Hey so OP is back with a similar question (idle issues) but on a different car... just picked up a second E46, an '01 325xi with about 155k miles on it from a friend. Thinking back to this post I pulled the ICV, DISA and throttle body to clean them out and make sure they were operating correctly and no leaks. Also checked entire car for any vacum leaks and replaced a couple hoses for the oil seperator. Also checked the plugs and replaced the VCG as well as installed a new battery. Now what is happening is that car starts fine (although cranks maybe a milisecond longer than my other car) and idles fairly normally... high at first then settles after about a minute at about 700-800 RPM. But, I'd say half the time when I tap the gas the RPMs drop and the car dies.
Original owner is a buddy of mine and never had this issue but the car sat for almost a year due to a broken drive shaft that he was too lazy to replace. Could this simply be a case of bad gas in the tank from sitting so long? I can hear the fuel pump prime when turning the ignition so doubt that's the issue and I also put in a quarter tank of Chevron 92 just in case. Any thoughts?
BTW, he was pretty obsesive about maintenance up until the drive shaft. Oil changes every 5000 miles using mobile one, all the services have been done, coolant system replaced. Not sure if fuel filter was ever done though. Which brings up a second question... is there any reason to pay the dealer for their fuel filter versus one from an auto parts store for a third the price?