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5 ways air passes TB: throttle plate, base idle speed screw (biss), ISC, FIAV and egr. ECU moves ISC around all the time when driving. When let off throttle, ECU steps the ISC all the way open to "catch" the engine as the RPM's fall below 1000. Now we know this. First thing to do is set the ISC. Should be done before every season or once a year, especially after modifications or when more air flow. Do this when engine is hot. I cannot stress this enough or on a hot day as well. I don't know what year your vehicle is, but you will need to ground ignition timing adjustment with jumper wire. Then the engine will run steady and ECU can't fiddle with ISC. I have 95' eclipse and its the brown connector on fire wall. Now just adjust biss until steady run at 750 or if you have big turbo HP, maybe set at 750-1000. If you have DSM link match both. Now you will have tuned ISC and disconnect ground. Now you set the correct range for ISC. You may have to replace rubber o-ring for biss. If this does not work. Then you eliminated base idle failure along with the fact you checked EGR and replaced ISC. So then it is FIAV which you will have to replace or clean the throttle body. If none of this works the car is not running correct or vacuum leak, or even bad o2 sensor which you can check by smelling exhaust when car surging. check terry's talon troubleshooting tips under idle surge in vfact on dsm talk. I must mention most of the time its just adjustment of biss when you ground ignition timing. Allot of information, sorry about that. Hope you fix.
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