Re: Engine rattle around 1,500 rpm under load
Guys, I have finally got to the bottom of this. I have gotten myself a BMW diagnostic cable and been putting together some software (see my other thread on this topic). The software is choppy at best but I managed to reset the transmission adaptations incl. torque converter lock-up clutch adaptions and this has actually fixed the issue that seems like an engine-related issue initially.
My TC lockup used to occur at around 1,400-1,500 rpm and when trying to pull away this loads the engine too much at low speeds causing detonation (spark knock).
The advantage of automatics vs manuals is that the transmission can actually deliver more power to the wheel than the engine produces because of the way the torque converter works. The TC allows the engine to rev without actually turning the wheel faster because it's working with fluids. This however results in fuel economy losses hence TC lockup was created. (Any mechanics here, please correct me if I'm wrong.)
You can test your TC lockup by just getting off your foot at low revs (1,500-2,000 rpms) and then suddenly getting on the gas again - just enough so that the transmission does not downshift. If your revs stayed the same but you feel the engine purr, your TC has locked up. If your revs went up, then the TC has NOT locked up.
Giving more foot pressure makes the gear down shift, but when TC lockup occurs too early the gear takes a long time to shift due to the transmission trying to rev match on the new gear speed; hence you feel the engine revving on its own in-between gearshifts.
After I reset the adaptions my TC lockup now seems to occur at around 1,800 rpm and this has reduced, or I dare say almost completely stopped the rattle/detonation.
When giving more foot pressure the downshifts also happen quicker since I notice whenever it downshifts the transmission tries to match the rpm at 2,000-2,100 so the transition from 1,800 to 2,000 rpm is quicker than from 1,500 to 2,000 rpm.
I am pleased to have equipped myself with a valuable tool. This problem has been very difficult to resolve and I have even mentioned this to MA although I did not pursue because it is seriously very difficult to describe. To me, it just felt like a BMW shouldn't drive like this!
MA or PML might not do a reset even upon your request because BMW specifically advised that adaptations should not be reset unless there's a major software or hardware change.
I would like to disclaim myself here that if you car is relatively old you should not perform these resets yourself either. The re-learning process may put stress on an old transmission. My car is relatively new (under 1 year).
I hope this is informative for bros here who feel their 6AT gearbox are lethargic.
P.S. The put the key in, press start, hold throttle, etc. method did NOT work. Resetting TC lockup adaptations *needs* a computer tool.
