odd Idle issue

Well after many years of being away from DSM's I just bought a 1992 Evo.

just working out a few bugs before I put it on the road.

one issue I have is with the heat off and the interior blower fans off the car idles well @ 850rpm .. I blip the gas it revs up and drops off and almost stalls .

I turn the blower fans on and car idles at 15-1700rpm .. then i blip the gas and it settles rite down it dont dip down any lower then what it idles at ..

1st guess was ISC .. but then why would the heat / fans being on make the idle go up ! Im alittle lost.