I think you’ve eliminated the MAF & TB as suspects but maybe smoke testing the intake path is a good step as well. Anywhere un-metered air could be entering should be investigated, PCV for example.

      I have repaired low power & engine stumbling on an X3 that were fixed by replacing the VANOS unit and it never threw a code. It was a complete guess on my behalf. Maybe?

      Have you ever had the intake valves cleaned? Anyway, good luck!

  • avatar

    That solenoid deal looks way too easy.
    The Mini is actually German, right?
    Shouldn’t the first step be “remove engine” ?

    • 0 avatar

      I didn’t own my 2009 Mini long enough to need to dig into the engine compartment very deeply, but I can say it was the easiest oil/filter change of any car I’ve owned. Apparently there are some German engineers for whom maintenance isn’t an afterthought. Who knew?

      • 0 avatar

        Bear in mind the Prince is a Groupe PSA-BMW collaboration, so it may be a French engineer who deserves the credit.

        Sample size of one, but a friend did have a good experience with the R56 Mini. Had it for a few years, liked it, didn’t have any major problems, traded it in when she had a child and wanted something with four doors. I’m not sure if that was typical, but it was better than I would have expected given things one reads online.

        Not to dredge up a comments section argument from the previous Mini article, but I’ll also say that I like the two-box design. I’m not sure two genuinely tall people could sit back to back, but I (5’10”) was fine sitting behind her (~5’5″) with her husband (~6’2″) riding shotgun. These days, any back seat where a 5’10” person isn’t touching the headliner is an achievement.

      • 0 avatar

        “Apparently there are some German engineers for whom maintenance isn’t an afterthought.”

        Rest assured the person(s) involved were *severely* punished .

        I rode in the back seat of the first new Mini and it was snug .


    • 0 avatar

      BMW does seem to actually think about ease of service. Longevity of parts, hmm, I dunno.

  • avatar

    The problem reminds me of one with a car from the pre computer pre EFI days. Sticking EGR valve. Easy to test on that car, Dodge with Mitsubishi 4 cyl. Just plug the EGR’s vacuum line and if the problem goes away, that’s it.

  • avatar

    Could be a cracked vacuum line. Could be that the new throttle body went bad again. Could be loose intake piping. The VANOS suggestion sounds plausible, but I would also expect another code related to a VANOS malfunction.

  • avatar

    Check any intake hoses. While not the same car, I had the same symptoms in my 330i, M54. Turns out the accordion hose on the intake had aged and cracked. Engine movement would cause occasional leaks, make things inconsistent and tossed codes. The hose was definitely leaking at cracks in the accordion.

  • avatar

    We had a 2007 Mini Cooper S (turbo).

    I know yours is not an S, but ours suffered from similar symptoms. Rough idle, sluggishness and the like.

    Ultimately the PCV system was the cause. Apparently it was clogged, or failed in some manner, which necessitated replacement.

    On all previous cars I ever owned, a replacement PCV valve cost less than $10. However, the Franco-Germanic engineers involved with designing the engine in our R56 Turbo decided to integrate it into the valve-cover – a $750 part (installation not included).

    Good luck. Thankfully there are no more Minis in my garage.

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