I have a 1991 Cadillac DeVille sedan with about 150,000 miles, I purchased this at an estate sale. As soon as I bought it, I replaced the spark plugs, wires, distributor cap, rotor, serpentine belt, power steering pulley, and of course changed the oil and oil filter. I was getting about 14-15 mpg (compared to 12 when I picked the car up). And the car ran good, not like brand new, but good. About a couple of weeks ago (as it started getting hot) the car would start chugging, backfiring and stalling, only after about 20 miles of driving (my commute to work is 35 mi), so I would spend the other 15 miles driving with four-ways on going at most 30 mph. But, on the drive home (at 2 am) I would have no issues. So, I got sick of that fun and changed the PVC valve and fuel filter (which looked like the original) and with every fill-up in addition to using 93 octane gas (as directed) and fuel injector cleaner, the gas mileage is around 10 mpg and the chugging persists after heating up. Two things to note, 1) the fans for the radiator do not turn on at all, and 2) the catalytic converter doesn’t smell like rotten eggs. I’m getting tired of guessing, as a faithful listener of Car Talk for almost 15 years, I’m really hoping Tom, Ray, or anyone else can help me!!!
The 4.9 is a solid motor. Could be a plugged/failed cat.