Atypical shudder/misfire

I've got a very strange issue with my daily (E91, RWD, manual) that I'm hoping y'all can help me with. Basically, it feels like the clutch is shuddering, but I don't think that's what is really going on - just a good way to describe it. When coasting down for a turn (in gear), it will shutter/misfire if I get back on the gas lightly at around 2,000rpm. The rpm-dependence is why I doubt the clutch is slipping. At WOT and higher rpm, there is no slip, even in higher gears. The other day, I even got it to shudder when coasting in 6th and then abruptly going to ~30% throttle at around that 2,000rpm mark.

So that's the most repeatable case. A less-often case is when starting from a stop, it'll occasionally buck like crazy until I either clutch it or power through. This is at closer to 1,000 rpm with the clutch being let out on throttle. 9.5 out of ten times, this is just a nice, smooth start, but, especially if the engine is not fully warm, it'll buck. 

One theory a friend offered is that there could be a small leak onto the friction surface that gets blown off at higher rpm. This idea can fit (with the aid of a shoehorn), but it doesn't seem quite right. These cars do use a dual-mass flywheel, whatever that means. 

No codes, but misfire codes are a black art anyways. Fuel consumption is fine, and it doesn't seem to take long to crank. One possible theory is an issue with the injectors and/or regulator, but I have no other symptoms of anything, and I regularly hit WOT and redline. Same for anything related to PCV; no strange noises, idle is smooth without AC on and just a bit rougher with it on. 

Thoughts? I bought it at 120k miles and it currently has 127k, so whatever I call 'normal' might not be. But it is fair to assume that it does not have any other issues I didn't list above. I do have a new air filter and spark plugs coming in since the current ones are unknown, so I figure that's when I can really dive into things like if there is oil in the intake or signs of injector failure on the plugs. 

N52 or N54?

Byrneon27 said:

N52 or N54?

N52, non-CA. 

Sounds similar to the dual-mass flywheel failure I just dealt with.

I would do plugs, coils, PCV, and injector service first though.

vacumn leak (?)

Javelin said:

Sounds similar to the dual-mass flywheel failure I just dealt with.

I would do plugs, coils, PCV, and injector service first though.

Googling it, most DMF failures seemed more prominent and at more rpm. Can elaborate more on your failure? My only experience with DMFs is this car, and most of my transmission knowledge is book-wise anyways, so I'm sure I have lots of gaps to fill. 

Purple Frog (Forum Supporter) said:

vacumn leak (?)

I can't think of a vacuum leak that would only occur at those exact driving conditions and no others. Especially idle is fine, and there are no odd sucking or whistling noises. 

Use a global OBD2 tool with data monitoring what do the short and long term fuel trims look like? How about the short term when the fault occurs? 

I too am leaning DMFW failure leading to a phantom/false misfire that the DME is compensating for. 

I know nothing about BMWs but the last vehicle I had that acted like that had a screwy TPS sensor. 

This was pre fly by wire though. 

Byrneon27 said:

Use a global OBD2 tool with data monitoring what do the short and long term fuel trims look like? How about the short term when the fault occurs? 

 

I too am leaning DMFW failure leading to a phantom/false misfire that the DME is compensating for. 

I didn't think of a phantom misfire, and that fits quite well. As I think about that angle though, the coast-and-tip-in shudder on the interstate could possible make sense. When the torque through the driveline reverses, something ticks off the ECU. Knock sensor would be my first guess since knock won't throw a code and does reduce torque. Anything else I can think of [should] throw a code. I'll log some data and see what I can find. 

Toyman! said:

I know nothing about BMWs but the last vehicle I had that acted like that had a screwy TPS sensor. 

This was pre fly by wire though. 

Yeah, DBW is a different beast and then VANOS takes that to a whole other level. Most errors will run like garbage rather than the little hiccup I'm getting. Doesn't mean that it isn't the top of the iceberg though, but that makes me think it isn't a leading suspect. 

Welp, I went through a bunch of maintenance this weekend - spark plugs, coils, CAI, cleaned MAF, oil change, new brake pads, and rotated tires. Pretty sure it was the new pads, but the misfire appears to be gone! Everything looked good. The plugs had some age on them, but to my eyes looked alright. MAF and air filter looked fine, too, so I have no idea what the actual issue was, but at least it wasn't the clutch!

In reply to cyow5 :

On a BMW, I wouldn't be a bit surprised if new pads cleared up a misfire.