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As quite a few MINI owners can tell you the recently released V36 of the engine management software was a welcome addition. However this recent software update didn't fix one problem that has afflicted a good percentage of Cooper S owners – the dreaded Yo-Yo. Ryan Malcolm (Ryephile on North American Motoring) has researched this subject extensively and gives us the following insight into what exactly the Yo-Yo is: “The Yo-Yo is the oscillation of the supercharger bypass valve, which is excited by an under-damped P.I.D. throttle body servo controller. The parameters of the Yo-Yo occurring are simple: typically during medium acceleration, on-boost, and with the throttle pedal reasonably steady. Amplitude of the Yo-Yo seems to vary per vehicle, ranging from imperceptible to an almost violent bucking.”
Further, Ryan is in a good position to understand this process as he works at Motorola SPS Automotive group and has access to powertrain engineers on a regular basis. He goes on to explain the solution he and a few of his co-workers came up with:
“Tying the bypass valve shut, or implementing a stiffer rate bypass valve return spring will eradicate the Yo-Yo. Furthermore, a retuned PID that is critically damped can and may result in usage of the stock bypass valve return spring, and may not excite the physical instability. Recommended solution is implementation of the stiffer bypass valve spring plus a critically damped PID controller.
The process of determining the above is a collaboration between Motorola SPS Automotive Groups volunteered expertise in engine management, combined with research carried out by myself, consisting of known-good parts swapping, fabricating prototype supercharger bypass valve springs, sampling a plethora of various other Cooper S vehicles, and using a variety of ECU versions and Maps. Deductive logic was exclusively implemented to ensure the highest quality datum possible.
My contact (who wishes to remain anonymous so he doesn't risk his contact at MINI) says that MINI Engineering is aware of and impressed with my research, though have not officially investigated the problem further. I don't like to push this hearsay, but it's the only communications with MINIUSA I'm aware of!
A big thanks to Ryan for all the work done on this topic – not to mention contributing so much to this post!
<p>Thank YOU Gabe for putting this article up for everyone to see. If anyone is interested in reading the whole drama, or want to add your MCS to the Registry of the Yo-Yo afflicted, check out the thread on NAM <a href="http://www.northamericanmotoring.com/modules.php?op=modload&name=phpBB_14&file=index&action=viewtopic&topic=21063&177">here</a>.</p>
<p>I'm got tired just reading about it.</p>
<p>Got the CD36 “FIX” today. Not sure if anything is better? Not hot enough (60's) in SoCal yet. My problem was the stumble that was exterior temp related. Start from 1st go into an idle then take off. </p>
<p>Not sure about a yo yo.</p>
<p>Thanks fo the update though.</p>
<p>RB</p>
<p>What do you mean when you say critically dampened. Critically sounds to me like you mean kill the damping, yet you say that in stock form it is under-dampened. Does the solution mean having a new throttle body designed with a different servo (what is doing the damping I presume). Additionally I think you recommend a bypass valve return spring that is stiffer. is that for faster action or so it will open only under greater pressures? I don't have any education in this field but would like to better understand your evaluation and solution.</p>
<p>It would be of some comfort to hear that MINIUSA is concerned in addressing this issue. My '04 MCS has experienced the 'yo yo' since day one.</p>
<p>I've been watching Ryephile's posts over on NMA with excitement. V36 definitely fixed my stumble, but the yo-yo is as bad as ever. It is very strange that the yo-yo is violently present on some cars and not even detectable on others. I've heard hearsay that it is more pronounced on '04 MCS's so maybe that will drive MINI to some action.</p>
<p>According to the Registry, most of those that have the Yo-Yo are indeed 2004's.</p>
<p>—>obeyond: the term “Critically damped” is where the output exactly follows the input with no overshoot (underdamped), and not overdamped. The PID controller is within the ECU, and can be programmed via the OBDII port. Also, the stiffer bypass valve spring pushes the acoustics of the mechanicals from resonating; a byproduct is it also keeps the bypass valve closed under all but the highest vacuum operations (like idle and engine braking). I hope that helps out!</p>
<p>I've had it on my July build 2003 MCS from day one, and version 36 didn't help. Hopefully there will be something to fix this soon, because the yo-yo is very annoying (occurs every time I accelerate in 2nd gear).</p>
<p>I have read all the posts on the stumble and yo-yo since ordering my MCS in August and was beginning to worry about what I was going to receive. After I received my 04 at the end of Oct. I was pleasantly surprised that my MINI didn’t have these problems and I was confused about what everyone was talking about. Then I tried different driving styles to see if I could induce this behavior and sure enough I could easily reproduce the yo-yo under part throttle. The reason I had not experienced the yo-yo before was my driving style was foot on the floor through 1st and 2nd then cruise in 3rd or 4th, maybe 6th on the freeway. I had just never had the part throttle condition in the right gear to yo-yo. I still don’t experience the yo-yo today at 3000 miles, I guess I just drive too aggressively. And so there is a possible solution to those who have the yo-yo. Keep your foot on the floor and no yo-yo.</p>
