If there is a fault with the O2 or any other sensors, a scan tool should be able to pick that up.
Have a scan performed to check if there are any trouble codes and if possible get the freeze frame data.
The roaring at 4000 rpm you decribed should be the engine rpm cut, designed to prevent over-revving of engine but it is lower than specifications which should occur at the redline of the tachometer.
The servomotor, I believe you are referring to the IAC ( Idle Air Control valve at throttle body). This component would affect engine idling but not the throttle response.
The item most likely to cause the symptoms would be the Throttle Position Sensor, it groundin circuit connections or the engine computer.
When did the problem first surfaced? Was any repairs done before it occurred?
Please consider a to help us answer more questions.
Wednesday, April 13th, 2011 AT 5:24 PM