Owner's Problem Description
The owner reports that their 1992 Plymouth Acclaim idles too fast, behaving as if the gas pedal is pressed even when parked. They mention being able to reach speeds of 30mph without pressing the gas pedal while driving. Previous attempts to fix the issue included replacing the throttle position sensor and the automatic idle speed motor, but the problem persists. The vehicle currently has a high idle and excessive fuel consumption.
Professional Technical Analysis
The 1992 Plymouth Acclaim, equipped with a 2.5L 4-cylinder engine and an automatic transmission, presents with a high idle condition, estimated mileage of approximately 150,000 miles. The owner describes the car's idle speed as excessively high, leading to premature acceleration and significant fuel consumption. The Automatic Idle Speed (AIS) motor and throttle position sensor (TPS) have been replaced, but the issue remains unresolved. A thorough analysis of the engine management system is required to identify potential causes. One primary suspect is a vacuum leak, which could lead to an increase in idle speed by allowing unmetered air into the intake manifold. Testing for vacuum leaks should involve a visual inspection of hoses and the use of a smoke machine if available. Additionally, the AIS motor's operation can be verified through a DRB-II scan tool, which is essential for monitoring the idle speed parameters. The TPS, although replaced, should also be checked for proper calibration and functionality, as it plays a critical role in engine management. Proper voltage readings should be observed from the TPS, with 0.5V to 1.0V at idle and approximately 4.0V at wide open throttle (WOT). Moreover, the idle speed can be affected by the Engine Coolant Temperature (ECT) sensor, which, if malfunctioning, can cause the engine to maintain a higher idle. An inspection of the ECT sensor and its connections is warranted. Overall, the vehicle's condition suggests an interconnected issue potentially stemming from multiple sources affecting the idle speed, necessitating a systematic diagnostic approach to pinpoint the exact cause.
Possible Causes
Most common causes (ordered by frequency):
- Vacuum Leak: A vacuum leak allows unmetered air to enter the intake manifold, potentially causing the engine to idle high. Common leak sources include cracked vacuum hoses, loose clamps, or faulty intake manifold gaskets. A smoke test can effectively identify leaks. Proper idle should be around 600-800 RPM. - High idle speed, hissing noise from engine bay, inconsistent RPM fluctuations when throttle is released.
- Faulty Automatic Idle Speed (AIS) Motor: If the AIS motor is malfunctioning or improperly calibrated, it may fail to adjust the idle speed correctly. The AIS motor should receive a stable voltage signal (typically around 12V) from the PCM to function properly. Testing should confirm the motor's operational status. - Continuous high idle after AIS replacement, no change in RPM when AIS is disconnected, voltage output from PCM during operation.
- Throttle Position Sensor (TPS) Calibration Issue: A new TPS may still be miscalibrated, leading to incorrect readings sent to the PCM. The TPS should provide a voltage of 0.5-1.0V at idle and approximately 4.0V at full throttle. Testing the TPS with a multimeter is essential to verify its functionality. - High idle with new TPS, voltage readings outside expected range during testing, potential stored codes related to TPS.
- Engine Coolant Temperature (ECT) Sensor Malfunction: If the ECT sensor is faulty, it may provide incorrect temperature readings to the PCM, causing improper fuel management and high idle. The ECT sensor should read approximately 180-220°F when the engine is at operating temperature. Testing should include verifying resistance values and voltage outputs. - Inconsistent idle speed during engine warm-up, ECT readings outside normal range, potential stored codes indicating ECT issues.
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 Diagnosis: Begin with a comprehensive scan using an OBD-II scanner or GeekOBD APP for efficient code retrieval and live data analysis. Check for any stored codes related to the idle speed or engine management system, and assess freeze frame data for additional context.
- STEP 2 - Vacuum Leak Inspection: Conduct a thorough visual inspection of all vacuum hoses and connections. Use a smoke machine if available to identify any hidden leaks. Pay special attention to the intake manifold gasket and any areas where hoses connect to the intake.
- STEP 3 - AIS and TPS Verification: Verify the operation of the AIS motor and TPS using a multimeter. Check that the AIS motor receives a stable voltage signal and that the TPS outputs the correct voltage range at idle and WOT. Document all measurements for reference.
- STEP 4 - ECT Sensor Testing: Test the ECT sensor for proper function by measuring its resistance at various temperatures. Ensure that the sensor reads within the expected range (approximately 180-220°F at operating temperature). This will determine if the sensor is affecting idle speed.
