Help! Engine malfunction:reduced power. stuck in Chicago

[Sophisticated Redneck]

Originally Posted by wagonguy

I had a very similar problem recently. 2011 M, bought in California, on the drive home to Nevada, got the engine malfunction/reduced power alarm. Pulled over, shut it down and restarted and all seemed good. Over the next two weeks kept getting boost codes and torque reading codes. It would shut off the boost and it would drive like a slug. Ended up being an old battery. Put a new battery in and registered it, ran flawlessly since.

Doug

Thank you for bringing this up. I forgot about it. This can happen in rare instances where the battery is not quite completely dead (just enough to start the car) but has very little capacity left. BMW has designed the charging system to only kick on the alternator during braking or when the voltage falls to an extreme low level or if certain high-drain accessories are turned on like headlights and foglights.

Have you ever noticed when you slow down and brake during the day you can hear your ac/heater blower fan increase in speed? This is why. The alternator turns on when coasting or braking. Now if you have a bad battery and decide to gun the throttle, it can drop so fast in voltage your computer goes nuts before the alternator kicks in and the voltage is so low the coils can't effectively fire, especially if they have some age on them.

So Always drive with your headlights and foglights on if your going to be stepping on it or if you think you have a weak coil or two. This causes the alternator to immediately come on and will keep the voltage at 13.5+ volts, otherwise it can drop as low as 11.3 before it forces the alternator to come back on. Grab a cheapy cigarette lighter voltage meter if you would like to see this insane charging system in action. It's how I learned that headlights + foglights forces the alternator on and voltage jumps nearly 30 percent in some instances.

Whoever designed the charging system was an absolute imbecile. All this for less then 1 percent gas savings.

TLDR version: drive with your headlights and foglights on if your racing or your battery is going south or as the first step to see if it has an effect on misfire codes.