Cold start issue

[RustyTheDriver]

Now that the cold weather is here I've come across a problem with my 318is. When I start the car in the morning, and the temperature is anything below 65 degrees f, I seem to have issues warming the engine up. Now this could just be impatience or paranoia, but I have literally sat for 15min trying to warm it up. Turning on the car, I let it sit until the water temp gauge clicks on. I then attempt to bring the engine up to 1500rpm to warm it up. Here's where the problem starts. Something happens (I'm assuming a misfire?) and it drops back to about 800 with a low, throaty exhaust sound, and it shakes the car quite a bit. No matter how much gas I give, I cannot get the engine to turn above 1500 until the water is out of the blue. I looked at the intake bellows and it looks like it has been previously patched but even with some reinforcement (don't worry I am looking into a new set) it still has this issue. Any ideas? I'm assuming either the thermostat or this bellows issue (creating an unstable air/fuel ratio).

[strad]

Yes, if there are any large vacuum leaks, the motor will run like crap, so definitely cure those if you think there are any.  The DME also takes a signal from the coolant temperature -- you probably should look into testing that sensor.  As for warming up, I've got a brand new thermostat and no air in the system, and it does take longer for my M42 to warm up than it does for my M20.

[DZick]

i had the same problem, but after i would let off the throttle, the rpm would drop so low it almost dies (~50rpm) then it jump back up to 1200.
 
Fixed it with vacumm lines (& throttle body heater delete so now i have i  only have 4 hoses instead of that mess)

[RustyTheDriver]

The first time this happened I simply assumed it was because that having a straight-4 heat up is naturally going to take a while. Yeah, the intake bellows are on the list. I'll look into the DME sensor, thanks for the suggestion.