Клапан EGR завис или заклинил

The valve no longer moves freely through its range — bound by carbon, a worn spindle, or a failed actuator. The direction it sticks in decides the symptoms entirely, and they look like unrelated faults.

Stuck open dilutes the intake charge at idle and light load, exactly when the engine needs clean air. That produces a rough hunting idle, hesitation pulling away, black smoke and sometimes a stall at junctions — the classic report of "terrible at the lights, fine on the motorway". It also makes the engine produce far more soot, which loads the particulate filter faster than it can clear.

Stuck closed costs nothing in driveability, which is why it goes unnoticed. Combustion temperature rises, nitrogen oxide output climbs, and the fault is discovered at an emissions test or through a flow code.

A valve that reports closed and is not is the case worth knowing about. Deposits on the seat stop the pintle seating fully while the position sensor still reads home — so the feedback is satisfied and the flow is not, which is why flow codes and position codes can disagree.

Ask for cleaning before replacement, and expect a position relearn afterwards on most vehicles — skipping it leaves the fault apparently unfixed.