Start with the vacuum hoses and air cleaner, then replace the spark plugs, wires, distributor cap, and rotor. Check for oil leaks, oil burning, and any history of overheating, which can keep this engine from passing DC emissions.
A 1989 Cadillac Sedan DeVille with a 4.5L V8 and 85,000 miles failed a DC emissions test with a reading of 246 versus a 220 passing score. The owner asked about the least expensive fix and whether the catalytic converter could be cleaned or flushed. The expert said this engine historically burns dirty by modern standards and that aluminum-engine oil consumption or a prior overheating event can prevent a pass. Recommended starting points were vacuum hoses, the air cleaner, plugs, wires, distributor cap and rotor, plus checking for obvious oil leaks.
