HedonisticFrog

Hi, I recently started fixing up a 1996 S420 and it developed a new problem. It ran and drove well when I bought it. One very peculiar thing is that nothing will trip the check engine light and the bulb isn't burnt out. Even with the MAF and an oxygen sensor unplugged it still doesn't light up but turns on when the key is in the on position. This makes diagnosing the car a lot more difficult. I replaced the PCV hoses since they were super hard and leaking oil, and the passenger side downstream oxygen sensor with one I had on hand from a 2001 ML430 that I dismantled. It seemed to run better after that since the old one kept reading lean for a long time when the driver side went back to .460 quickly. It felt like it had more power and idled smoother. I then decided to change the spark plugs with NGK iridium ones and used dialectic grease on the boots. I then drove 50 miles to my parents and back in which it was putting out huge clouds of white smoke but it was too late to turn back by the time I noticed it. It seems to run a lot better at constant speed or just idling. It puts out the most smoke when accelerating from a stop. It's not losing coolant, and there's no mixing of fluids so I doubt it's a head gasket. The other telling thing is that the fuel economy is terrible right now and is high single digits so it's just dumping fuel into the exhaust. I used about six gallons driving 50 miles. Unplugging the MAF didn't help. Unplugging the air temperature sensor didn't help. Unplugging the new oxygen sensor didn't help. Another odd thing is that the live data still showed the oxygen sensor at .460 even though it was unplugged. I even swapped the old spark plugs back in and removed the grease but nothing changed. It idles well and still accelerates quickly as well. I'm just lost at this point.