2009 Ford Escape V6. Struggling Idle with intermittent stalls
The car in question is a 2009 Ford Escape XLT V6 3.0 Duratec with ~125,000 miles.
The issue is when the car is in drive and comes to a stop, the idle drops below its nominal idling RPM. The idle drops and struggles to keep the car on for a about 2-3 seconds before idling normal. The car idles perfectly with no drop with the car in park or neutral, and also accelerates and drives very strong; this only is happening under Drive load at a stop. Two or three times a week the idle struggles so bad it stalls at a stop or in stop and go traffic.
The Ford dealer has looked at the car twice with no success. There are no codes or check engine light. All they did was replace a recalled throttle body assembly. My next step will be to check voltage and resistance on the Idle Air Control Valve connector harness and connector on the part itself respectively.
I know this issue can be a variety of issues from the fueling system, ignition, and vacuum systems. Would it be a good idea to start somewhere else?
P.S., I checked the transmission fluid and realized it was overfilled by 0.75-1 quart. I drained the excess and topped it off to proper level at operating temp with car on. The car seems to be have driven with this overfilled for about 3 years. Would a failing transmission also be a culprit?
Thank you for any help!