My daughters accord idles really rough after driving it for about 10 minutes, but not as you are driving it, only when you stop. Then, when she stops, the car usually shuts off. It normally starts fine, even after shutting off. We have checked the codes and it comes up with something for the transmission (shift control valve I think). We have checked the transmission and it is fine...no problems shifting, etc. If she turns the A/C on, it makes the problem worse....if the a/c is not on, or we shut it off when the car idles high, it usually helps, but doesn't always keep it from shutting off. The car runs smooth, drives really well, no other weird noises, knocking or smells. We have checked for vacuum leaks, cleaned the throttle body, and checked the ignition switch. Any ideas?
The Idle Speed Control (ISC) valve, also called an Idle Air Control (IAC) valve, is used to regulate idle speed. The IAC valve opens a small bypass circuit that allows air to bypass the throttle. Increasing the volume of air that flows through the bypass circuit around the throttle increases idle speed. Reducing the bypass airflow decreases idle speed.
The ISC valve is controlled by the engine's computer (powertrain control module or PCM). The computer monitors idle speed by counting ignition pulses from the ignition module in the distributor or crankshaft position sensor when the throttle position sensor or throttle switch signals the computer that the throttle is closed and the engine is at idle. When the engine's idle speed is above or below the preset range in the computer's program, the computer tells the ISC valve to either increase or decrease the bypass air flow. Other sensor inputs from the coolant sensor, brake switch and speed sensor may also be used by the computer to regulate idle speed according to the operating condition.
**Idle speed may also be increased when the A/C compressor is engaged.
Hope this helps you, I would make sure that the computer is not receiving false readings from the sensors I mentioned above.
This is a clear sign of a dirty/carbon-ed up throttle body. This video will show you how to fix it:https://www.youtube.com/watch?v=eQpPcdvSBks
A vacuum leak will not cause this problem, it will just idle faster with a leak on this engine. Because it runs well above idle it is not likely that any of the ignition parts are at fault. Although the transmission fault code could indicate a problem that could cause idle problems that is very rare. Cleaning the throttle body usually fixes this problem, but you did that with no result. Cleaning and testing the idle air control valve is next. You need verify that all the cylinders are sealing well, a leaking intake valve can cause and will show symptoms first at idle and then later at higher power settings.
Source: 30 years automotive diagnostics.