The critical issue is resistor versus not resistor, which everyone overlooks.

Most aftermarket catalogs call out resistor plugs for Mercs originally equipped with NON-RESISTIR plugs. You will likely NOT find non-resistor plugs at parts stores, and they will try to sell you a resisitor type plug, which can cause idle quality and emission problems.

Bottom line is to use ONLY the NON-RESISTOR Bosch OE type plugs - for example H9DC for M103 engines, which are readily available from Mercedes parts specialists like Fastlane.

Even if you buy from a store that sells Bosch plugs, they will likely hand you a set of HR9DC - WRONG PLUG! It's a resistor type.

Any plug you buy that has an "R" in the number is likely a resistor plug, and it's likely to cause problems.

Duke