You need to give a lot more information as to how the car is actually running - what is cold idle at first start up, how/when the idle drops, what your warm idle is, whether your car reaches proper running temperature, etc. By IACV, I assume this is what we call the idle speed solenoid (ISC) on this forum, one of whose jobs it is, is to regulate the idle when you have put a load on the engine. As this is new - perhaps we can assume that it is in good condition, hopefully. Which of course doesn't mean that the connections or wiring to it are ok, or that the air hose isn't blocked up, etc.
On the 16V its called an Idle Air Control Valve.
There is a wire from the A/C unit to the ECU that tells it to increase the idle speed when the A/C is on, you should check that the signal is reaching the ECU. What market was your vehicle built for? does the ECU have two connectors or three? Was the IAC duty cycle adjusted when the new valve was fitted?
While my info will probably not be applicable for the 16v - you should check Rhino's wiring suggestion above. In the 8v, a yellow/black wire runs from the A/C amplifier to the ECU signalling that the A/C is on.
When you replace the IACV you need to set the idle such that it is idling at the correct speed with a duty cycle of around 50%. If this isn't done then the IACV may not be able to flow enough air to raise the idle under load.
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