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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