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Ice forming in the throttle body from condensation, holding the plate or narrowing the bore. Air accelerating past the throttle plate cools sharply, and in damp conditions near freezing the moisture it carries deposits as ice on the plate's edge — the same effect as carburettor icing.

The conditions are specific and that is the diagnostic clue: cold, damp, near but not far below freezing, at light throttle. The fault appears on a particular sort of morning and never in a workshop. The crankcase breather is usually implicated, since it feeds warm moist air into the intake immediately upstream — a breather passing more moisture than it should, or a heated breather whose heater has failed, makes an unremarkable morning enough. Check the breather and any intake pre-heat before the throttle body, which is rarely at fault.