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Deposits between a valve and its seat, preventing it from closing fully. It is the specific version of carbon fouling that produces the more noticeable symptom, because a valve that cannot close is passing flow continuously — whereas one stuck shut merely fails to do its job when asked.

On a recirculation or purge valve this means gas entering the intake all the time, including at idle where the engine can least tolerate it. The result is a rough, hunting idle and a hesitation pulling away, easing at higher load where the leaked fraction is proportionally smaller. An engine that idles badly and pulls cleanly is describing exactly this.

The module may report the valve as closed, since position feedback measures the actuator rather than the seal — so a valve reporting closed while held a fraction off its seat is precisely this fault. Check the seat and the seal by hand with the valve removed rather than trusting the reported position, and expect the deposit to return unless what produces it changes.