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      01-30-2022, 01:57 PM   #1
2500AB
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M235i 6MT RWD - Something feels loose in the driveline when at speed

Hi there.
Hoping someone can chime in to help...

I'm facing some strange issue with my 2016 M235i 6MT RWD. No mods. 30K miles.(well, it is 40K, but issue was already noticeable at 30K).

When cruising at higher speeds (60+ mph) and higher gears (4-5-6) there is a noticeable thud in the driveline when I accelerate (20% throttle or more) or decelerate (just get off the throttle).

This is not happening at higher gears/speeds and there are no clunks or unusual vibrations. Also does not happen if I ease it on/off throttle.

One noticeable thing is that engine RPMs seem to free-float by about 100 rpm every time this happens. In other words, I can be going at 3000 rpm in 6th gear and off throttle, then I hit the throttle - engine would go to 3100 rpm instantly, after which I feel that thud from the driveline and then steady acceleration begins. Same applies when I let off the throttle - goes from e.g. 3000 rpm to 2900 and thud happens.

Does feel very mechanical and there are no codes.
All bushes seem OK and there is no play in the driveline at low speeds or when trying to check things around with pry bar.

See vid describing "free" jumping rpm bounce when going on/off throttle https://youtu.be/KJX1NFxz9m0

I suspect it could be the flywheel, but not sure how to test... Maybe a guibo, but why would it feel solid at lower speed with more torque?
Diff gear play and bushes are also on the list, but why is it not happening at higher gears?


Any help would be much appreciated.
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      02-05-2022, 10:21 PM   #2
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If it helps to anyone - I was planning to install a short shifter and stiffer shifter bushing (turner polyurethane).

Well, got it installed today and driveline no longer feels loose. It still bounces those 100 rpm's, but I can no longer feel the thud on accel/decel. So, party time 😁

Rear shifter bushing was pretty much destroyed, despite relatively low mileage.
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      02-07-2022, 03:13 AM   #3
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Interesting, take a look at your shifter when you load/unload the engine. Does it lean to the right when you load it? Mine does, moves maybe 3/4- a full centimeter over when I give it spool up a bit in second gear. No clunk or thump but I do notice that in 4-5-6 gear if I’m below 3400 rpm and whack the gas it will surge up a small bit, maybe 100rpm, I’ll hear my BOV hiss a little air and the tach will fall lower to the appropriate rpm. I always just thought this was an overboost situation because of the aggressive ots tune + lugging situation but it might be the same as yours- if we are actually just spinning the trans+diff combo on their soft stock mounts.

Can you post a video of the behavior? I’ll get one tomorrow and post it too.
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      02-07-2022, 03:21 AM   #4
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Oh- I just saw the video. That’s weird how its bouncing for you. I would be interested to see if that is an rpm change the cars dme is registering or not, it almost looks like its a notchy tachometer needle that is trying to display instant changes in the display commanded rpm. Mine is definitely more load related almost like a slipping clutch but way way more mild and there is no loss of power, and it never happens in lower gears (2nd gear 3500 stg2+ antilag won’t even chirp my tires in summer they are hook so well) or higher rpm which tells me its a mount problem and not a clutch problem.

I have seen noticeable thunk feelings arise in motorcycles when drivelines are too loose, either the chain adjusted too loose or a super worn and spread out sprocket. I think even though your mounts feel good to a hands on inspection, I would like to see a gopro mounted recording your driveline as you load and unload it. You may find a decent shift if its bad enough to be visible all the way on your tach like that.
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      02-07-2022, 09:01 AM   #5
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Quote:
Originally Posted by 2500AB View Post
If it helps to anyone - I was planning to install a short shifter and stiffer shifter bushing (turner polyurethane).

Well, got it installed today and driveline no longer feels loose. It still bounces those 100 rpm's, but I can no longer feel the thud on accel/decel. So, party time 😁

Rear shifter bushing was pretty much destroyed, despite relatively low mileage.

