[MegaMatt3]

Hi all,

I have '16 M235i with about 9000 miles. Manual transmission. Drove perfectly until about 5000 miles, when one day when I went to drive it and I noticed something had changed.

When I'm right at the transition point between throttle and no throttle (whether getting on throttle or coming off throttle), the car will sometimes noticeably hesitate and not transition cleanly. This is NOT only when I'm shifting gears - it can happen when I'm in a gear and cruising, giving it throttle or coming off it. The worst cases are when the car does a slight lurch, generally when coming off the gas. It's like a hesitation from throttle to no throttle where the car can't decide if it wants throttle or no throttle, and ends up sputtering between the two for a second, which results in a quick *bump bump bump* lurch before it comes completely off. Almost like what would happen if you pull out the clutch too quickly when downshifting. That same feeling.

This doesn't happen every time. And when it's not happening, I can almost always feel a much more noticeable *bump* a half second or so after coming completely off the throttle, especially if I do it quickly. It's basically just not driving as smoothly or as cleanly as it did for the few few thousand miles. And I probably don't need to say that this is incredibly obnoxious. I'm generally pissed off when driving the car now because I can almost always feel these throttle transitions not happening cleanly.

So I took it to the dealership under warranty, probably didn't explain it as well to them as I just did here, and had them check it all out. They couldn't find anything wrong, which is pretty much what I expected, unfortunately.

What I need is a couple of possible specific things that it could be that I can tell the techs at the dealership, to focus the diagnostics a little better. Any possible culprits folks here can provide would be extremely helpful, for my car and my sanity.

Thanks!

[Anthony235]

I have an auto, but have had plenty of manual turbo cars before and can't recall that kind of issue in any of them.

Sorry I can't be of more help, but have you tried to duplicate the problem with the different throttle settings? Sport, Sport+, etc....

Only does it in certain gears?
Certain speed?

Half the process is just being able to duplicate the problem for the dealer.

[XutvJet]

Any modifications? Have you played around with anything under the hood? Please be honest. It's the only way we can help you.

Which gears is this most noticeable?

Is there a particular rpm range that this happens?

Does it happen coming off of part-throttle, full throttle, or both?

Does it happen in the first 5-10 minutes of driving or potentially anytime?

Is there a thud from the chassis when it happens? If so, where does the sound appear to come from?

How often is this happening?

If you go back to BMW, it would be best to present them with a piece of paper describing the issue in detail. The tech will greatly appreciate it.

[overcoil]

Ideally a flight data recorder to record the hesitation as opposed to going back in latter to pull codes - but that's up to the dealership.

If you look at the tach when this happens and if the tech stumbles at the moment it hesitates then you look at fuel system, if the tach very fast drops then electrical.

Things you can do, change gas both brand and station. Do some sustained high speed and some high-rpm acceleration runs.

Hopefully someone who really knows these drivetrains can answer better.

[MegaMatt3]

Originally Posted by XutvJet

Any modifications? Have you played around with anything under the hood? Please be honest. It's the only way we can help you.

No, I wouldn't know where to start under the hood. Haven't touched a thing, and that's the truth. All stock.

Which gears is this most noticeable?

Maybe not any one particular gear, but it's more noticeable at lower RPM's because then engine is lower and the hesitation is more pronounced.

Is there a particular rpm range that this happens?

Again, lower RPM's, maybe somewhere between 1000 and 2000.

Does it happen coming off of part-throttle, full throttle, or both?

When coming off part throttle, when coming off slowly so I can find that transition point from there being SOME throttle and there being NO throttle, that's when I can feel it most. Coming quickly off full throttle (or lots of throttle) produces less hesitation and more of a thud from the chassis about a half second or so after I've come off. The thud is delayed just a bit.

Does it happen in the first 5-10 minutes of driving or potentially anytime?

Anytime. I've tried to determine if it's more noticeable when I first start driving, after I've been driving a while, cool mornings, hot days, and nothing seems to make it more pronounced at one time than another. I feel the thud more at highway speeds and the hesitation at lower speeds around town. I guess both of those times are when the car's warmed up a bit.

