* Mass Airflow Sensor (MAF) Sensor: A dirty or failing MAF sensor will provide inaccurate air readings to the computer. This leads to a rich or lean fuel mixture, especially pronounced when cold. The engine may run better once it warms up because the sensor might start working a little better with heat, but performance will still suffer.
* Throttle Position Sensor (TPS): Similar to the MAF sensor, a faulty TPS will give the computer incorrect information about the throttle position. This can cause poor fuel delivery and misfires, especially noticeable under acceleration (hills, heavy throttle).
* Fuel Injectors: Clogged or failing fuel injectors can spray fuel inconsistently. Cold starts are often worse because the fuel might be thicker, exacerbating the problem. A rich mixture under acceleration from clogged injectors could explain the misfire with too much gas.
* Ignition System (Worn Spark Plugs, Wires, Distributor Cap/Rotor): Worn spark plugs, cracked or worn spark plug wires, or a bad distributor cap and rotor (if it's a distributor ignition system) can cause misfires, especially noticeable under load (hills). Cold temperatures can also amplify problems in the ignition system.
* EGR Valve: A sticking EGR valve can cause rough running, especially at low speeds and idle. It might improve as the engine warms up but generally leads to poor performance.
* PCV Valve: A clogged positive crankcase ventilation (PCV) valve can cause a build-up of crankcase pressure, affecting engine performance and potentially leading to rough running.
Troubleshooting Steps:
1. Check the obvious: Start with the easiest and cheapest:
* Inspect and replace spark plugs and wires: Worn plugs are a very common cause of these symptoms.
* Check the air filter: A clogged filter restricts airflow and will cause poor performance.
* Check the fuel filter: A clogged filter can restrict fuel flow, especially noticeable during acceleration.
2. Advanced diagnostics: If the simple checks don't solve it, you'll likely need to:
* Obtain diagnostic codes: Use an OBD-II scanner (available at auto parts stores) to read any trouble codes stored by the engine control unit (ECU). This will give you a good starting point.
* Check the MAF sensor and TPS: These can be tested with a multimeter (requires some electrical knowledge) or replaced as preventative maintenance. Cleaning the MAF sensor is sometimes helpful but not always a permanent fix.
* Consider professional help: If you're not comfortable working on the fuel system or ignition system yourself, take it to a mechanic. They can perform more advanced diagnostics and pinpoint the exact problem.
Remember safety first! Disconnect the negative battery terminal before working on any electrical components. If you're not experienced working on cars, it's best to consult a professional mechanic, especially with fuel injection and ignition systems. Improper repairs can cause further damage.
