I have a 91 brougham with the 5.7, 105k miles. The car missed at light pedal under load and would run terrible at full throttle, almost to the point of stahling. So I replaced the following; cap,rotor,sp wires and plugs,air filter,fuel filter, exhaust system for the cat back. No help ran the same. So I retrieved the codes, got rich o2 sensor and evo sys failure. Replaced o2 sensor. Runs the same, crappy! Also I couldn't clear the codes per procedure. Previous owner said car has been doing this for 2 years on and off. Please help me the car has a app on tues. , but I really would like to fix it myself, but I don't want to keep putting on parts it doesn't need. Thank you.-Black Deville

Check all your vacuum lines.

change the catalytic converter. or just knock a hole through it.

It was the cat! thanks!