I have a 99 saturn sl2 is running rough i`ve replaced plugswires &coils still has A rough idle once you get going it runs great

the sensor that controls the mixture is the heated oxygen sensor (HO2S) in the exhaust manifold
compression is low as it should be no less that 125 psi that is if the readings were taken with the throttle wide open
the noise in the exhaust indicates unburnt mixture accumulating in the muffler to flash off
check for a crack in the flex section of the exhaust near the manifold

SOUND LIKE BAD EGR VALVE.IT WILL CAUSE ROUGH IDLING AND STALLING OUT ENGINE WHICH WILL CONTAMINATE OXYGEN SENSOR WITH RICH FUEL MIXTURE.

Try replacing a valve called the Idle Air Control Valve, if your car has one. See if that solves it. Not sure where it is located on your specific vehicle.

If previous to this issue you were seeing:

A) a rather noticeable rattling almost like a marble in a coffee can type sound

B) the engine running a bit warmer than usual after driving, say 210-225 or so, but not so much if it's only allowed to idle.

C) a check engine light, intermittent or not, reading an egr system code or maybe even something with the map sensor.

I would start to look at a plugged up exhaust system.

Outside of that you may wish to look into having your coils and/or ignition control module tested. Most parts stores will do it for free if you take them off the vehicle and bring it in.

Sounds like an IAC motor( Idle Air Control) easy proof test is to bring it to Auto Zone or Advanced Auto and have them scan it for any codes, with a rough idle something should be set in the computer.
Once a code pulls up they can identify what it is where it is and how much it costs and if it is readily stocked.Thank you and have a great day!!!

I would look at the egr system. First pull the valve and clean it. If you have a vacuum tester this would be a good time to test the valve itself as well. When under load you should be able to see the diaphragm engage and hold and then release as you release the vacuum. If that doesn't work check and/or replace the egr solenoid. It's located on the driver's side of the valve cover and usually runs about 60$.

Yours is a common complaint and the above is the most common remedy. Outside of that you might want to check and clean the idle air control valve and the port where it's housed. If it is extremely dirty or something managed to get stuck in there that could affect you vehicle running at idle as well.

Another thing, even though you just changed the plugs, pull one back out and look at it. If is fouled with oil you might need a new valve cover gasket which if it's leaking badly enough will impede spark.

Try replaceing the ECT sensor. Mine was doing same thing, this SOLVED my problem!!

Could be fuel pressure regulator, fuel injector, sensor, or gasket leak.

Most likely a Coolant Temp Sensor & Connector.Under upper radiator hose at cyclinder head.Yellow & Black wires.Hope this helps

its an old car..............................