Most Likely Culprits:
* Mass Airflow Sensor (MAF): A faulty MAF sensor provides inaccurate air/fuel mixture readings to the computer. This often manifests as problems at lower RPMs and lighter throttle positions, where the sensor's readings are most critical. At higher RPMs, the engine might compensate enough to mask the issue.
* Throttle Position Sensor (TPS): Similar to the MAF, a bad TPS gives incorrect information about throttle position. This can cause erratic fueling and misfires, especially at low RPMs and during throttle transitions.
* Idle Air Control (IAC) Valve: The IAC controls air flow at idle. A dirty or malfunctioning IAC can lead to rough idling, stalling, and hesitation, particularly noticeable at low speeds.
* Fuel Delivery Issues (Low Pressure or Dirty Injectors): While less likely given the high-RPM power, a problem with fuel pressure (fuel pump weakening) or dirty fuel injectors could cause a low-speed miss. At higher RPMs, the fuel pressure might be sufficient to overcome the issue. Check fuel pressure.
* Ignition System (Worn Distributor Cap, Rotor, Wires, or Spark Plugs): These are always suspects in a missfire, but if the high-RPM performance is good, the problem is likely less severe (e.g., a slightly cracked distributor cap or a single failing wire only affecting low-load operation). Nonetheless, they should still be inspected.
* Vacuum Leaks: Small vacuum leaks can significantly impact low-speed drivability by disrupting the air/fuel mixture. They often become less noticeable at higher RPMs where the engine's demand for air is greater.
* Computer (ECM): While less common, a failing Engine Control Module (ECM) can cause intermittent issues. This is usually a last resort diagnosis after other components have been checked.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Your Mustang likely has a diagnostic port. Use a code reader to see if the computer has stored any codes. This will give you a great starting point.
2. Visual Inspection: Carefully inspect all vacuum lines for cracks or leaks. Check the condition of the distributor cap, rotor, spark plug wires, and spark plugs. Look for anything obviously damaged or corroded.
3. Test the MAF Sensor: Use a multimeter to check the sensor's voltage output according to factory specifications. A malfunctioning MAF often shows erratic readings.
4. Test the TPS Sensor: Similar to the MAF, test the TPS voltage output with a multimeter to verify proper operation.
5. Clean the IAC Valve: Remove the IAC valve and clean it thoroughly with carburetor cleaner.
6. Check Fuel Pressure: Measure fuel pressure using a fuel pressure gauge to ensure it's within specifications.
7. Perform a Compression Test: This will help rule out mechanical issues within the cylinders, though it's less likely given the strong high-RPM performance.
8. Check for Vacuum Leaks: Use a vacuum gauge and carefully check all vacuum lines while the engine is running. A drop in vacuum indicates a leak.
Important Note: If you're not comfortable working on your car's electrical system or fuel system, take it to a qualified mechanic. Incorrectly diagnosing or fixing these issues could cause further damage to your engine. The intermittent nature of the problem makes a professional diagnosis easier.
