pt:

The temperature range determines the actual operating temperature of the tip of the spark plug. Hotter plugs have longer, thinner insulators, colder plugs have shorter, fatter ones. This determines how much heat gets transferred away from the tip.

Hotter plugs than necessary in an engine will lead to pre-ignition and possibly engine damage if they get so hot the porcelain insulator cracks off!

Set mixture correctly before changing heat range. Then, if you still get fouling problems, move up one heat range. Check the plugs for signs of overheating, too -- insulator should be light gray or tan (although platinum plugs sometimes stay bright white). If the plug is too hot (or the mixture too lean), the insulator will be bright white and may have small blisters where it is starting to melt in small spots. If you see this, richen the mixture and/or use colder plugs.

If, however, the plugs stay dark brown and collect black soot or crusty brownish stuff, and the mixture is right, use plugs one heat range hotter.

The vast majority of the time, the factory recommended range is fine. However, if you do ALL stop and go city driving, no expressway at all, they may foul. MBs prefer some hard driving regularly to keep the engines working correctly!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!