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The fine passages carrying assist air to the injectors, fouled so atomisation is lost. Air assistance exists to break fuel into a finer mist when the engine is cold, and the passages doing it are small enough to close with the deposits every intake accumulates.

So the fault is a cold-running complaint that disappears once warm — rough idle, smoke and stalling in the first minute, with an engine that is faultless by the time it reaches a workshop. The control valve is usually blamed and is usually fine: command it and confirm air actually reaches the injector, because a valve that operates correctly into a blocked passage produces the identical code.