Originally posted by inspyral
The IACV increases idle speed when the engine is cold in order to warm it up faster. The idle speed should be constant around 1000-1200. If it's fluctuating, then there is something wrong. Usually, a fluctuating idle is cause by one or all of the following:
1) vac leak
2) air in your cooling system
3) bad IACV
If you're sure you don't have a vac leak, then I would wager you either have air in your cooling system, which throws off the IACV(the IACV relies on coolant running thru it in order to determine how fast to idle the engine) which could cause a fluctating idle. To bleed any air that m ay be in your cooling system, you need to open the radiator cap, and then put it back on loosely. Then, start the car, and let it idle until it completely warms up. During this time, you should have the heater blasting full-hot in order to circulate out any air that may be trapped in the heater core. Once the car is warmed up(radiator fan comes on at least twice), shut it off and check the coolant level in the radiator. If it went down, fill it up to the top and repeat the process. When you're done, put the radiator ca back on tightly. If this doesn't solve the fluctuating idle problem, then your IACV is probably toast.