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The seal between an injector and the cylinder head, leaking combustion pressure back past the injector body. This is a compression leak rather than a fuel leak, which is the distinction the name does not make obvious and the one that explains its symptoms.

The noise is the classic sign — a sharp tick or chuff at idle, present from cold and often easing as the parts expand, sometimes described as a diesel "blowing back". Soot deposits around the injector base are the visual confirmation, and they mark exactly which cylinder. That black crust is combustion products escaping, and once it forms it erodes the seat further.

The seal is usually a copper washer and it is not reusable. An injector refitted with its old washer, or with the washer left in the head so that two are stacked, will leak — so this fault is frequently self-inflicted during earlier work. Check the seat is clean and the old washer removed before fitting a new one, because a washer left in place is the commonest reason a fresh injector seal fails immediately, and the height error also alters the injector's protrusion into the chamber.