The banjo fitting is only outfitted on the turbo models. Some of the compressed air that is forced into the intake is routed through this fitting, then over to the switchover valve, and on to the aneroid compensator, where it signals the injector pump to feed the engine more fuel. A turbocharger makes little sense if this mechanism isn't working properly. It's easy enough to remove and clean(it sits at the firewall end of the intake manifold), but be careful not to overtighten when reinstalling. Furthermore, if this fitting is badly clogged, it's a safe bet that the switchover valve is similarly clogged. When cleaned, it will make a world of difference.

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Matt
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1995 E300 Diesel (Die Blau Frau)