My daughter has a 86 230E. Its a Euro 300E with the 2.3 engine. It has developed a spark knock. Moving the distributor does not have any effect on the timing. I was told that the 2.3 has an EZL ignition with a trimming plug. A round black plug with the letters KAT or EZL written on it. This is supposed to adjust the timing for different fuel octanes by giving a reference resistance. You adjust it by pulling the plug and turning it to the next number. Any comments? Where is this plug located?

John
84 300SD
82 380 SEL
86 230E