Owner's Problem Description
The owner reports that their 1996 Chrysler Concorde is idling at an excessively high RPM of 2500. They recently replaced the MAP and TPS sensors but still experience high idle. When shifting gears, the idle drops significantly to around 10 RPM. The owner is seeking further diagnostic assistance after these sensor replacements.
Professional Technical Analysis
This 1996 Chrysler Concorde is experiencing a high idle condition at approximately 2500 RPM, which is significantly above the normal operating range of 600-800 RPM. The vehicle is powered by a 3.5L V6 engine with an automatic transmission. The owner has replaced both the MAP sensor (OEM P/N 4606249AB) and the Throttle Position Sensor (TPS, OEM P/N 4608884AB), yet the high idle persists. A typical idle RPM for this model should be around 650-750 RPM in park. The observed symptom of high idle can be influenced by several factors, including vacuum leaks, incorrect sensor signals, or issues with the engine control strategy. Notably, the drop to 10 RPM when shifting gears indicates possible stalling or miscalibration of the idle air control (IAC) system, which maintains idle speed by regulating airflow. A thorough examination of potential vacuum leaks, particularly in the intake manifold area and any associated hoses, is crucial, as they can cause erroneous readings at the MAP sensor, leading to improper fuel metering and high idle conditions. The PCM (Powertrain Control Module) relies on accurate data from the MAP and TPS to adjust fuel delivery and ignition timing. Therefore, any discrepancies in sensor readings, or damage to wiring harnesses, can further exacerbate the high idle issue. Current diagnostic checks should include using the GeekOBD APP to retrieve any fault codes, monitor live data for MAP and TPS voltage outputs, and verify proper operation of the IAC valve under various conditions.
Possible Causes
Most common causes (ordered by frequency):
- Vacuum Leak: A vacuum leak in the intake manifold or associated hoses can cause the engine to receive more air than intended, leading to a high idle condition. The MAP sensor relies on accurate vacuum readings to determine engine load and adjust fuel delivery accordingly. Common areas to inspect include the intake manifold gasket, vacuum hoses, and connections. A vacuum leak can cause the MAP sensor to report a lower-than-expected pressure, which may lead to an incorrect air-fuel mixture and subsequently high idle. Performing a smoke test can help identify leaks effectively. - Observations of fluctuating fuel trims, high idle RPM, and possible hissing sounds from the intake area during operation.
- Idle Air Control (IAC) Valve Malfunction: The IAC valve is responsible for regulating airflow during idle conditions. A malfunctioning IAC valve may stick open or closed, causing erratic idle behavior. Testing the IAC valve operation involves checking for proper resistance (typically 6-12 ohms) and ensuring that the valve responds correctly to PCM commands. If the IAC valve is faulty, it may need to be replaced (OEM P/N 04891592AA) to restore normal idle operation. - Inconsistent idle behavior observed during testing; IAC valve resistance measuring outside normal range.
- Improper Throttle Position Sensor (TPS) Calibration: Although the TPS was replaced, incorrect installation or calibration may lead to improper voltage readings. The TPS should provide a voltage between 0.5-4.5V based on throttle position. If the voltage is outside this range, the PCM may not receive accurate information for adjusting fuel delivery, resulting in high idle. Using the GeekOBD APP, monitor the TPS voltage to ensure it operates within specifications during throttle movement. - TPS voltage readings inconsistent with expected values during testing; potential misalignment of the sensor during installation.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Scan: Begin with a comprehensive scan using the GeekOBD APP to retrieve any stored DTCs and monitor live data. Pay close attention to MAP and TPS readings, as well as any codes related to the IAC system. This initial step will help identify any underlying issues affecting idle speed.
- STEP 2 - Vacuum Leak Check: Perform a vacuum leak check using a smoke machine or by visually inspecting hoses and the intake manifold for any signs of wear or damage. Ensure that all vacuum hoses are securely connected and replace any that show signs of cracking or deterioration.
- STEP 3 - IAC Valve Testing: Check the IAC valve for proper operation. Measure the resistance across the IAC terminals (should be 6-12 ohms). Also, verify that the valve responds to commands from the PCM by monitoring its operation during idle conditions using the GeekOBD APP.
- STEP 4 - TPS Voltage Verification: With the throttle positioned at various angles, use the GeekOBD APP to monitor TPS voltage outputs. Ensure that voltages fall within the expected range of 0.5-4.5V. If the readings are outside this range, consider re-calibrating or replacing the TPS sensor.
