Fuel System:
* Fuel Injectors (if equipped): If it's fuel-injected, clogged or leaking injectors can cause excessive fuel delivery.
* Intake Manifold Leak: A vacuum leak can cause a rich condition, especially if it's pulling unmetered air. This is a common problem on older vehicles.
* EGR Valve: A stuck-open Exhaust Gas Recirculation valve can allow excessive exhaust gas into the intake, leading to a rich mixture.
* Fuel Pump: A faulty fuel pump might deliver too much pressure or inconsistent flow. While less common than other issues, a failing pump is a possibility.
* Throttle Position Sensor (TPS): If equipped, a malfunctioning TPS can send incorrect signals to the computer, leading to an overly rich mixture.
Ignition System (beyond the components you listed):
* Ignition Coil: A weak or failing coil might not provide sufficient spark, causing misfires and leading the computer to add fuel to compensate.
* Crankshaft Position Sensor (CKP): If the engine is not properly sensing crankshaft rotation, the fuel delivery can be affected. This is more likely on fuel-injected models.
* Computer/ECM Issues: The Engine Control Module (computer) itself could be faulty and delivering incorrect signals to the fuel system.
Other Possibilities:
* Oxygen Sensor (O2 sensor): If equipped, a faulty O2 sensor provides incorrect feedback to the computer about the exhaust gases, leading to improper fuel adjustments. This component is crucial for maintaining the correct air/fuel ratio.
* Vacuum Lines: Check all vacuum lines for cracks, leaks, or disconnections. A small leak can cause significant issues.
Troubleshooting Steps:
1. Start with the basics: Check the easy-to-access items you mentioned (fuel filter, spark plugs, wires, distributor cap, fuel pressure).
2. Check for vacuum leaks: Use a propane torch (carefully!) to check for leaks around the intake manifold and vacuum lines. A hissing sound indicates a leak.
3. Inspect the fuel injectors (if applicable): If you have fuel injectors, they might require cleaning or replacement. You'll need a fuel pressure gauge to evaluate their performance.
4. Scan for trouble codes: If your vehicle has an OBD-I system (check your owner's manual), a code reader can give you valuable clues.
5. Consult a repair manual: A factory service manual is invaluable for troubleshooting your specific vehicle.
Remember safety first when working on your vehicle. If you're not comfortable working on your vehicle's fuel or ignition systems, take it to a qualified mechanic.