<p>The stumble was another issue; I can only reproduce this scenario by engaging the clutch at just over idle then putting my foot to the floor. This is asking the engine to perform in an area of the torque curve where the torque is non-existent, thus no-go. I don’t leave a stop at anything under 2000 rpms with a smooth clutch engagement and have never experienced the bog off the line. This behavior is consistent with all small engine vehicles that rely on horsepower to get the job done and not torque. These types of vehicles require a different driving style when compared with car with good low end torque. An excellent example is the famed Honda S2000, a great performing car with many accolades for it’s acceleration and performance, but it is terrible off idle. This vehicle doesn’t like to be run under 4000 rpms. Way more fussy than the MCS, but still a lot of fun if driven properly. What is it the magazines say about the way to drive an MCS “Drive it like you were running from the cops”. I find this analogy to be 100% correct. The driving style for the MCS is totally different than a Civic or Corolla or any other small stick shift car. High performance cars, and that is what the MCS is, have to be driven differently than normal sedate daily drivers. Just my 2 cents worth.</p>
<p>I had CD 36 downloaded to my '04 MCS a couple of days ago and I am happy to report that the car is running smooth as silk.</p>
<p>I really never had a problem with the yo-yo to begin with.</p>
<p>COOPR SS: In case you haven't done it, I highly recommend having your dealer update your DME software to version 36. According to the MINIUSA service bulletin, Cooper/Cooper CVT and Cooper S models built prior 10/2003, do not have version 36 installed from factory. My '04 MCS is a September '03 build and it came with the “interim” version of software 35 which was intended to be a stop gap until CD 36 became available in September. Version 35 is a pretty clean software release, hence the majority of '04 owners really never got to experience the aweful stumble that most 2002 and 2003 MCS owners endured for a very long time.</p>
<p>Once you get CD 36 downloaded to the car, you'll notice that it is virtually impossible to make the car stumble going by the technique you provided in your post. I believe the air/fuel mixture is richer in the lower RPM band with version 36 to eliminate the problem.</p>
<p>Also, CD 36 is giving me 2MPG better fuel economy in the city than with version 35. Now I am getting 26MPG versus 24MPG from before the update.</p>
<p>I meant to say:</p>
<p>“COOPR SS: In case you haven't done it, I highly recommend having your dealer update your DME software to version 36. According to the MINIUSA service bulletin, Cooper/Cooper CVT and Cooper S models built prior 11/2003, do not have version 36 installed from factory. My '04 MCS is a DECEMBER '03 build and it came with the “interim” version of software 35 which was intended to be a stop gap until CD 36 became available in September. Version 35 is a pretty clean software release, hence the majority of '04 owners really never got to experience the aweful stumble that most 2002 and 2003 MCS owners endured for a very long time.</p>
<p>Once you get CD 36 downloaded to the car, you'll notice that it is virtually impossible to make the car stumble going by the technique you provided in your post. I believe the air/fuel mixture is richer in the lower RPM band with version 36 to eliminate the problem.”</p>
<p>Please ignore above post.</p>
<p>Also, CD 36 is giving me 2MPG better fuel economy in the city than with version 35. Now I am getting 26MPG versus 24MPG from before the update.</p>
<p>Frank, thanks for the info, I have been afraid to get the V36 installed because mine runs so well. But based on your recomendation I will try to get it installed soon before our run to the Vegas meet next week.</p>
<p>Man, I need a vacation..keep making mistakes with the build dates!</p>
<p>here is what I TRULY meant to say!:</p>
<p>COOPR SS: In case you haven't done it, I highly recommend having your dealer update your DME software to version 36. According to the MINIUSA service bulletin, Cooper/Cooper CVT and Cooper S models built prior 11/2003, do not have version 36 installed from factory. My '04 MCS is a September '03 build and it came with the “interim” version of software 35 which was intended to be a stop gap until CD 36 became available in late December. Version 35 is a pretty clean software release, hence the majority of '04 owners really never got to experience the aweful stumble that most 2002 and 2003 MCS owners endured for a very long time.</p>
<p>Once you get CD 36 downloaded to the car, you'll notice that it is virtually impossible to make the car stumble going by the technique you provided in your post. I believe the air/fuel mixture is richer in the lower RPM band with version 36 to eliminate the problem.</p>
