Erratic idle
 

My ’91 S2 has developed a very strange problem just recently. Intermittently upon startup, it idles horribly, as if it’s only running on two cylinders. It shakes, coughing and spitting but then I put it into gear and drive and it seems to clear out about 2500 rpm and runs just fine after that, even returning to a normal idle the next time I have to stop. The maddening thing about this is the fact that it doesn’t do it at every startup. Sometimes it starts perfectly normally. It also doesn’t seem to matter if the engine is warm or stone cold. I haven’t got a clue where to look first. Anybody out there ever seen this before?

First off.. I'm no expert but I would check the obvious, cheaper (free) things first.
I had my car laid up for over a year changing parts, taking guesses... My car would barely start then stall within seconds... most experts here said to check for a vacuum leaks... I found a possible one... on the rear end of my balance shaft (a common leaking point)...but that still wasn't the main problem. I changed all of my vacuum lines. Yesterday I disconnected my exhaust just before the catalytic converter and Wha Lah! She finally started! I took the cat section off and inside it looked like a bunch of lava rocks.
She couldn't take any air in because she couldn't get any out!
So if I were you I would first have somebody check your exhaust tail pipe to make sure that your engine is breathing properly... if it is then check out your vacuum lines. You can take a pvc 2 1/2 plumber end cap(or whatever fits in your air intake boot) and drill a hole in it and screw in a air compressor hose end. and fill the system with 7- 10 lbs. of air listen and/or squirt soapy water on all of the connections.
btw... I bought a nice vacuum line kit at lindsay racing
Do you know about Clarks Garage? You can find all kinds of info on that site! It's a must have if you own one of these cars! That and this site.. for all of the great guys that like to help and give support! LOL
Good luck!

The idle control valve is clogged or dead? Under the intake manifold.