I hope Ive got this right,its bloody hard to understand what you are trying to say,so here we go.The IAC valve,why disconnect it,of course the engine will change revs(rev higher or lower) if its not connected.It cant control the idle speeds without the IAC valve connected. But if you mean that the IAC valve cant control the idle speeds when it is connected,then theres either something wrong with the IAC valve itself,or something wrong with the with the IAC electrical wiring or connectors.If the IAC valve and its wiring and connectors are all ok,then it could also be a faulty ECU,but thats very rare and highly unlikely,so Id say its more than likely to be a vacuum leak somewhere.It could be around the engine inlet manifold gaskets,it could be a leak in the vacuum lines somewhere,or even behind the dash area as the heater controls are all controlled by vacuum from the engine..Id also check the brakes as the brake booster uses vacuum too.You can test the booster by turning the engine off and pumping the brake pedal a few times.It should go hard,it should take 3 or 4 pumps of the brake pedal with the engine off to deplete the stored vacuum in the booster.Then whilst holding the brake pedal in,start the engine,if the pedal sinks in further as the engine starts,then the booster is considered to be working ok..As for finding a leaking vacuum hose,you just have to check every part of the vacuum lines around the engine bay for splits and cracks etc.Theres also a vacuum reserve tank under the left front guard behind the bumper.It looks like a black plastic ball thing,it should have a vacuum line connected to it,so check it out too.
With the engine light.Id check the bulb first if its not coming on when the key is turned on.Its probably blown.You really need it working so you can do an error code check,the codes will probably lead you to whats causing the trouble with the idling.It takes about 5 minutes to remove these dash clusters to change the bulbs in the warning lights,so thats what Id be doing first.