Consider cleaningg the EGR
One thing to help the engine burn "clean" would be to take the EGR valve off the top of your engine, take it all apart and then with one of those wire brushes you can buy at the dollar store, take all those years of caked on, crusted exhaust emmissions. It'* the easiest project in the world and it helps the car idle really smooth.
Don't use any solvents, just a wire brush, maybe some water and a bit of lubricant, (WD-40 etc) for the air valves.
Good Saturday morning project.
1990 Pontiac Bonneville LE
250 000 KM
NEW: Gas Tank, Fuel Lines, Brake Lines, Front Axle, Brakes, Rotors, Calipers, HI-Beam switch, 02 Sensor.......all done by me.