<p>Also, CD 36 is giving me 2MPG better fuel economy in the city than with version 35. Now I am getting 26MPG versus 24MPG from before the update.</p>
<p>Please ignore previous posts time stamped at 11:27AM and 11:31AM.</p>
<p>Sorry for the triplicates.</p>
<p>Does this DUNCAN effect mainly happen on the CVT models?</p>
<p>No, the Yo-Yo is only applicable to the Cooper S, because it's the only model using the supercharger and the bypass valve.</p>
<p>I had v36 flashed two weeks ago with no effect on my 2003 MCS, good or bad. I did not have the stumble but the yo-yo was and still is present. I'm waiting for a fix but getting impatient and frustrated by the lack of support or acknowlegment from MINI an issue even exists. How long must we endure this issue?</p>
<p>Somebody please help those of us with this annoying problem on an otherwise great car.</p>
<p>I have a 11/03 built 04 (presumably V36) and it has no stumble and what I'd characterize as mild to occasionally moderate yo-yo.</p>
<p>It sounds like the problem is being looked at from a lot of angles, but I'm still a skeptic that the ultimate solution is at hand. If it was something easy like putting a little more damping in a circuit, I would have thought BMW would have had a handle on this a long time ago. If it is something simple that a handy guy in his garage can discover, then a pox on BMW for letting it fester this long.</p>
<p>I could focus on the problem, but it is really not a huge deal – I prefer to think of it as part of the “character” of the car. Every turbo or supercharged car I've ever driven has suffered to some extent with uneven power delivery and bootstrapping. But I won't presume to know how bad the problem might be in all of the cars out there.</p>
<p>It has always been my experience that expecting perfection — especially in wifes, children, and cars — will always result in disappointment and lead excess stomach acid.</p>
<ul>
<li>Mark</li>
</ul>
<p>I have an 11/03 build 04 MCS. There is no question that the Yo-Yo exists. I can reproduce it at will. For me, it occurs at moderate throttle worst in second gear and more evident when going up hill. It amazes me that MINI simply denies the problem exists. This is clearly not an issue of driving technique nor one of skill at standard transmission/clutch operation. Unless I'm willing to use full or near full throttle accelerating through lower gears, I have Yo-Yo.</p>
<p>My suspicion is that since this is a drivability rather than a safety issue, MINI is not as interested in solving the problem as they were with the stumble which had significant potential safety concerns.</p>
<p>For those of you who wish to be more informed of the details in Ryephile's quest for this solution, please visit the NAM website and read “The Yo-Yo Chronicles” in the “Beneath the Bonnet” section.</p>
<p>You will then understand that it took quite a bit more than “a handy guy in his garage” to find the cause of the ECU problem and the mechanical fix that negates the effect of that bug. There was some solid research there and quite a bit of deductive reasoning.</p>
<p>Why MINIUSA hasn't oficially acknowledged or addressed this issue speaks more to the “character” of the corporation rather that that of the car.</p>
<p>YMMV,</p>
<p>Theo</p>
<p>I got the CD36 install yesterday.
I don't have a yo yo thing but a stumble. No stumble this time of year but even with this “FIX” don't see any real improvement in launch……RB</p>
<p>Does this affect only MCS's made in the US as I am from the UK and am awaiting my 04 MCS in early March</p>
<p>I have not heard of any Yo-Yo issues in Europe. While I doubt a different bypass valve is used, I believe European-spec MINI's use a different ECU code than U.S.-spec.</p>
<p>Actually, Europeans have the same Yo-Yo issues ;-( Forums are full with it…</p>
<p>MINI was deligent in resolving the stumble issue. I am confident it will resolve the yo-yo problem as well. These issues are not very easy to implement in a large scale and require planning and strategy.</p>
<p>Please be patient.</p>
<p>markjenn said:
“It has always been my experience that expecting perfection — especially in wifes, children, and cars — will always result in disappointment and lead excess stomach acid.”</p>
<p>My experience says that expecting perfection leads to many ex-wives!</p>
<p>I'm afraid that I have to classify yo-yo and stumble right up there with the rear fog lights. Given any driving style, I can find lots of little things that Mini does that are not nice. My answer is to not use that driving style! Call it what you will, yo, stumble, bog… the answer is accelerator therapy.</p>
<p>(Mind you, I also believe in 'retail' therapy.)</p>
<p>I believe my car is more of the severe occurance. If I get stuck behind a truck on a hill on my way home It Yo-Yo's so bady my headrest acts like a catchers mit.</p>
<p>It is the perfect place of hell for Yo-Yo. Stuck in 2nd gear, first will launch you into the vehicle, 3rd bogs down and you have to mat it to keep up.</p>
<p>The easiest answer is to always drive above 3K rpm. Speeding tickets, gas mileage, tire wear, rotor/brake wear and passengers clutching to everything in the car would have to become afterthoghts.</p>
<p>Mostly it is just really annoying. I am patient. Dealership is a long way away and will take a full-day for me to deal with when the 'fix' is available.</p>
<p>Ryan, deserves much applaud for his dilligence in the matter.</p>