Could you please post what short shifter, and bushings you bought ? (I am interested in improving my shifter feel). Thanks
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      02-09-2022, 09:04 PM   #6
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Not an ad, but this is what I got.

https://www.ecstuning.com/b-turner-m...a~dk/?pdk=AQcI

https://www.ecstuning.com/b-turner-m...hoC0XsQAvD_BwE



Shifter feels really nice and helps dramatically to have smoother shifts, while bushing ended up adding some NVH and I'm not 100% if I like it when not pushing it. Waiting for some warmer temps to see if it improves.
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      02-09-2022, 09:12 PM   #7
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Quote:
Originally Posted by rtl32 View Post
Interesting, take a look at your shifter when you load/unload the engine. Does it lean to the right when you load it? Mine does, moves maybe 3/4- a full centimeter over when I give it spool up a bit in second gear. No clunk or thump but I do notice that in 4-5-6 gear if I'm below 3400 rpm and whack the gas it will surge up a small bit, maybe 100rpm, I'll hear my BOV hiss a little air and the tach will fall lower to the appropriate rpm. I always just thought this was an overboost situation because of the aggressive ots tune + lugging situation but it might be the same as yours- if we are actually just spinning the trans+diff combo on their soft stock mounts.

Can you post a video of the behavior? I'll get one tomorrow and post it too.

Hi, thank you for chiming in. I should add that I did try full solid polyurethane transmission mounts and while that did further improve the feel, it was way way way too harsh so I reverted back to stock for the mounts. Rpm bounce did not change.


I do suspect that rpm bounce is related to flywheel/spring compression - there was absolutely no play in driveline after the tranny... Prob need to test drive another manual 2-3 series to see if it does the same.
As mentioned, rear shifter bushing seems to have made the most noticeable impact on how tight everything feels.
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      02-22-2022, 08:11 PM   #8
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Quote:
Originally Posted by 2500AB View Post
...
Shifter feels really nice and helps dramatically to have smoother shifts, while bushing ended up adding some NVH and I'm not 100% if I like it when not pushing it. Waiting for some warmer temps to see if it improves.

Still happy with the shifter action and the NVH ?
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      02-25-2022, 11:00 PM   #9
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Quote:
Originally Posted by Wheelwright View Post
Quote:
Originally Posted by 2500AB View Post
...
Shifter feels really nice and helps dramatically to have smoother shifts, while bushing ended up adding some NVH and I'm not 100% if I like it when not pushing it. Waiting for some warmer temps to see if it improves.
Still happy with the shifter action and the NVH ?
Short shifter is excellent. Very happy.

Ended up removing the poly shifter bushing and putting stock one back - miss the absolute notchiness of poly, but prefer to hear exhaust and engine instead of transmission gears and syncros .
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      10-14-2022, 08:06 PM   #10
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Alright, sounds like it is time for an update, in case this helps anyone in the future.

Some background for starters:
- Car came with LSD (dealers were installing those pre-sale, and touching/re-boling the drivetrain) .. maybe an important factor for later.
- Ever since I bought the car (not the first owner, but under 30k miles) - there was that weird thud noise any time I transition from acceleration to coasting and back, as described in the first post.
- I installed a short-shifter and poly shifter bushing. That helped with this strange "clunk" a lot, but not for long. More on that later.
- Another observation was related to vibration at speed (above 70 mph). I never connected the two issues and always thought I had wheel balance issues or a bent rim.


Fast forward, vibration issue got worse and all the way to the point that I no longer enjoy driving the car on a highway. Everything is great at slower speeds and when accelerating, but as soon as I let off the throttle I get that thud/clunk and vibration. As soon as I give it even slight throttle - we are back to normal.

Tired of this situation I went for all 4 wheel refurb/straightening and road-force balancing. Two were indeed slightly bent and were redone to 100%. Excited me went on a highway to only discover zero change in experience, ugh...

