You will need
- a set of tools, clamps, new gaskets, manual.
Should immediately say that in a cold engine immediately after switching on for some time at elevated idle. This is necessary in order to have it warmed. That is, it is not difficult to guess that in the winter time of year the engine after switching on will take longer to run at higher idling speed than in summer. A normal level of speed for most cars is about 1000 rpm. The exact level recommended for your car, you will be able to find in the manual. If the revs idling your car does not fall after the engine is warmed up or "float", you need to look for the fault.
First we need to know what type of engine installed in your car fuel - injected or carbureted. If you have a carbureted engine, you can easily to clean it. If the car has a considerable age, the carburetor should be removed and cleaned, as often clogged the carburetor is the cause of the high idle engine speed. If you have no experience tuning and washing of carb, it is better to ask knowledgeable people who can quickly and quality to repair it.
Carefully inspect all rubber seals and hoses. A torn gasket can also cause high idle, as the system will flow more air than it should be. Pay special attention to flanges and gaskets, leading to the intake manifold. Try gently through the rubber hoses, parallel to follow the momentum. If you press on any hose they will fall, it means that you found the cause of the problem. Note on all connections. Worn clamps should be replaced, as they loosely attach the hoses and let the air leak out.
If your car is set injection engine, a mechanical way to change the rpm level is almost impossible. The fact that the speed depend on the firmware, which "poured" into your car. That is, the level control is programmatically. You can set the onboard computer to monitor the level of turnover in the online mode. If you want to downgrade, you should contact the specialists, who will be inundated by new firmware. But you should consider that too low idle speed may lead to premature wear of the generator.