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      06-29-2018, 04:40 AM   #1
Tomsmith1980
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M3 not pulling away smoothly

Hi All,

I have noticed an issue which has started this week with the car giving a jolting sensation when pulling away. This is what I do to make it happen:

> Brake to a stop e.g. in traffic.
> Car is in gear auto or manual.
> Accelerate normally there is minor pause.
> Then a clunk from the rear like the rear brake has just released. It gives the sensation what you have been pushed in the back. Another way to describe it is when you have cleaned the car and the brakes have rusted slightly you get that sudden release sensation once (the brakes are not rusted in this case).

This does not happen if you blip the throttle slightly to make it roll forwards. It appears to stop if you lift the hand brake on then off whilst the car is stopped - I need to run some more tests whether this is valid every time > this may not be the issue at all.

Any idea what this is before I take it to BMW?

2012 E92 M3 DCT - Frozen Silver - 28k Miles.

Thanks.

Last edited by Tomsmith1980; 06-29-2018 at 09:46 AM.
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      06-29-2018, 06:20 AM   #2
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Sounds similar to my new-to-me 2011 E90 M3 DCT
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      06-29-2018, 08:51 AM   #3
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Easy solution: Get a manual.

Possible causes:
1. Sounds like a normal hill start assist.
2. 90% of the time clunks in the rear are the result of bad shock mounts.
3. It could be your differential giving backlash as well. If you floor the car when drivetrain components aren't warmed up-which takes a lot longer than warming up the engine-then you put tremendous wear on transmission and differential. Your driving style may have affected this.
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      06-29-2018, 09:50 AM   #4
Tomsmith1980
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kyrix1st Thanks for that, I made a couple of notes below.

1. Sounds like a normal hill start assist.
This is happening on level roads.

2. 90% of the time clunks in the rear are the result of bad shock mounts.
Would be surprised if it's this as it's only on pull away and you can be in traffic. I'll get it check though.

3. It could be your differential giving backlash as well. If you floor the car when drivetrain components aren't warmed up-which takes a lot longer than warming up the engine-then you put tremendous wear on transmission and differential. Your driving style may have affected this.
This is my concern, I have only had the car a month and warm the car up, although previous owner may not have.

I'll test the car again today, sounds like a trip to BMW might be required.
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      07-04-2018, 02:40 AM   #5
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What shift mode are you in? S3 or higher in constant start stop traffic can get a lil jerky despite how awesome you think your right foot is. Try S2 in traffic as it's the first shift setting to use 1st from a stop. It just drives like grandmas M, so don't freak out.
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      07-05-2018, 06:12 AM   #6
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Quote:
Originally Posted by Tomsmith1980 View Post
kyrix1st Thanks for that, I made a couple of notes below.

1. Sounds like a normal hill start assist.
This is happening on level roads.

2. 90% of the time clunks in the rear are the result of bad shock mounts.
Would be surprised if it's this as it's only on pull away and you can be in traffic. I'll get it check though.

3. It could be your differential giving backlash as well. If you floor the car when drivetrain components aren't warmed up-which takes a lot longer than warming up the engine-then you put tremendous wear on transmission and differential. Your driving style may have affected this.
This is my concern, I have only had the car a month and warm the car up, although previous owner may not have.

I'll test the car again today, sounds like a trip to BMW might be required.
Hill assist works on all kinds of roads. On my manual car, if I brake to a complete stop and then keep the car in gear with the clutch depressed, the brakes hold the car for a moment after I release the brake. If I do a rolling start or keep it in neutral, then release the brake and put it in gear and go, the hill start assist does not hold the car.
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