2000 Ford Taurus Engine Accelerates Suddenly

I have a 2000 Ford Taurus with a 3.0 DOHV Duratec Engine. The other day, after taking my foot off the gas pedal and while coasting to a stop, all of a sudden the engine accelerated to about 2,000 RPMs. I put the car pulled over to the side of the road and put the car in neutral and the engine accelerated to 4,000 RPMs and remained there. I goosed the gas pedal several times and finally the engine returned to a normal idle speed of about 800 RPMs or so. This has happened several times over the last several days. Each time it has been harder and harder to get the engine to return to normal operation. The last time no matter how many times I goosed the accelerator, the engine just kept turning 4,000 RPMs. Finally, I opened the hood and kept pressing on the accelerator linkage and the engine finally returned to normal operation. I have cleaned the Idle Air Controller (IAC) and the Throttle Body with Throttle Body/Carburetor cleaner and that hasn't made any difference. Should I replace the Idle Air Controller as my next step?

may want to check for a vacuum leak.a large one at that....check for
a hose that has come off

When you pressed on the accelerator linkage did you notice if the linkage/throttle plate moved just prior to the engine speed slowing? With the engine off I would suggest you work the throttle to determine if the return spring is functioning and if the linkage or throttle plate appears to stick or bind. You should be able to pretty easily disconnect the throttle cable so you and actuate the cable independently from the throttle plate and spring at the throttle body.

You may also want to connect a scan tool with datastream feature or backprobe the Throttle Position Sensor to see what kind of linearity it has.

-Rod