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.

 

The only difference to your issue is that my MIL works fine. I was in the process of getting my vehicle to pass smog test. It failed twice. Using using an Actron CP9650, I found no MIL light, and no codes, only INC for all 5 evap systems.

Attempting a drive cycle for 5 consecutive days did nothing to resolve this issue. So, rolling up my sleeves, I attacked the complete evap system, and associated components. First, I found the vacuum hoses to the EVS Air and Absolute Pressure Sensors. Tracing these lines I discovered they were brittle and broken all the way to the back of the engine.

Additionally, the pressure sensor was fried. Also the 2 temp sensors were rusted. The hard plastic hose to the purge valve was cracked and shattered when touched. All of the PCV hoses were hard as a rock and broke when touched. The O2 front sensors were slow to react (lazy?)

All of the afore-mentioned components were replaced with Mercedes factory parts. Now, I went on a drive cycle trip. Sure enough, it passed!

The next day, I started the car and proceded to the store. Within 2 miles, I noticed a huge amount of smoke constantly coming out the tailpipe. More so from a dead stop. Then the MIL lit and began flashing. I was able to limp home as is. I connected my diagnostic meter and code P0308 showed. This indicated a failed #8 coil or spark plug. It turned out to be the coil. After another trip to Mercedes to buy the coil and replacing it, the MIL light went out.

Now, I discovered a leak on the driver's side of the head. So far, no oil in the radiator, nor bubles in the reservoir. SHEESH! When it rains, it pours. I really like this car, so I want to fix this problem. Can anyone suggest how best to proceed?

Any and all suggestions, recommendations, and constructive criticism is greatly appreciated.

 

Smoke that smells of fuel or oil from the tailpipe?

Vacuum lines that go to a fuel pressure regulator.... Unplug and look/sniff lines. Smell/see gas? Replace fuel pressure regulator.
Look closely at crankcase ventilation. If plugged up (common in the bad old days before full synthetic oils) crankcase vapors will build pressure in the crankcase for smoke and it will blow oil seals, push the dipstick out of the hole.

Wiring in your vintage of car likes likes to have the insulation turn to dust inside the wiring harness. ESP anywhere the harness bends. Bare wires touching each other in the harness make all manner of nightmares up to and including the dreaded Carbeque.

Air mass meters fail. She’s not young. Expect more than one issue. Being a ‘96 you can plug into OBDII for codes.