I must give enormous credit to Arthur Dalton for helping me through this, with innumerable private posts back and forth - thank you for your patience, Arthur.

The old plugs were the proper Bosch F8DC4 units, and while they had obviously seen better days, they weren't in terrible shape so I cleaned, re-gapped and reinstalled them. The car ran very well, much better than before, but there was still a hesitation and surging under full throttle.

I had cleared all the codes and decided to ride it out for a while to see if any codes came back and whether the drivability improved. They did and it didn't, in fact the drivability deteriorated to the point where the car was chugging and missing and making me so disgusted that I must admit that I harbored thoughts of switching to a Toyota (not really, but almost) and continuously cursed myself for violating my own Prime Directive about nothing being as expensive as a cheap Mercedes, so I decided to try replacing the plugs as a "what could it hurt" step while I prepared myself for having to get deeper into this problem.

After a futile search for another set of the Bosch F8DC4 plugs, Arthur advised that some guys have had success with NGK BP5ES plugs, so I placed an order for a set down at my local NAPA and waited for them to arrive. Finally, they showed up and when I tried to install them, I found that they could not be installed because they require a 13/16" socket that won't fit down into the plug tube.

Drats - foiled again!

After some more research, Arthur forwarded to me a copy of the NGK part number decoder and we identified the part number for the 5/8" socket configuration: the BCP5ES.

Another wait while my local NAPA ordered the new ones, and when they finally arrived I gapped and installed them and it worked like you wouldn't believe! The car now runs like I knew it should, pulling like a freight train with nary a stumble or a cough all the way out to redline under full throttle. The kinship to my C36AMG is very clear, once again.

Who woulda thunk all it would take is a $12 set of plugs?!?

Thank you , Arthur!!!

Best Regards,
Rob

__________________
Robert Fini

'12 ML350 BlueTec, 100k
'06 E320CDI (x2)
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 70k
'97 C36AMG, 319k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)