Okay here is what I have done. New all AC Delco parts, wires, plugs, rotor, distributor cap, egr, cleaned EGR tubes. Still getting the stumbling, hesitation on the highway. Cat okay. I have only 3 things left to replace, ECM (AC Delco for $75.00) TPS (AC Delco for $83.00) and MAP (AC Delco for $41.00). Anyone taken their 89 Cadillac 4.5 litre into the dealership to see what they had to say about this problem so I can select the ECM, TPS or MAP to replace next? The AC Delco distributor guy says GM was famous for bad ECM's in the 80's and 90's! Anyone else want to take a guess at what should come next? My hands are raw from changing those plugs at the back of the engine. What a job!!!!! HELPPPPP!!