Why does my bimmer have a rough idle?

[Dopplemire]

Hi guys! 2nd time BMW owner, first time poster: I need some help.

I just bought a 1993 318is w/ 156k.
Overall a solid vehicle, I'm pretty happy with my purchase, but I need help diagnosing engine problems.

When cranked, the car idles very rough, jumping between <500 and 1200 rpm. When you get on it and give it gas the motor pulls strong and smoothly, but once you remove the load from the engine (push the clutch in/shift into N) it will drop off and choke unless you blip the throttle to keep it alive. It runs VERY rich and drinks gas.

I unbolted the intake manifold, checked every hose and fitting that I could get my hands on and cleaned the ICV. I then pulled off the MAF and the TPS and cleaned them. When the MAF sensor is unhooked the idle smoothes out completely. Not perfect, but it doesn't choke at all. When driving with the MAF unhooked, the engine idles smoother but runs even richer, and eventually will refuse to accelerate above 2k (I'm thinking the car puts itself into limp-mode).

I've tried to do as much homework as I can, but I can't figure this one out:

After doing the above to alleviate the rough idle (to no discernible improvement) I parked the vehicle, and unhooked the MAF and plugged it back in (with the car off). The bimmer started right up and ran PERFECT. No rough idle, no trouble at all, good fuel/air mix; it was purring like a kitten and driving like the day it came off the assembly line. It stayed like this for about 2 hours of short trips around town. Then, unexpectedly while driving, it went back to the rough idle, running rich, stalling when the rpm dropped etc.

I pulled over, turned the vehicle off, unhooked the battery, then tried to replicate what I did earlier (disconnected/reconnected the MAF) and got about 30 seconds of 'perfect engine' performance, then it was back to the rough idle and stalling.

I have not been able to replicate the 'perfect engine' after that moment. Tonight I cleaned the MAF again as well as most other sensors with contact cleaner. Their is most certainly a vacuum leak, as I can hear a slight hissing coming from somewhere under the intake manifold/behind the crank case.

When I completely seal off the air intake (which should stall the engine) I can hear the noise of a leak pulling in enough air to keep the engine alive, but I can't locate it. It sounds to be under the intake manifold (but I already checked all the hoses) or from behind the engine block.

Is my MAF bad, or is the vacuum leak solely to blame for the rough idle?

What is likely the source of the leak? How can I pinpoint it? Any non-obvious or non-visible hoses that I should look for? Gaskets?

I've tried spraying water around the engine bay, but was unable to get any reaction out of it.
This E36 is OBDI and throws the CEL occasionally, but it never stays on longer than a minute or so.

Sorry for writing a book, but I need this car as my DD so any and all help is appreciated!

[JE BMW]

I cleaned my maf and still had the same problem.
i called a junk yard, they let me try the maf they were gonna sell to me before i bought it. well i went, swaped it and problem solved..
sometimes if you touch the inside of the maf while cleaning it, it gets messed up.

[KimiSan]

I have nothing helpful to offer you except praise for your pretty thorough description of the prob.; I hope it works out/ someone here has the solution.

[slocar]

If it's flashing a check engine light, there are codes being stored: have someone with a BMW scanner scan it or do the "stomp test" ... that would be a good start to at least know what the car thinks is wrong with it.

ps. "Cleaning the MAF" does nothing on your car. It has an AFM ... it's just a potentiometer with a door, there's nothing to clean ... you run more chance of ruining something spraying things in there than making anything better.