ok, first , the ecm runs off the scan tool when the tool is hooked up so yes the idle will smooth out, so if there are no codes then you are going to have to go after common issues, throttle body leaks, fuel injectors dripping, idle air control valve sticking, throttle body base gasket, egr sticking, heres what happens, the engine
starts , the ecm sees low idle or excessive air, so adds fuel, then the engine smooths out and the o2 sensors see to much fuel, so it lessons the fuel with gives lower rpms, and it keeps doing this trying to find the proper air fuel ratio, so the issue is with fuel or air. hope this helps,Please take a second and click the box and rate my service GOOD OR EXCELLENT SERVICE if you feel I have been helpful and please request me in the future. If you feel I have not been helpful please DO NOT RATE BAD OR POOR AND WASTE YOUR MONEY , I will opt out to see if another expert can provide more or better assistance so you're not wasting your money!!. Thanks for choosing just answer. Lou. also , if you feel i deserve one , BONUSES are GREATLY APPRECIATED ! 😉Keep in mind that rating me does not end the chat, it simply credits me for assisting, i will still be here to help if follow up information is needed , Lou.