1. Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS): These sensors tell the computer how much air and throttle opening is present. A faulty sensor can lead to incorrect fuel mixture calculations, resulting in hesitation, especially at consistent speeds.
* How to diagnose: You'll likely need a scan tool to check for error codes related to the MAF or TPS. A visual inspection for damage or dirt on the MAF sensor is also recommended. Replacing these sensors is relatively straightforward but requires some mechanical aptitude.
2. Fuel Injectors: Clogged or failing fuel injectors can cause inconsistent fuel delivery, leading to hesitation. While the fuel filter is changed, the injectors themselves might be the problem.
* How to diagnose: This is harder to diagnose without specialized tools. A fuel pressure test will check for sufficient pressure, but doesn't pinpoint a faulty injector. A fuel injector cleaning might be attempted, but a more thorough diagnosis usually involves testing the injectors' spray pattern and flow rate on a bench test.
3. Intake Manifold Gasket Leaks: Leaks in the intake manifold gasket can cause a lean air/fuel mixture, leading to hesitation and rough running. This is more likely if you notice vacuum leaks or unusual sounds from the intake.
* How to diagnose: Visually inspect the intake manifold gaskets for cracks or damage. You might hear a hissing sound if there's a leak. A smoke test (introducing smoke into the intake) can reveal leaks.
4. EGR Valve: The Exhaust Gas Recirculation valve is responsible for recirculating exhaust gases into the intake. A stuck open or closed EGR valve can lead to poor engine performance, including hesitation.
* How to diagnose: You can try cleaning the EGR valve (relatively simple, look up a tutorial for your specific engine). Again, a scan tool might reveal relevant error codes.
5. Vacuum Leaks: Anywhere in the vacuum system, a small leak can disrupt the proper operation of various engine components. This is a broad category, and tracing the leak can be challenging.
* How to diagnose: Again, a visual inspection, listening for hissing sounds, and a smoke test are helpful. Carefully examine all vacuum hoses and connections.
6. Ignition Control Module (ICM): While you changed the distributor cap and rotor, the ICM itself could be failing. This is less common, but possible.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scan tool to read any stored codes. This will often point you towards the most likely culprit.
2. Visual Inspection: Carefully inspect all components related to the ignition, fuel system, and vacuum system. Look for anything obvious like broken wires, loose connections, or damaged parts.
3. Systematic Approach: Address the issues listed above one by one, starting with the most likely candidates based on the information you gather from the DTCs and visual inspection.
Important Note: If you are not comfortable working on your vehicle's engine, it's best to take it to a qualified mechanic. Improperly diagnosing or repairing engine problems can lead to further damage.
