This is the exact same problem I had on my 90 LS. First pull the ECU under the right hand dash and look to make sure it is not cooked (upper right hand corner of the circuit board)you will be able to see it. If it is burned then you need to get a new one or have it repaired. The next thing is to replace the IAC (idle air control) 25 bucks or so. The IAC can burn up the ECU so make sure it is replaced quickly. If you replace the IAC and the ECU is burned the car will still not run correctly. Good luck!
1. ECU check
2. Replace IAC (25$ and 5 mins)

Seattle WA... (Everett)
1990 Montero LS OME complete, 2"BL,spray-paint.
1990 Montero LS in the 1st stage of rebuild.
2009 Pilot, Bone stock.
"Let me put on my slightly larger glasses".