Even though the O2 sensors regulate how much fuel is injected through feedback, the sensors can only control the average fuel injection. If one or more injectors are clogged, the flow among the injectors is not balanced, resulting in rough idle.

When cold, the injectors are injecting excess fuel (more than needed for combustion). This masks problems with injectors that are partially clogged. Once warm, the dirty injectors are injecting less than optimum amounts of fuel, resulting in rough idle.

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95 E320 Cabriolet, 169K