Cold idle when hot?

[Deadduck]

I have just finished doing a full rebuild on my g16b started it runs and drives fine timing ect all good when warm it was idling at 800-850rpm as normal but after about 30min of driving I stopped to change into 4wheel drive and the idle was at a constant 1500rpm where it normally sits when fully cold after turning off letting it rest for 30min and starting again still the same would this be the Engine Coolant Temp ECU Sensor or is there something else that would cause that?
Thanks