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Old 01-11-2013, 11:27 AM
Piers1989 Piers1989 is offline
Officially Welcomed to the 'Fest
Location: Newcastle, UK
Join Date: Mar 2012
Posts: 302
Mein Auto: E65 745i
Originally Posted by DFEL750I View Post
If idle is not fluctuating when in Neutral or Park then its something in your transmission ...most likely the torque converter.

When the car is acelerating or decelerating the RPM of the Engine Flywheel and Transmission input (Drive shaft) are at different speeds.... This means the torque converter has to do torque manipulation to make sure the car operates transparently to the user.

RPM fluctuation happens in this case when the torque converter is not doing enough torque manipulation or its slipping and having issues cancelling out the difference in RPM between the flywheel and the transmission input shaft. It doesnt happen past a certain RPM because the torque converter locks up ..basically connectiong the input shaft of the drive shaft directly to the flywheel. This is done for better gas mileage once the car has gotten up to speed in that gear...saves on gas by eliminating ineffeciencies within the torque converter

If your torque converter is failing your car can also feel very sluggish from the start...its its slipping then your RPM will go up fast but the car wont be moving like it should...if it is locking up prematurely and sticking then your engine will "bog down" when getting off the line because the torque converter is not "stalling" enough to allow the engine to reach its optimum power band before transfering power into the input shaft of the transmission.

Sometimes the torque converter is not working right because bad or low fluid. It is fluid controlled so changing or topping up the fluid can work. Other times its a mechanical problem and the torque converter needs to be replaced.
Thanks for the information.

As the box was just replaced, the fluid is new and has been filled to the correct level. Also there are no leaks.

The RPM seems to stay ABOUT level when idling, but if im honest it doesn't idle as smoothly as my old 745i did, you can feel it occasionally through the car.

I haven't really tired putting my foot down since I got the car back, but it seems to set off fine.

From doing more driving and being aware of an issue I've realised to power deliver when it does fluctuate does feel a bit unsmooth, and I was thinking that if rather then "bouncing around" on cruise, the revs are actually just dripping then rising then is it possible that the engine is causing the fluctuation and the torque converter is actually masking some of the issue by allowing some slip from normal operation?

I know in most auto cars as you adjust load when under very light load the rev needle doesn't stay directly synced with the speed. (My old volvo, proton, mates 335i)

Does that make sense or am I talking out of my ass xD

Last edited by Piers1989; 01-11-2013 at 11:33 AM.
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