Spark plug wire arcing will not be visible unless it is totally pitch black unless the wires are really bad.

Your M119 W140 is definitely a LH-SFI not the CIS-E. Same would be true even if it was a 92.

While you are on Jim F's site, you should read about and then do his "cool harness" mod. He tells you how to do it yourself (It's very easy) or you can buy his plug in kit. I swear by the mod and many guys who are way more knowledgeable than me swear by it too.

California cars got the self diagnostics system and the "Check Eng" light that goes with it before the rest of the country did so it's possible that your car didn't come with it especially if your car is an early 93. All 92 and later 500Es supposedly got this system regardless of whether or not they were Cali cars but my buddies 92 500E doesn't have it. It is an early 92 BTW. The point is if your car doesn't have it, it doesn't mean something is wrong.

Another crazy thing that has popped in my head is the possibility that the cats are beginning to plug up. I don't think this is "it" but it's something to keep in the back of your head if all else fails.
Regards, Eric

__________________
89 300E "Benzer1" 15.924 Uncorrected
93 400E "Benzer3" 14.200 U.C.
95 E420 "Benzer4"
92 300E "Benzer5" 16.299 U.C. Future turbo CNG
87 300D "Benzer7"
87 300D "Benzer8"
87 300D "Benzer9"
87 300D/70 AMC Javelin "Sidewinder-Benzer"
87 300TD "Benzer11"
06 E320 CDI "Benzer12"
05 E320 CDI "Benzer12A"
71 AMC Javelin AMX 401 "Sidewinder"
74 AMC Hornet 401 "C.K.10" 13.63 U.C.
74 Bricklin SV1 "Presto" AMC 360 pwrd.