hi
my car is a 92' STS Seville with 4.9L engine (not norhtstar). It starts and runs without any bigger problems. The only thing is that it runs a little rough when it idles. There are no errorcodes in the OBD, but i cant read ignition specific ECM Data in theOBD like "ignition point in ° before TDC". This points are just not listed anymore in the OBD.
A few weeks ago, I had to replace the ECM. But the new one is exactly the same. The Part number is the same as the original ECM.
Anyone who knows something about this problem? I'm glad for every helpful information.

4.9 will has slight rough idle normally. If you have already done tune up,cap,rotor,wires & checked for vacuum leaks.Check TPS -1.0-1.0 & MAP less than 40 at idle. Do power balance and injector drop tests, fuel pressure should be 28-38 psi at idle. Check for carbon build up on throttle plates. Timing set at 10 btdc. Should have relearned idle and tps when ecm was replaced.

Thanks for the help
I haven't done everythingyou mentioned yet, but I checked injection time, TPS and ignition point. I saw in the OBD that ignition point in idle is set 25° to 30° before TDC. Is this normal?
Ignition time was 3.6ms onall injectors and TPS is 2.9 in idle.I will check the other things too when I have time.
Thanks again for the information.

Is there a possibility to check all cilynders are working without pulling the spark plugs off? Or is the only way to be sure they're working to pull the plugs and controll the spark?
Thanks for answers

At a shop a scope can do it. On your own Factory shop manual can tell you how.