| Re: Engine oscilation !
This is a Northstar car, right...???
It sounds like the idle speed control may be having problems when you are coasting with your foot off the throttle. This could be due to several potential problems.
The first thing that I would do is to take the inlet air duct off and hold the throttle blade wide open (engine off) and spray the backside of the throttle blade and the perimeter of the throttle bore with carb cleaner and scrub it with an old tooth brush to clean off any deposits. The deposits may be thick enough to be partially blocking off the air flow around the perimeter of the blade when it is nearly closed while coasting causing the RPM to dip low then the idle speed control extends the throttle to prevent a stall. Clean the throttle body thoroughly, disconnect the battery negative terminal for 60 seconds to reset the idle learn offsets and drive it and see if it is OK.
The other thing to do is to check the operation of the closed throttle switch. With the engine running and in gear, enter into the onboard diagnostics by simultaneously pressing and holding the OFF and WARMER buttons until the onboard diagnostics activates. After any codes display the system should prompt you with PCM? Answer yes with the HI button. Now that you are in the PCM mode the icons on the climate control panel take on new meanings. The LO icon represents the status of the closed throttle switch inside the idle speed control motor. If your foot is on the accelerator pedal pressing it even slightly the LO lite should be out indicating the switch is open due to you opening the throttle. If you have your foot off the accelerator pedal idling or coasting then the LO icon should be on indicating that the throttle lever is resting on the idle speed control motor plunger. Do your coast maneuver where the RPM wants to hunt and watch the LO icon while coasting. If the LO icon is blinking on and off it indicates the closed throttle switch is opening and closing which may be what is causing the problem. Something is causing the switch to open and close intermittantly which is throwing off the idle speed control signals to the idle speed control motor. Either something is restricting the throttle travel or dragging on the throttle cable or pedal when you lift off of it or the closed throttle swithc inside the idle speed control motor is wearing out and sticking and going intermittant when it is closed. Stop and idle for awhile and watch the LO icon to see if it ever goes out when just sitting and idling...if it does then I would definitely suspect the closed throttle switch inside the throttle body. Replacing the idle speed control motor is most likely the cure. | 
