Owner's Problem Description
The owner of a 1993 Plymouth Colt reports an idle surge occurring after the engine warms up. The idle speed fluctuates between 1100 RPM and 1900 RPM. The vehicle starts fine but experiences this surge consistently when reaching operating temperature. Various components have been replaced or repaired, including the computer, ISC motor, temperature sensor, O2 sensor, fuel pressure regulator, and all sensors tested within specifications. The timing is confirmed to be good, and new plugs, cap, rotor, and fuel filter were installed. All vacuum hoses have also been replaced. Notably, the ISC motor disconnection does not stop the surging, and there are no fault codes stored.
Professional Technical Analysis
This 1993 Plymouth Colt, equipped with a 1.8L engine and approximately 150,000 miles, presents a complex idle surge issue after warm-up. The surging occurs within a range of 1100 to 1900 RPM, suggesting a potential air/fuel metering issue rather than a mechanical fault. The owner has replaced multiple components including the computer (ECM), idle speed control (ISC) motor, temperature sensor, and O2 sensor, indicating a thorough approach to resolving the issue. Despite these efforts, the vehicle continues to surge without any diagnostic trouble codes (DTCs) present. The ISC motor's disconnection allows the vehicle to maintain a surge, indicating that the issue may not be directly related to the idle control system but rather a broader air-fuel mixture problem. An analysis of the fuel system shows a pressure reading of 47.5 PSI, which is within operational specifications (should be 35-45 PSI); however, the behavior of the ISC could be influenced by other factors such as vacuum leaks or throttle position sensor (TPS) inaccuracies. The absence of EGR and MAP sensors complicates the diagnosis as they are often integral to fuel mixture adjustments. Additionally, the absence of fault codes during diagnostic checks suggests a potential intermittent fault or a condition that isn't triggering the onboard diagnostics. Components such as the throttle position sensor and intake air temperature sensor should be checked again, including their wiring and connections, as they can directly influence the PCM's ability to modulate idle speed effectively. The current state of the vehicle, with no fault codes and a new ISC motor, indicates the need for a more in-depth analysis of the air intake system and potential sensor inaccuracies.
Possible Causes
Most common causes (ordered by frequency):
- Throttle Position Sensor (TPS) malfunction: The TPS, which should provide a voltage signal proportional to the throttle position, may be sending erratic signals to the PCM. A faulty TPS can cause incorrect air-fuel mixture adjustments, leading to idle instability. Testing should confirm that the TPS provides a voltage between 0.5V (closed throttle) and 4.5V (wide open throttle) under dynamic conditions. If the TPS is not within this range, or if there is any fluctuation when the throttle is held steady, it should be replaced. Common failure patterns include internal wear that affects the potentiometer's output. - Fluctuating idle speed, inconsistent TPS readings during testing, potential stored codes P0120-P0123 related to TPS failure.
- Vacuum Leak: Despite the replacement of all vacuum hoses, the possibility of a hidden vacuum leak remains. A vacuum leak can cause excess air to enter the intake manifold, leading to a lean condition which affects idle stability. Testing should be conducted using a smoke machine or propane enrichment method to identify any leaks that may not be visually detectable. Pay special attention to the intake manifold gaskets and throttle body mount. - Lean idle condition, potential hissing sounds from vacuum areas, testing results showing abnormal air-fuel ratios.
- Idle Air Control (IAC) System Issues: The IAC motor may be functioning improperly or may not be receiving the correct signals from the PCM. Although the IAC motor was replaced, issues can arise from poor electrical connections or a faulty PCM. A thorough check of IAC operation, including response times and voltage supply, is necessary. The IAC should ideally operate between 0.5V and 12V. If the IAC is not responding as expected, further diagnosis on the PCM and wiring harness is required. - Inconsistent idle adjustments, IAC voltage readings outside of normal range, possible intermittent connection issues.
- Fuel Pressure Regulation: Although the fuel pressure is reported at 47.5 PSI, further testing of the fuel pressure regulator may be needed. A malfunctioning fuel pressure regulator can allow incorrect pressure fluctuations, leading to erratic fuel delivery. The fuel pressure should be maintained consistently between 35-45 PSI with a maximum drop of 5 PSI over 5 minutes after key-on. If fluctuations exceed these specifications, replacement of the fuel pressure regulator is recommended. - Fuel pressure readings inconsistent with specifications, possible pressure drops observed during idle conditions.
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 - Comprehensive OBD-II Scan: Utilize a professional OBD-II scanner or the GeekOBD APP to check for stored codes and freeze frame data. Even though there are no codes, reviewing the freeze frame can provide insight into the vehicle's operating conditions at the time of the last fault occurrence. Focus on parameters such as fuel trims, IAC position, and TPS readings. This step is crucial for establishing a baseline of the vehicle's performance prior to further diagnosis.
- STEP 2 - Live Data Monitoring: Use the GeekOBD APP to monitor live data parameters while the engine is idling and during the surge. Pay particular attention to the fuel pressure, TPS voltage, and IAC position. The TPS should show a voltage between 0.5V and 4.5V, while the IAC should be adjusting as needed to maintain idle speed. Document any abnormalities in these readings that correlate with the idle surge.
- STEP 3 - Check for Vacuum Leaks: Conduct a thorough inspection for vacuum leaks using a smoke machine or by applying a propane enrichment test. Focus on areas such as intake manifold gaskets, throttle body seals, and any connections that may have been overlooked during previous repairs. A vacuum leak can significantly affect idle stability, so this step is vital for isolating the cause of the surge.
- STEP 4 - Test and Verify Components: Test the TPS voltage output and response time to ensure it is functioning correctly. Measure the IAC voltage and check its operation under varying idle conditions. Also, verify fuel pressure readings under load to ensure the fuel system is delivering consistently. Document all findings for further analysis.
