I don't believe the diffuser ball glows, as they're always black when you remove the injectors. Their purpose is to help atomize the fuel spray and reduce ignition delay. The reason Mercedes has such a complicated combustion system is to make the engine quiet enough for a luxury car.

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'83 300D, 126K miles.

The Ball Pin should get nearly as hot as combustion. Between the combustion cycle there is some time for it to get cooler but there is a limited area for the heat to transfrer from it.

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84 300D, 82 Volvo 244Gl Diesel