Why do the RPMs drop when idle?

-I drive a BMW 350i or something like that. Late 80's to early 90's. 5 speed manual.



When I start my car the Tac rises and drops between 500 and 1500 rpms constantly. If I'm sitting at a light or in a parking lot, the same thing. When it drops below 1000, it sounds like it might shut off, but then it revs-up to around 1500 and then back to around normal 1000. Repeating the cycle. Just started recently, like around 3 days or so ago.



Some of the changes in my driving include later shifting to reduce jerking on my car, down-shifting to reduce riding the clutch when slowing down (including rev-up in neutral during down-shift to catch the clutch smoothly). I also have stopped parking in first gear.



I don't know what has happened to cause the engine to go up and down like this in idle.Dang, dawg- do you own this car or what? Now check it - you need a toothbrush and a can of choke cleaner. We ain't choke'n no chicken or nutten funny like that, we just gone'a clean your intake. Remove the big ho, I mean hose at the throttle body, and with that screwdriver, wedge open the butterfly, and spray, spray away. And scrub all-up-in-it with your roomates toothbrush. Now, get in all the holes, and any place things might touch, spray some more- youain'tt hurt'n it. Install the air hose crank it up (its going to puff out a li'l black smoke,that'ss cool), and ride sally ride. Now if it returns, you got a fitty fitty chance of replacing the TPS (throttle position sensor) or IAC valve (idle air control valve), all/ any of which you can do yourself brother. Now dig this - 1st thang is to make sure ain't nutt'n come loose, like a hose, even a small one, talk'n bout jerken and yanken before. Don't sweat it. you got it.Lots of possibilities.

The air pollution system takes feedback from the O2 sensor, and tried to make changes in order to reduce pollution. But if something is wrong, it will swing back and forth wildly, from one extreme to the other. One possibility is an intake manifold leak. Most common is a cracked, loose, or broken vacuum hose. But your PCV valve from the valve cover, will cause oil and water foam to build up on the Mass Air Sensor, making it think there is no air moving into the engine. It works on heat being cooled, and the foam insulates it from the wind of the air. It can easily be taken off and cleaned. If fuel pressure is low from a dirty gas filter, that can also do it. But you could also just have a misfiring spark plug wire.