When in diagnostics mode, and Idle screw turned all the way down, the idle is at 1000 rpms, I can't find any vacuum leaks any where. With the jumper out in regular mode, if I turn the fan switch on with the a/c, the Rpms jump to 2000 then slowly fall down to 1000. I have replaced the ECU, the Idle air control valve and inspected the Air valve on the intake manifold, on this 1.6l 1990 Mx-5 Any help would be awsome.

 

go to the hardware store and buy a propane torch. WITHOUT lighting it turn it on LOW, so there is no flame but just gas comming out. with the car running slowly move it around your vac lines, intake, and anyother area that might be letting air into the eng. if the eng rev's up, you found a vac leak.

go to a local parts store that will read your eng codes for free ( not all codes show, call the store first to see if they charge), might be a code that don't keep the light on. hope this helps, good luck.