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Old 12-11-2012, 01:44 PM
Squidget Squidget is offline
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Location: Boston
Join Date: Jul 2011
Posts: 459
Mein Auto: 2011 335i E90 (MT,Sport)
Probably clutch related. The pros can check my line of thought:

There are three independent spinning assemblies in play. From front to back:

engine + crankshaft -> clutch assembly -> driveshaft + wheels

Things with pluses are linked together, so can be considered one unit. Simplifying, that's three chunks of metal. Let's number them:

1 -> 2 -> 3

The arrows are the boundaries where we can disconnect the spinning metal:

* 1 and 2 are separated by pressing in the clutch pedal

* 2 and 3 are separated by shifting into neutral

To pull away from a stoplight, the steps are:
* At start: 1 is rotating (idling engine). Since clutch is out, 2 is locked with 1 and also rotating. 3 is connected to wheels, not rotating.
* Push in clutch. 1,2,3 are all now unlocked. 1 is still idling, 2 is rapidly slowing down to zero, 3 is still stopped.
* Put shifter into 1st. gearing locks 2 and 3. 2 may or may not be spinning, 3 is not. The the synchro eats any extra energy in 2. Once locked, 2 and 3 are stopped.
* Release clutch. As clutch plate grabs, 1 locks to 2, bringing it up to speed. Since 2 and 3 are already locked, this drives the wheels and the car starts to move.

Diagram of above:
1 + 2   3
1   2   3
1   2 + 3  
1 + 2 + 3
If you have a clutch problem, then 1 and 2 would still be somewhat locked after you push in the clutch. When you put the car into 1st gear, the synchro tries to lock 2 and 3. But that means that 1 is trying to keep 2 spinning, and 3 is trying to make it stop spinning. If the clutch is just a little grabby, the the sychro grinds a bit, but wins and locks in as usual. That could cause what you see.

Diagram of above:
1 + 2   3
1 + 2   3 <- grabby clutch
1 + 2 + 3  <- grind grind!
Something to try: Begin with the car turned off and cold, to make sure nothing is rotating. Start the car, keeping the clutch pushed in the WHOLE time, and shift into 1st.

If it there is ANY grinding at all, then it is definitely the clutch. It can't be a synchro problem. If the clutch works then 2 and 3 would be at zero RPM so there's nothing to synchronize. If there's grinding, then 2 is spinning, which means that the clutch is being grabby.
BMW CCA Member

Last edited by Squidget; 12-11-2012 at 01:48 PM.
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