A car’s engine can surge for many reasons. Generally the ignition, wires, spark plugs, or the distributor rotor and cap will cause this problem. Another possibility is that your fuel filter is clogged. Injectors could also possibly be clogged on a high mileage vehicle. This is where a tune up can help your car. A vacuum leak can also cause surging problems. First inspect all hoses and fittings then around the carb or throttle body. Run the engine and spray carb cleaner on areas that could be leaking, the engine will die or speed up when you spray if there is a leak. A vacuum leak may or may not make your check engine light come on. Surging could come from a sensor problem or a leak causing the sensor to not operate correctly. It is always good to check your computer codes first if the light is on. This will help you to identify the problem area.
The idle air control is a control system used on fuel injected cars. The idle air control or IAC valve controls the idle speed of the engine. The IAC valve is controlled by the ECM (computer). The computer gets it’s information from the rpm which is supplied by the distributor, crank or cam sensor. The IAC valve moves in and out to bleed air behind the throttle plate. This air bleeding is the same as opening the throttle plate. The idle speed can be controlled precisely the throttle plate which isn’t controlled as well. Air conditioning will drop the idle speed down and will kill the engine if the IAC doesn’t work well. The idle air control will be located on the side of the throttle body. The IAC can become sticky after time and will cause the the engine to surge, rev or stall the engine. The valve can be removed and cleaned if not too severely worn. Usually the valve needs to be replaced. Make sure the gasket looks alright. Also make sure the idle air control is secured well and doesn’t leak air, or the engine will rev up.
The idle air control is used to adjust idle speed on electronic fuel injected cars. Back before electronic fuel injection was used, carburetors would use a screw to adjust the throttle plates. The more the throttle plates were open the more air and the higher the idle would go. Fuel injected cars use a throttle body to control the air coming into the engine. The idle air control allows air to enter the engine instead of holding the plates open.