2006 Mercury Mariner 3.0L, after reaching normal temperature, starts a continual, erratic drop in rpm due to IAC valve slamming shut. White/red lead has a continuous 12V, other lead has 10V and intermittently loses voltage enough to see meter needle move. Can Hear Valve slamming even better with air filter holder open.
My car has 141500 miles.
My car has an automatic transmission.
You probably have a faulty idle air control valve that is not adjusting properly as the engine reaches operating temperature. Sometimes these valves can be cleaned to reduce the amount of carbon built up inside of them, but replacement of a faulty valve usually comes soon after anyway. Consider hiring an experienced technician like one from YourMechanic who can come out for a closer look and offer a more personal diagnosis of your rough/low idlle situation.
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