Next - took it to two shops for inspection as I ran out of ideas on jackstands. Shops came back with the following suggestions: engine mounts, trans mounts, clutch/flywheel, diff clutches, subframe mounts, tires. I mean - can't prove any of this and all seems to be just a shot in the dark. Did not move forward on any of this.

Further reading, I found numerous topics about older F series cars, mostly 5 series with driveshaft issues. They had a different connection to the rear diff than F22 and now having a lot of issues with vibrations due to driveshaft lisalignment.... Well, between that and the idea of broken diff mounts, I got the camera going and tried to just bounce the car in 1st and reverse while standing on the brakes - see the results below. My hope was to see at least something.

My two observations and thoughts (please comment if you see anything else):
1) Clearly, driveshaft is not firmly connected to the diff. You can see it is shaking and loosely moving around while not under load (I guess 3-5 mm play). I did compare it to another F chassis and it should be absolutely solid).

2) Rear diff bushing seems to be moving quite a bit. Going to inspect it further, but did not find any issues yet.

https://youtu.be/kiGn9z_8RQU


What is next?

I ordered a new 50mm insert nut to connect driveshaft to the diff along with 50 mm wrench. Ordered new guibo (mine looks good, but it is factory Aluminum one and BMW superseded it with the rubber one - some say that aluminum may have failed and can't be diagnosed until removed).

Plan is:
-take off the driveshaft (check insert nut torque before removing).
- replace guibo along with 6 new bolts.
- inspect driveshaft central support bearing and splines for wear + re-lube.
- put everything back together and tighten the new insert nut.
- further review diff bushings for damage.


Will post an update when parts arrive.


P.S.
- The fact that dealer touched the rear diff, could this be the reason why driveshaft is loose on the diff? Was something not tightened properly?
- when I installed the shifter and shifter bushing, all this work involved a lot of driveshaft touching as it was very hard to reach those parts... I suspect that I moved it just enough so it was in the right spot to correct the problem for a few miles.
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      10-17-2022, 10:30 AM   #11
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It's the Guibo
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      11-03-2022, 07:03 PM   #12
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Time for closing update! 🙂

Upon disassembly, I confirmed that the insert nut was loose, allowing for excessive play in the driveline any time car transitioned from acceleration to deceleration.

Here is the visual - you can clearly see the nut was not tightly attached to the diff, causing all kinds of play. No surprise there would be a lot of vibration at higher speeds.

https://youtu.be/QaY4hkuMLh8


So, how we fix that?
Order of operations was pretty simple and ended up costing about $70 in parts (new insert nut, new guibo, 6 guibo bolts/nuts).

1) Safely jack up the car, remove exhaust and heat shields.
2) Loosen up the insert nut (it had play, BUT it was really really tight!) With 50mm wrench.
3) Loosen up center support bearing and guibo.
4) remove driveshaft.
5) clean everything up.
6) Put new guibo (not required, but I replaced original aluminum one with the revised rubber one as it supposed to be better - and it did add smoothness to when I shift gears).
7) Lube and torque as needed following the BMW manual ( start with insert nut, then guibo, then CSB).
8) Enjoy the smooth ride!

Some pictures attached for your viewing pleasure.


I put about 700 miles since the work was done - The car never felt this tight and organized.


Upon further review, the threads on the driveline had two types of locktite - I suspect one was from the factory and 2nd from when dealer installed the LSD (aka dealer did not clean up original sealant).
Dealer did likely follow the torque spec to install the LSD, but the nut reached the torque spec before it fully secured the driveshaft.


Work took about 4 hours and was a rather satisfying process as things came together quite well - especially considering how many parts needed to be removed.


P.S. - I did first try to tighten the original nut to test if this would fix the issue. Indeed, it fixed it, allowing me to go full confidence on making a proper repair.
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      11-03-2022, 09:55 PM   #13
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Great update!
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