1990 Oldsmobile 98 Runs rich

A bad MAP sensor will make it run rich on cold startup. But, I would be more concerned with the ECM first. In your posting you "replaced module", which module? Please advise.

Well, without engine codes to go by, I would have followed the same routine you have. I'm going to give you a few assumptions.
Then you start the car cold, it "chugs" until it warms up, then it runs fine. Yes?
At a cold start, if you rev it up a little, it kinda' clears up a little, but back at an idle, it chugs again. How am I diong so far?
A theory, engine temp sensor is telling the ECM the engine is cold and richens up the fuel pulse to compensate. It should also open up the idle air control valve to compensate for the richer mixture, but it's not. Have you cleaned the IAC control or checked it for operation? Please advise.

Charlie, if you raise the rpms or after the engine warms up, do the lights go off? They both get their signal from the ECM. If they are on all the time, I would suspect a problem in the wiring between the dash panel and the ECM. Let me know.

Charlie, if the IAC valve is just "grunged up", you can clean it with a can of spray carb cleaner. This seems to work the best.

This is a "toughy". With no engine codes, let's try to create one. Try unplugging the O2 sensor and see if the computer set a TC for an O2 problem. I think if we find out why the ECM isn't registering codes, we'll find the problem. Let me know. Also, if the motor is running as rich on startup as you say, you may have fouled a plug or 2.

Everything still points to the IAC. With engine cold, pull the IAC out. When you turn the key to "on", the IAC motor should pull the valve back, or to open. If not, do you have voltage to the connector and continuity through the motor?

OK, when you start the cold, and while it's doing it's chugging thing, pull off a vacuum line somewhere on the intake manifold so you create a gross vacuum leak. Your checking to see if the rpms rise and the "chugging" improves. In doing this we are doing the same thing as the IAC does.

Does the "chugging" seem to be less with the vacuum hose disconnected?

There has to be a problem with the wiring between the ECM and the IAC unit. Do you have, or have access to a fax # and I can ship you a copy of the diagram. I can attach it to this post, but it won't come through very clear. There are 4 wires connecting to the IAC. One open circuit and one to close. Apparently yours is closing alright, but it won't open. Please advise. I wish I had a diagram for the intermnals of the ECM, but I don't.

Charlie, the only thing I can think of is the CPS must have been sending a late signal to the ECM therefore, retarding the ignition timing and injector pulse. Something in my guts tells me this is going to come back to haunt us, I hope not. Glad you found it. I'll keep this one for future reference.