Hope you're sitting down; this is going to be real involved!
Go out and drive the car, then, when the engine is warmed up, leave it in gear like normal, and from highway speed, coast for at least seven seconds without touching the pedals.
The glaring clue is the battery was disconnected. Thank you for including that. The Engine Computer lost its memory. Most of the learned data will be relearned as soon as you start driving, except for "minimum throttle". The computer needs to see a specific set of conditions met while coasting, that tells it your foot is off the accelerator pedal. It relearns the signal voltage from the throttle position sensor, and from then on, it knows it must be in control of idle speed any time it sees that same voltage. Until that is relearned, the engine might not start and run unless you hold the accelerator pedal down 1/4", you won't get the nice idle flare-up to 1500 rpm at start-up, and it will tend to stall at stop signs, (mainly with automatic transmissions).
For many people, the relearn takes place while coasting down an off-ramp, then the problem of low idle speed mysteriously disappears.
Apr 12, 2019 at 4:38 PM
