The reason why your car was idling between 1500 and 2000 RPM is because your car was running above its normal temperature. When its hotter, it'll make the engine run faster to turn the fan faster to help cool the radiator down... spose it doesn't work if theres no coolant hehe
Probably best to take it too a mechanic, they'll sort it out in a jiffy.
I had problem with mine where the head gasket had carketed it and was leeking coolant (no oil nor was the coolant getting into the oil), so yer, could be head but yer, i'm no mechanic.