Climbing down the wormhole but thought I'd post to see if there might be something I haven't thought of. 1995 E420, 171,000 miles, out for a drive for approx. 15 minutes when the car suddenly felt like it was running on 4 or 6 cylinders. Felt similar to a caps failure but they're 3 mo. old so I ruled them out. Limped the car home and replaced all the plugs. Same issue. (no CEL) ran the codes anyway and the DM showed an ignition failure on bank 1 so I went down the train and replaced the Ignition coils. Same problem. Next down the line, replaced the EZL. Again, same issue. This time though the car threw a CEL and SRS light. Tried running the codes through Xentry and showed the DM as clear of faults. Also, the onboard diagnostic module holds the red LED when pressed and nothing after. Replaced the fuel filter and confirmed fuel pumps are good. Pressure regulator was replaced last year so that's good.

Live data shows proper voltage to MAF, O2 and ignition is stable at 30v / cylinder so it's not an ignition issue as far as I can tell. Based on the fact that the DM is unresponsive am thinking there's a wiring issue but the main harness has already been replaced with updated wiring and looks good. Lower harness is original but don't know if that might be the culprit considering it isn't controlling much outside of the the starter and alternator.

Car smells running rich but no rotten egg smell so I don't think the cats are bad.

Pretty much stuck at this point so if anyone has any insights let me know.

Thanks in advance!

__________________
1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL

Recommend a compression check on all cylinders. I had a similar experience with a 1995 E320 … running fine, then suddenly started running rough while stopped at a traffic light. To make a long story short, turned out there was little/no compression in one cylinder. Not exactly sure what happened, but I was not about to spend big bucks repairing a vehicle that I bought for $4k, drove 100,000 miles, and had 200,000 miles on the odometer.

Good luck with the diagnosis. If your 1995 E420 is in pristine condition you may want to consider extensive/expensive repairs. That is a very personal decision … and can be very emotional.

__________________
Fred Hoelzle

I know you recently replaced the caps but did you also replace the insulator cups? Those are a known source of poor running condition when warm. They typically get moisture behind them. Also what’s the condition of your throttle body? Does your car have ASR?

I would also recommend replacing the blade fuses in the base module. I spent weeks chasing a similar issue on my former 93 400E and one of them was blown.

I'll check the insulator caps today. If those were causing issue wouldn't the ignition voltages be wacky on the cylinders during firing? No ASR on this particular vehicle and the throttle body was replaced approx. 8 or 9 years ago I believe.

The car is getting close to DNR and I was hoping to get it through the winter before getting the new car out of storage. My goal either now or in a few months is to get the car to someone who can part it.

__________________
1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL

Had a similar, but infrequent problem on my 94 E420 with all new caps, plugs and both coils plus previous year new fuel filter, pumps, pressure regulator. Problem most often was during monsoon type NW rain storms. New plug wire set was the fix. Car had approx. 275,000 miles and old plug wires about 100K.

__________________
John 5:25