i have a 91 740 gl. in good weather it runs fine, but throw in cold or especially rain and it wont idle for crap. I can put the car in neutral and keep it running by reving it up, then puting it into gear and giving it some gas to get going. but even once im moving, if i floor it, it goes really sluggish. iv replaced my air idle valve. could it be the maf? or a vacume leak? why does it run good with the warm, dry weather?

 

Have you cleaned the throttle body?
Yes it could be the Mass flow sensor.

 

Also check the accordian looking hose for holes. Unmetered air will make it run bad. It rubs against the fender and get a hole worn in it usually.