Is there a thud from the chassis when it happens? If so, where does the sound appear to come from?

YES, absolutely. The thud is noticeable. It's less a sounds and more of a feeling through my seat. It's hard to say where it's coming from because I can really only feel it, but I know I do feel it through my seat, if that's any indication.

How often is this happening?

The thud when coming quickly off throttle happens pretty much all the time now. The hesitation when transitioning throttle at lower RPM's is more pronounced sometimes than others -- I'll have perfect transitions sometimes, I'll have noticeable lag sometimes when I start to give it gas, and sometimes I'll get that jerky transition. It seems to be random, and again, not related to shifting gears.

If you go back to BMW, it would be best to present them with a piece of paper describing the issue in detail. The tech will greatly appreciate it.

Thanks for all the probing questions. Answers inside the quote block.

[MegaMatt3]

Originally Posted by ScottSinger

Ideally a flight data recorder to record the hesitation as opposed to going back in latter to pull codes - but that's up to the dealership.

If you look at the tach when this happens and if the tech stumbles at the moment it hesitates then you look at fuel system, if the tach very fast drops then electrical.

Things you can do, change gas both brand and station. Do some sustained high speed and some high-rpm acceleration runs.

Hopefully someone who really knows these drivetrains can answer better.

Scott - I'm trying to follow your comment about how to tell if it's electrical, but I'm having a hard time. Can you give a little more detail on that process?

Also, can I do my own flight recording?

[overcoil]

You can't do the flight recorder data w/o the expensive computers.

About the electrical/fuel comparisons using the tach.
If your car was stalling or dying while running. By looking at the tachometer the moment it stalls you can get a few clues: if it's fuel starvation as the fuel dries up while going to the injectors the tachometer will gradually drop to zero.
If it's an electrical component that's turning of the engine the tachometer will quickly drop to zero - as quick as turning off a switch.

Knowing that, if you view the tach while the car hesitates, a very rapid swing of the tachometer MIGHT mean electrical, like when you turn the car off. On the other hand if the tachometer needle drops or moves at a slowish speed like pumping a gas pedal then it could be fuel delivery or something fuel related like valve deposites.

Did the dealer describe what kind of diagnostics they did, computer scan, search for loose piping around intake or turbo?

But I don't know these particular cars very well, so I'm just telling you the kinds of things a driver can do to help clue into a solution.

[MegaMatt3]

I haven't noticed the tach doing anything abnormal when it happens, but I also haven't been paying close attention. I'll keep my eye on it. The hesitation ("stalling" was a good word you used) is subtle enough where I'm not sure I'd see the tach move much anyway. It's not like the whole car nearly turns off. Just more like it's struggling to get all the way off throttle (or onto it).

[XutvJet]

Is this your first manual?

[Anthony235]

Perhaps low rpm bucking.

[XutvJet]

Originally Posted by Anthony235

Perhaps low rpm bucking.

That's what I'm wondering. He's experiencing the issue on on/off gear throttle applications at near idle and below 2,000rpms. Those are pretty low operating rpms for a 3 liter motor when it's off-boost. I know these are powerful motors, but they lack the torque when off boost and are in a tall gear at low rpms. If don't downshift and simply keep adding gas, you start spooling the turbo and may put the motor into a dangerous high load, low rpm situation. Depending on conditions (cold/hot/muggy conditions or heat soaked motor), inducing a high load, low rpm situation can cause a big boost spike and the DME (ECU) may yank timing or abruptly kill boost to keep the motor from destructing from detonation. Automatics will "unlock" and slip the torque converter to compensate for the lack of torque or will downshift to keep from inducing a high load, low rpm situation.

I can feel a very minor drivetrain thud when letting off the throttle while in gear in 6th on the highway (rpms 2000-2500 or so) and I can feel something similar through the seat when I'm in a tall gear and running at a low rpms (i.e., below 2,000rpm). I notice it more now than I did when I first got the car, but I believe it's because I'm paying more attention to things after 7 months and 7K miles of ownership. I also understand that things have loosen up a bit in the drivetrain.

