Owner's Problem Description
The owner reported that the 1996 Plymouth Breeze experiences engine stalling after the cruise control is engaged for a while. The issue first appeared while driving at speeds of 65-70 mph with cruise control activated, and has been noted to occasionally stall even without cruise control at similar speeds. Previous repairs include an idle air control (IAC) valve replacement, which was recently done. The vehicle currently shows intermittent stalling behavior during highway driving.
Professional Technical Analysis
The 1996 Plymouth Breeze, equipped with a 2.0L inline-4 engine and a 4-speed automatic transmission, has an estimated mileage of approximately 120,000 miles. The owner has reported that the engine stalls after engaging the cruise control for a while, raising concerns regarding the cruise control system's interaction with the engine management. Given the symptoms, the stalling may be related to several interconnected systems: the idle air control valve (IAC), throttle position sensor (TPS), and possibly the cruise control module itself. When cruise control is engaged, the PCM adjusts the throttle position to maintain speed, which can lead to stalling if the IAC is not functioning properly. The IAC valve, if improperly installed or malfunctioning, can lead to incorrect air-fuel mixture adjustments, causing stalling. Additionally, the TPS must provide accurate readings to the PCM; a faulty TPS can mislead the PCM, disrupting engine management. Current vehicle conditions suggest intermittent stalling, with potential safety risks when driving at highway speeds. The owner also noted that the vehicle can stall even when the cruise control is not activated, suggesting a deeper underlying issue with the engine management system or fuel delivery. Monitoring the vehicle's fuel pressure while under load may provide insight, as a drop below 35 PSI during operation can lead to stalling. Additionally, checking for any stored OBD-II codes will help identify any faults in the system that may be contributing to these symptoms.
Possible Causes
Most common causes (ordered by frequency):
- Idle Air Control (IAC) valve malfunction: The IAC valve controls the amount of air entering the engine during idle conditions. If it's improperly installed or malfunctioning, it can lead to stalling when the engine requires a stable idle air supply. A typical IAC valve should have a resistance of 10-14 ohms. Improper calibration or a stuck position can prevent the PCM from maintaining the desired idle speed, especially when cruise control is engaged. This could cause significant drops in RPM, leading to stalling, particularly when the throttle position changes due to cruise control adjustments. - Engine stalling at idle and while engaged in cruise control, IAC valve resistance measured at 12 ohms indicating potential malfunction, check engine light may illuminate with related fault codes.
- Throttle Position Sensor (TPS) failure: The TPS provides critical input to the PCM regarding the position of the throttle plate. If the TPS is malfunctioning or not calibrated correctly, it may provide incorrect data leading to inappropriate fuel delivery and stalling, especially when the cruise control alters throttle position. TPS should show a voltage between 0.5V at closed throttle to 4.5V at wide open throttle (WOT). Erratic readings may signal a need for replacement. - Erratic engine performance when cruise control engaged, TPS voltage readings inconsistent, potential stored codes P0120-P0123.
- Cruise Control Module failure: A malfunctioning cruise control module could incorrectly interpret vehicle speed signals or throttle position, leading to stalling. This module is responsible for maintaining the desired speed and may prevent the throttle from returning to idle properly when disengaged. This could result in a failure to maintain idle RPM, which is critical for engine stability. Checking the module for proper connections and functionality is essential. - Inconsistent vehicle speed control, stalling when cruise control is disengaged, visual inspection of module connections shows signs of wear or damage.
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: Use a professional OBD-II scanner or GeekOBD APP to retrieve stored fault codes and freeze frame data. This initial scan will help identify any existing issues related to the engine management system or cruise control module. Pay attention to any codes related to the IAC, TPS, or cruise control system, which can provide insight into potential malfunctioning components.
- STEP 2 - Live Data Monitoring: Utilize the GeekOBD APP to monitor key parameters such as IAC position, TPS voltage, and engine RPM during cruise control operation. This real-time data can help identify irregularities that may cause stalling, particularly focusing on IAC response and TPS readings under varying throttle conditions. Document any significant deviations from expected values.
- STEP 3 - Component Testing: Test the IAC valve and TPS using a multimeter to measure resistance and voltage values. The IAC should have a resistance of 10-14 ohms, while the TPS should show a voltage range of 0.5V to 4.5V. If readings are outside the specified range, consider replacing the faulty component. Additionally, inspect wiring and connectors for signs of damage or corrosion that could affect performance.
- STEP 4 - Road Test with Monitoring: Conduct a controlled road test while monitoring live data with the GeekOBD APP. Observe the vehicle's performance during cruise control engagement and note any stalling occurrences. Evaluate RPM drops and throttle response during the test, ensuring to document any anomalies for further analysis.
