Ok, this is my first post here. So Hi. :smile:

I haven't gotten my S yet. "Estimated" build date for my S is 9/12/03. Naturally, I'm worried about this stumbling nonsense, and I've given it a lot of thought and reading. So I've come up with a theory. If you decide to read this, keep in mind that I don't have the car, and everything I know about this stumble is from reading everything I can find about it. I'm not in any way an expert, but I've had some experience rebuilding older cars, and have a pretty good idea of how everything works.

So here goes:

Basically, my idea is based on one issue that I read in another thread somewhere. If that's not true, then this is in vain. The precept is that there's a boost limiter on the supercharger that is engaged when in first gear, below around 2000 RPM. Supposedly, the boosted air is shunted elsewhere until that rev is hit, at which point, the valve opens and dumps the boost into the intake. So, we'll assume that's true.

Now, when this computer controlled release occurs, it makes sense that the computer would be programmed to anticipate it. In it's anticipation of a sudden increase of air mass, it will dump a larger amount of gas in order to keep things as close to stoicometric as possible, right? Otherwise, you end up with a super lean condition for a moment.

Now, assuming the computer richens the mix just barely before the boost release, this should work great, and all that additional 02 has plenty of room to burn. The problem is that if the computer richens the mixture right before the boost release, there won't be any additional air in there. Why? Because there will be a lag while the pressurized air moves into the unpressurized intake. It will take a brief moment for the intake itself to become pressurized. So what happens is the anticipatory fuel dump causes a super rich mixture for a moment, slightly before hitting the 2000 RPM mark. Which is exactly when the stumble occurs. Given the symptoms I've read about, it sounds like an engine that is in fact running very rich for a moment. At least based on the older cars I've worked on. After a split second, the pressure increase is there, the MAF has taken over from the anticipation routine, and the engine recovers before stalling.

2 other opservations that seem to support my theory:

1. I've noticed that a lot (but not all) of chipped mini's no longer display the stumble. It would make sense that perhaps some of the chips disable this boost limiter.

2. I've noticed that a lot of people claim that the stumble is less pronounced, or even gone after the engine has warmed up. If the engine's warm, and the car's not moving very fast (As it wouldn't be when the stumble happens.) then the air entering the intake is warmer, and therefore the computer will back off the gas to compensate for the lighter air, making the mixture less rich, and the stumble less pronounced.

So that's the theory. I dunno, I think it makes sense. Perhaps my idea of how computers and superchargers work is off. Like I said, I won't claim to be an expert in any way.

Thoughts?

 

Same boat as you (getting a 04 MCS this winter), but I think your thories seem to make sense.

My friend had problems on a Grand Prix GTP with this, he further modified the
air-intake for better flow and the problem was reduced. Perhaps the folks who put higher-flow intakes in seem to have less???

Let me know how the stumble is when you get yours, very curious.

hope your stumble free!

-Matt

 

There is no such "boost limiter." Only a bypass valve.


--
Cheese

 

I had a problem with my MINI-S being hard to start when it was cold. The problem was getting progressively worse so I took the car to CREVIER in Santa Ana, CA. They determined that it was a software problem and they re-flashed the ECU. After I left the dealer I noticed the "stumble" for the first time. It was consistent at any temp and at any acceleration rate. The dealer could not isolate the problem so they called BMW. BMW asked that the ECU be removed from the car and sent to them for evaluation, since this problem had not been solved and may other complaints had been logged. BMW had the ECU for almost two weeks. Fortunately, the dealer loaned me a 325i (5 speed) for the duration. They got the ECU back from BMW and noticed that the problem was actually worse than before. It was finally decided that a new ECU should be installed. That seemed to solve the problem. The only thing I have noticed is a split-second delay in the throttle response. I will bring that up at my 10K oil change, which will be in amout 1200 miles. My point is, the "stumble" seems to be electronic, not mechanical...

 

One other point to the theory proposed here is that the MINI does not have a mass airflow sensor.

Graham

 

Hhhhmmm,

the MINI does have the bypass valve, yes, but it operates as a boost limiter at higher boost levels, so I would assume the bypass valve opens to restrict boost pressure to a given value, at a specific rpm and above, but not below. This is because with a mechanically driven supercharger, boost is a direct function of rpm. I do not know at which rpm the bypass valve opens to limit boost pressure, does anyone know?

Also, I was of the impression that the MINI does indeed have a mass airflow sensor, can someone verify this or correct me?

M.