Fuel System:
* Fuel Pump: A weak fuel pump might provide enough pressure at high engine speeds (higher RPMs create more pump pressure) but struggle at low speeds and idle. A cold engine also requires more fuel. Listen closely for the pump whirring when you turn the key on (before cranking). A weak or failing pump might make a quieter or different sound.
* Fuel Filter: A clogged fuel filter restricts fuel flow, especially noticeable at low idle and under load. This is a cheap and easy component to replace.
* EGR Valve (Exhaust Gas Recirculation): A sticking or malfunctioning EGR valve can cause rough idling, especially when the engine is warm. It reintroduces exhaust gases into the intake, affecting the air/fuel mixture.
* Fuel Injectors: Clogged or leaking injectors can lead to a lean or rich mixture, resulting in poor performance. This is more likely a culprit if the problem is gradual. They might need cleaning or replacement.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate information to the engine control unit (ECU), leading to poor fuel delivery. This can manifest as problems across the RPM range.
* Mass Air Flow Sensor (MAF Sensor): A dirty or malfunctioning MAF sensor, which measures the amount of air entering the engine, can cause incorrect air/fuel mixture calculations.
Ignition System:
* Ignition Coil: A weak coil may struggle to provide sufficient spark at low speeds and idle, causing misfires.
* Distributor Cap and Rotor: These components wear over time and can cause misfires if cracked or corroded, especially noticeable at low RPMs.
* Spark Plugs and Wires: Worn or fouled spark plugs and damaged wires can prevent proper combustion. Check for cracks, corrosion, and appropriate gap settings.
* Crankshaft Position Sensor (CKP): A faulty CKP sensor can prevent the engine from starting or running smoothly, especially when cold.
Other Possibilities:
* Vacuum Leaks: Small vacuum leaks can disrupt the engine's operation, especially at idle. Listen for hissing sounds around intake hoses and vacuum lines.
* IAC (Idle Air Control) Valve: This valve regulates idle speed. A dirty or malfunctioning IAC valve can cause unstable idle.
* Computer (ECU): While less likely, a failing ECU can cause a variety of problems, including those you've described.
Troubleshooting Steps:
1. Check the easy stuff first: Fuel filter, spark plugs, and wires. These are relatively inexpensive and quick to inspect/replace.
2. Listen for unusual noises: Pay close attention to the fuel pump, and listen for any hissing (vacuum leaks).
3. Inspect the distributor cap and rotor: Look for cracks or corrosion.
4. Check for codes: If your vehicle has a check engine light, retrieve the diagnostic trouble codes (DTCs) using a code reader. This will pinpoint potential problems.
5. Test fuel pressure: This requires a fuel pressure gauge. Low fuel pressure confirms a fuel system issue.
6. Consult a mechanic: If you're not comfortable diagnosing these issues yourself, a qualified mechanic can properly diagnose the problem and perform necessary repairs.
Remember safety first. Work on a cool engine and disconnect the battery negative terminal before doing any work under the hood. The symptoms you've described suggest multiple possible issues; a systematic approach is key to finding the root cause.
