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?

  • check plugs, wires, coil, distributor cap/rotor (make sure oil isn't leaking under cap), and IAC valve
    – user4546
    Apr 29, 2015 at 0:57
  • Checked and/or replaced all of that this weekend, still wants to shut off. May 4, 2015 at 19:21

3 Answers 3


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

  • Thank you, but we have already done that, as well as cleaning out all sensors and filters. Apr 27, 2015 at 16:42
  • Sorry, I should have read your question fully. In this case the only thing could be clogged fuel injectors. I would suggest taking it to a mechanic before spending any more money with guesses.
    – rana
    Apr 29, 2015 at 16:31
  • The OTHER "only" thing it could be is poor spark for some reason or another. May 28, 2015 at 9:30

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.

  • Another thought: when the throttle body was cleaned I assumed that the idle speed adjuster screw was removed and cleaned. If not it is the next step. Find the screw, under lots of gray paint if it has never been removed. Scrape out all the paint, remove it, clean the screw and the passages behind it. Reinstall it till it bottoms out and then turn it out one turn. Aug 25, 2015 at 14:38
  • When the idle system is working correctly as the rpm comes down from speed to idle the rpm will stop about 100 rpm above the 700 rpm normal idle speed and then slowly settle down to the normal idle speed. If the speed drops below the normal idle and then climbs back up to the 700 rpm something is still wrong. Aug 25, 2015 at 14:38

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