Owner's Problem Description
The owner reports that their 2000 Ford Contour with a 2.0L engine is experiencing rough idling, especially when the A/C is on, and it stalls under full throttle. They've replaced the PCV valve, IAC, TPS, and fuel pressure is reportedly fine at 30-45 PSI. All fluids are full and vacuum lines checked, but the engine exhibits spitting and sputtering when attempting to rev. No engine lights are present, and a scanner shows no issues. They seek help diagnosing the root cause.
Professional Technical Analysis
The 2000 Ford Contour, equipped with a 2.0L engine and a non-returnless fuel system, is showing symptoms of rough idling and stalling under load, particularly with the A/C activated. This vehicle, typical at around 150,000 miles, often experiences issues related to the idle air control (IAC) and throttle position sensor (TPS), both of which have been recently replaced. The intermittent rough idle and stalling can indicate a potential vacuum leak, improper fuel delivery, or ignition timing issues. The charging system should maintain a stable voltage between 13.8-14.4V; fluctuations could lead to PCM malfunction. Fuel pressure tests indicate 30 PSI at idle, rising to 45 PSI under throttle, which is within normal operating range but could mask issues with the fuel pump or regulator. Analyzing the spark plugs for signs of rich or lean running conditions could also prove beneficial as they may reveal misfiring patterns. Additionally, checking the EGR system for clogging or improper operation is crucial, especially if the vehicle has a history of carbon buildup. The rough idle when the A/C is on suggests a potential load issue or insufficient air/fuel mixture, possibly exacerbated by minor vacuum leaks that were overlooked. Overall, a systematic approach to diagnosing electrical signals from the IAC and TPS while also monitoring for vacuum integrity is recommended to ensure accurate diagnosis and repair.
Possible Causes
Most common causes (ordered by frequency):
- Vacuum Leak: A small vacuum leak may not be evident during inspection but can significantly affect idle quality. Check for leaks in the intake manifold, around the throttle body, and in vacuum hoses. Use a smoke test if necessary to identify hidden leaks. - Rough idle with A/C on, fluctuating RPMs, possible hissing sounds when revving.
- Ignition Timing Issues: The timing at 11 degrees at idle may be too retarded, affecting combustion efficiency. Verify timing with a timing light and ensure the timing is set according to specifications, which is typically around 10-14 degrees for this engine. - Sputtering and misfiring under load, poor acceleration response.
- IAC Valve Malfunction: Even after replacement, the IAC valve may not be calibrated correctly or could be faulty. Check the IAC for proper function using a scanner to monitor its position and response to throttle input. - Fluctuating idle, stalling when A/C is engaged.
- Fuel Delivery Issues: Although fuel pressure seems fine, consider testing the fuel pump's flow rate and the fuel filter for clogs. A partially clogged filter can restrict fuel flow, especially under high demand. - Inconsistent fuel pressure readings during acceleration.
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 - Verify Fuel Pressure and Flow: Use a fuel pressure gauge to confirm that the fuel pressure remains consistent at 30 PSI at idle and rises to 45 PSI during acceleration. Additionally, assess the fuel pump flow rate to ensure it meets specifications. This can be done by measuring the amount of fuel delivered in a specific time frame.
- STEP 2 - Inspect for Vacuum Leaks: Perform a thorough inspection of all vacuum hoses and connections. A smoke test is recommended to identify any hidden leaks that could be causing rough idling. Pay special attention to the intake manifold and throttle body gasket areas.
- STEP 3 - Check Ignition Timing: Use a timing light to verify that the ignition timing is set to the manufacturer's specifications, typically around 10-14 degrees. Adjust the timing if necessary and monitor how this affects engine performance.
- STEP 4 - Monitor IAC Function: Use a scanner to observe the IAC valve operation during idle and under load. Ensure it is responding correctly to throttle input and is positioned appropriately. A faulty IAC can lead to improper air-fuel mixtures.