I can't say this sensation/thud isn't uncommon to me though. My 2012 WRX 5MT did it, my 1996 Maxima 5MT did it, and all my prior manual Hondas did it. Some worse than others. What I felt was just the nature of operating a manual transmission at low rpms, the engine lacking some initial torque to overcome the load, and the drivetrain rocking a bit in its soft motor mounts.

[MegaMatt3]

Originally Posted by XutvJet

Is this your first manual?

No, I've been driving manuals since I was 16 and I'm 32 now. I'm a skilled manual driver, and this isn't happening when shifting.

[MegaMatt3]

Originally Posted by XutvJet

That's what I'm wondering. He's experiencing the issue on on/off gear throttle applications at near idle and below 2,000rpms. Those are pretty low operating rpms for a 3 liter motor when it's off-boost. I know these are powerful motors, but they lack the torque when off boost and are in a tall gear at low rpms. If don't downshift and simply keep adding gas, you start spooling the turbo and may put the motor into a dangerous high load, low rpm situation. Depending on conditions (cold/hot/muggy conditions or heat soaked motor), inducing a high load, low rpm situation can cause a big boost spike and the DME (ECU) may yank timing or abruptly kill boost to keep the motor from destructing from detonation. Automatics will "unlock" and slip the torque converter to compensate for the lack of torque or will downshift to keep from inducing a high load, low rpm situation.

I can feel a very minor drivetrain thud when letting off the throttle while in gear in 6th on the highway (rpms 2000-2500 or so) and I can feel something similar through the seat when I'm in a tall gear and running at a low rpms (i.e., below 2,000rpm). I notice it more now than I did when I first got the car, but I believe it's because I'm paying more attention to things after 7 months and 7K miles of ownership. I also understand that things have loosen up a bit in the drivetrain.

I can't say this sensation/thud isn't uncommon to me though. My 2012 WRX 5MT did it, my 1996 Maxima 5MT did it, and all my prior manual Hondas did it. Some worse than others. What I felt was just the nature of operating a manual transmission at low rpms, the engine lacking some initial torque to overcome the load, and the drivetrain rocking a bit in its soft motor mounts.

I wouldn't say I'm in a gear at too low of an RPM. For what it's worth, as you've probably noticed, the car wants me to shift to a higher gear anyway (via the number suggestion) after around 1500 RPM's, and that's still well within the range when I feel this behavior. And I guess I should clarify that low RPM's is just when it's more pronounced, but I feel the engine hesitation/stall at higher RPM's as well, just not as often or as pronounced.

[MegaMatt3]

This happened very noticeably again today. Cruising down the highway in some traffic, in 3rd gear, let off throttle, and just before I'd come completely off the throttle, at a little over 2000 RPMs, the engine sputtered/stalled a little before coming all the way back.

I was reading about throttle actuators and it sounds like a bad actuator could be a possible culprit. Any thoughts? Again, this doesn't happen consistently, so I'm not sure if that influences the answer.

[overcoil]

Originally Posted by MegaMatt3

This happened very noticeably again today. Cruising down the highway in some traffic, in 3rd gear, let off throttle, and just before I'd come completely off the throttle, at a little over 2000 RPMs, the engine sputtered/stalled a little before coming all the way back.

I was reading about throttle actuators and it sounds like a bad actuator could be a possible culprit. Any thoughts? Again, this doesn't happen consistently, so I'm not sure if that influences the answer.

Are you in 'SPORT' mode and are you getting exhaust farting sound ?

[MegaMatt3]

No, this is in Comfort. Definitely not the exhaust fart. This is abnormal. The car jerks as the engine briefly stalls.

[jpnh]

Originally Posted by MegaMatt3

No, this is in Comfort. Definitely not the exhaust fart. This is abnormal. The car jerks as the engine briefly stalls.

Have you taken it to a mechanic/dealer yet or you waiting for an online diagnosis?

[MegaMatt3]

In my original post I said that I'd taken it to a dealer and they were not able to find anything wrong. But I didn't describe it well at the time - I said that the engine was misfiring when transitioning between no throttle and some throttle, and that it had a rough idle. When I saw the advisor notes to the tech, it didn't mention the throttle piece, just that the engine was misfiring, so I don't think they took that into consideration during their check. And I've decided now that the engine could be idling roughly, but I'm not positive enough to say it out loud to a tech. I'd rather they focus on the things I know for sure.

