Fuel Delivery Issues:
* Fuel Pump: A weak or failing fuel pump might not deliver enough fuel pressure to meet the engine's demands, especially under load. This is a common cause of lean conditions.
* Fuel Filter: A clogged fuel filter restricts fuel flow, resulting in a lean condition.
* Fuel Injectors: Clogged or failing fuel injectors may not spray the correct amount of fuel into the cylinders. This could manifest as a lean condition and misfires in specific cylinders. You might have one or more failing injectors.
* Fuel Pressure Regulator: A faulty regulator could allow excessive fuel pressure, *or* insufficient pressure, leading to a lean condition. Usually it's the latter, but check both directions.
* Mass Airflow Sensor (MAF) Sensor: A faulty MAF sensor could read air intake incorrectly, causing the computer to deliver less fuel than needed, resulting in a lean condition and misfires. A dirty MAF sensor can also cause this.
Air Intake Issues:
* Intake Manifold Gasket Leak: A leak in the intake manifold gasket will allow unmetered air to enter the engine, creating a lean condition. This is a very common cause of lean codes on these engines.
* Vacuum Leaks: Leaks in vacuum hoses connected to the intake manifold or other components can also introduce unmetered air, leading to a lean condition. Check all hoses carefully for cracks or disconnections.
* EGR Valve: A faulty EGR (Exhaust Gas Recirculation) valve can sometimes contribute to lean conditions, although less frequently the primary cause. While not as common as other culprits, a stuck-open EGR valve can create issues.
* PCV Valve: A clogged positive crankcase ventilation (PCV) valve can cause a buildup of pressure in the crankcase, potentially leading to leaks and lean running.
Other Possibilities:
* Oxygen Sensor (O2 Sensor): A faulty O2 sensor provides inaccurate feedback to the engine control module (ECM), which can lead to improper fuel mixture. While less likely to be the *primary* cause of both lean and misfire, it can contribute to a worsening condition.
* Spark Plugs and Wires: Worn-out spark plugs or damaged ignition wires can cause misfires, and while not directly causing the lean condition, the resulting misfires can exacerbate the problem.
Troubleshooting Steps:
1. Check for vacuum leaks: Carefully inspect all vacuum hoses and the intake manifold gasket for leaks. This is often the culprit.
2. Check fuel pressure: Use a fuel pressure gauge to measure fuel pressure.
3. Inspect the fuel filter: Check for clogs.
4. Inspect spark plugs and wires: Look for damage or wear.
5. Check the MAF sensor: Clean it or replace it if necessary.
6. Scan for codes: Use an OBD-II scanner to get specific trouble codes, as this will help pinpoint the problem area. Don't just rely on the symptoms. The codes will tell you far more.
7. Check the fuel injectors: A fuel injector test is needed to fully rule this out. This often requires specialized tools.
It's crucial to diagnose the problem systematically. Starting with the easiest checks (vacuum leaks, fuel filter) and progressing to more involved diagnostics (fuel pressure, injector testing) is recommended. If you're not comfortable working on your vehicle, it's best to take it to a qualified mechanic. A lean condition can cause significant engine damage if left unresolved.
