![]() ![]() If the wiring harness appears to be good, and the ISC solenoid or motor is receiving a signal from the PCM - but is not responding - you can assume the fault is a bad ISC motor or solenoid. Replacing a perfectly good ISC motor or solenoid won’t fix the problem if the fault is in the wiring. Look up the ISC circuit wiring diagram for the vehicle, and check for voltage and ground at the appropriate terminals. Problems here can prevent the PCM command signal from reaching the ISC solenoid or motor. Ĭheck the idle speed control system wiring harness and connector to make sure it’s tight and corrosion-free.But if the ISC system still fails to respond to your command to increase idle speed, or fails to maintain normal idle speed, there’s a problem that will require further diagnosis before you replace any parts. If the engine now idles normally, and you can command idle speed changes, you’ve fixed the problem (a dirty idle bypass port or valve). ![]() Then start the engine and repeat the idle speed test. Let the cleaner soak for at least 10 minutes before giving it another blast of cleaner. Use the small plastic tube that attaches to the nozzle on the aerosol can to aim the cleaner directly into the bypass port(s). Aim for the little air bypass holes in the throttle opening. Don’t replace anything until you do the following:Ĭlean out the idle air bypass circuit with several shots of aerosol throttle cleaner. If idle speed drops down and does not come back up, the ISC system is not responding the way it should.ĭon’t jump to conclusions - even though the most likely cause would be a bad idle speed control solenoid or motor. If the ISC system is working correctly, the PCM should adjust idle speed to keep the engine from lugging down when the A/C compressor engages. If you don’t have a scan tool that can do an idle speed test, or the vehicle does not have this capability, you can still test the response of the ISC system by turning on the A/C to max. No response? Then you know the ISC system has a problem. But if it is, and you have a scan tool that can run the test, a good place to start would be to command an increase in idle speed to see if the ISC system responds as it should. If such a test is not available on the vehicle, then obviously you can’t check it this way. The idle speed control circuit can often be diagnosed with a professional-grade scan tool that has the proper software to command an idle speed control test. This can lug down the engine and cause it to stall.Ī problem with the idle speed control circuit would be the most likely cause of stalling if the problem only occurs when the engine is idling or when the car comes to a stop after driving. ![]() If the ISC valve is gummed up with fuel varnish or dirt, it may stick, preventing the engine from maintaining the proper idle speed when the throttle closes.Ī faulty idle speed control circuit may also allow the engine to stall when idling if it fails to compensate for idle loads placed on the engine by the charging system or A/C compressor. The PCM monitors idle speed and operates a solenoid or small electric motor to move a valve that regulates airflow through the air bypass circuit. The engine’s idle speed is controlled by how much air is allowed to bypass the throttle via the idle bypass circuit in the throttle body. But even then, you may not capture any usable data.Ĭommon causes of intermittent stalling may include a bad idle speed control (ISC) system, low fuel pressure, loss of ignition, vacuum or EGR leaks, or other problems we’ll get to later in this article. Or, if you have a flight recorder, you can hook it up to the car and send your customer home with instructions on how to catch a snapshot of the problem when it happens. You may have to keep the car and drive it for a few days to see if it stalls. So the trick to diagnosing an intermittent stalling problem is to catch the vehicle when it’s acting up - and that rarely happens when you have the vehicle in your shop. Many times, there will be no check engine light and no fault codes to give you a clue as to what might be causing the engine to stall.
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