So what I'm after is some possible culprits that I could take back to the dealer. Without those, they're just going to see if the car is throwing any codes (it's not) and call it a day.

Do you have any ideas?

[jpnh]

Originally Posted by MegaMatt3

In my original post I said that I'd taken it to a dealer and they were not able to find anything wrong. But I didn't describe it well at the time - I said that the engine was misfiring when transitioning between no throttle and some throttle, and that it had a rough idle. When I saw the advisor notes to the tech, it didn't mention the throttle piece, just that the engine was misfiring, so I don't think they took that into consideration during their check. And I've decided now that the engine could be idling roughly, but I'm not positive enough to say it out loud to a tech. I'd rather they focus on the things I know for sure.

So what I'm after is some possible culprits that I could take back to the dealer. Without those, they're just going to see if the car is throwing any codes (it's not) and call it a day.

Do you have any ideas?

Take it to the dealer and take them for a drive and replicate the problem so they see/feel it

[MegaMatt3]

It doesn't replicate often enough for that to work. And I did take it to the dealer and take it out with the shop foreman. It didn't happen on that drive.

[MegaMatt3]

What about my bad throttle actuator theory? Could there be a problem without the car throwing a code?

If you add up all the symptoms, throttle is the only consistent piece:
- Throttle seems to engage too hard and seems to disengage too hard
- Engine hesitation/hiccups/stalling at very low throttle, usually when coming off

[XutvJet]

The N55 does not use a throttle body under normal operations. The motor uses it's Valvetronic system to manipulate the valve timing and intake cam lift to adjust air intake volume. The motor does have a throttle body, but it's always fully open. Only if the Valvetronic system fails will the throttle body be used. I've never read of a Valvetronic motor failure causing an issue like this.

With respect to these "engine hesitation/hiccups/stalling at very low throttle, usually when coming off" issues, keep in mind that it is a manual thus a direct connection between the motor and transmission. If you're at really low throttle (sub 2000pms) and in the taller gear 3-6, releasing the throttle quickly and applying it somewhat quickly, can induce some stumble, hesitation, etc. especially if you're trying to use more than ~20% throttle. IMO, do not pay attention to the upshift indicator. It's too optimistic for a manual in many situations and can cause lugging. This is still a 3L motor with a turbo, not a high displacement V8 with a ton of torque down low. Sure, the N55 is a torquey beast, but only when the turbo is spooling. At low and part throttle, it's not spooling much at all (if any) thus far less torque is available. Getting on the throttle moderately or hard at a really low rpm can create a lot of boost and high load and flake out the DME. I strongly suggest using more elegant throttle applications at low rpms/high gear and to keep the rpms above 2000rpms when in gears 3-6, especially when approaching steeper grades.

It's really hard to say what the issue is at this point because it's not happening all the time. It could possibly be:

1) The way you're driving (i.e., driving at too low rpms for certain situations and using too much throttle)

2) Simply the direct connection of a manual and movement drivetrain in it's somewhat soft rubber mounts.

3) A very tiny vacuum leak that becomes more pronounced has the motor rocks in it's mounts at lower rpms. The most common leak is at the charge pipe connection at the throttle body (driver's side of the motor, easily observable). Look at it closely and see if anything looks cracked or loose. Also look at the intake pipe joint connection at the front of the passenger side of the motor.

4) Fuel type. If you shop at the same gas station, perhaps consider a different station.

5) Bad coil pack or injector. This would be odd because you'd think the issue would be more pronounced under higher rpms and load and wouldn't make sense in an off throttle situation.


I'd check all the intake piping has noted above, change gas stations, drive at slightly higher rpms (try and keep it between 2000-2500rpms) and consider plugging the negative battery terminal, technically resetting the ECU/DME. I'd also experiment with different driving modes and see if the car behaves the same way. If the problem persists, write a very detailed letter explaining what is happening, when, what it feels like, the driving situation, the gear(s), throttle application and amount of throttle given, ambient air conditions, etc. The more detail, the better.