Check the faq's in this forum. Could be vacumn leak. You can test this by spraying carb cleaner around the outside of the carb and if you notice a sudden change in noise/revs then probably thats where the leak is. Also clean out the throttle plate, check and adjust the Throttle position switch, and idle air control valve (IAC) which is located on the engine mount just below the 1st inlet manifold. It should have 2 vacumn hose and possibly an electrical connection. To get this off is very messy. You can test this by gently tapping the valve with the butt of a screwdriver, if the situation improves then you will need to renew this (it cost me £70 pounds sterling in the UK.

The unit you are trying to ID is probablt the coolant temp switch. You can test this by removing the switch and coolant should pour out, but do this when engine is cold otherwise you will sacld youself. Good luck!