hesitation/stumble under acceleration

[olds350]

I had posted in another thread about what I thought was a transmission slip on my 128. Just to level set, I have a '12 128 'vert with 89k miles on it and very little service history.

I few hundred miles ago I started getting what I thought was a transmission slip, but it was hard to duplicate. I started watch the rev counter with the thought that a tranny slip would cause the revs to climb and an engine stumble would cause the revs to drop. That bit of detective work didn't quite pan out as the gauge was too slow to respond to those quick stumbles or hesitation.

I had fears of a transmission rebuild, but I couldn't put my finger on the issue. Car seemed to run fine except when under modest acceleration. I even put some HEET gas dry in the tank thinking I may have water, but that made no difference.

I put my new to me Carly reader on the car and I found no codes.... frustrating. Took it to my trusty mechanic and asked him to put his high dollar code reader on the car. He came back and said there were no codes. This made no sense. After a bit of conversation, it was suggested to change out the plugs since BMW revised their maintenance and plug changes are scheduled for every 60k now. Couple that with the fact BMW no longer recommends the Bosch coils and they should now be the Denso brand.

With a new set of NGKs in hand I started changing out the plugs. As my luck turned out, I had a set of Bosch coils in the car. I know these motors like to eat these coils as I've already had to replace a set in the wife's '11 328 and that's the same motor.

As luck would have it, a local parts store had a set of coils. Not the brand I wanted to use, but I needed a set and they had a limited lifetime warranty, so I should be good for a few years at least. I started out to do the coil and plug swap. Wouldn't you know it, I dropped the first plug and it landed on the belly tray. There's an extra 15 minutes of frustration fishing the plug back out.

An hour later, all the plugs and coils are swapped and everything was buttoned back up. Key in, fingers crossed, hit the button and it started right up. Out for a quick spin for a few miles and no sign of the hesitation. I need to run out this gas mix with the HEET and then do another fill up and test from there. So far, so good.

[asiflicious]

You would definitely know and the car would tell you if your transmissions was slipping. Kind of sounds like you had slight misfires underload and freshening up ignition system fixed the problem. Good luck!