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The gasket between the throttle body and the intake manifold, leaking so air enters downstream of the airflow sensor. It is an unmetered air leak in one of the least visible places — the joint faces each other, so nothing is exposed to view and the leak is silent.

It shows worst at idle, which is what makes it look like an idle control fault. Manifold vacuum is highest at idle, so the leak flows most then and shrinks as the throttle opens; the module compensates by closing the throttle further and eventually reports that it is at the end of its authority. A high or unstable idle with positive fuel trims that fall away under load is the pattern.

Two causes account for most of them. A gasket that has hardened and taken a set with age, and one disturbed during a throttle body cleaning and refitted rather than replaced. The second is the more common and entirely avoidable — the gasket is inexpensive and single-use. Smoke-test the joint rather than spraying anything flammable near a hot engine, and check the fasteners are to torque while there, since an unevenly tightened throttle body will leak with a perfect gasket.