Originally Posted by 70GT6
First place to start is the vacuum lines, air pump line (which is know to come loose), and the egr valve & lines.
Any of these will play hell with the idle.
And most assuredly will turn the check engine light on in short order. You need to find somebody that knows his way around a scanner and have him check the base idle settings and if the ECM has been chipped. What ever is going on, the "system" think everything is normal or the MIL light would be on.
Is this an automatic or manual trany?