1st thing I'd check is the idle switch, make sure the throttle snaps shut to the stop, the upper part moves freely and contacts the switch, they can get a bit gummed up with age. If its sticky remove the 13mm nut on the top and carefully remove the top arm, give it all a clean then reassemble. Take care not to trap the spring in the plastic bushes as it will bind up, once workign the top arm should move freely and hit the switch every time you open and shut the throttle.
if that checks out, switch on the ignition with engine off and make sure the ISV buzzes. It shouldnt make loads of noise but it should have a nice strong humm, you'll feel it if you touch the ISV. Next check the 3 temp sensors on the side of the head, make sure no wires have snapped or fallen off the sensors.
If all that checks out run the car till hot, then swap the red/yel wire to the other 2 sensors 1 at a time, then go check the dash gauge reads about the same on all 3 sesnors. You dont have to have the engine on just run it till its hot enough for the dash temp gauge to move with the ignition on.
If all THAT checks out then you may need to adjust the base idle & CO, there is an FAQ post on how to do this
