Fuel Delivery Problems:
* Fuel pump: A weak or failing fuel pump can't deliver enough fuel pressure to the injectors. This is a common cause.
* Clogged fuel filter: A dirty filter restricts fuel flow.
* Clogged fuel injectors: Blocked or partially blocked injectors prevent the proper amount of fuel from being sprayed into the cylinders. This often manifests as a gradual lean condition.
* Fuel pressure regulator: A malfunctioning regulator might not maintain the correct fuel pressure.
* Leaking fuel injectors: Injectors leaking fuel *after* the injection cycle can lead to lean conditions in other cylinders.
* Fuel tank issues: A severely restricted fuel pick-up or debris in the tank could reduce fuel delivery.
Air Intake/Mass Airflow (MAF) Sensor Issues:
* MAF sensor: A faulty MAF sensor provides incorrect air mass readings to the computer, leading to insufficient fuel injection. Often shows up as a lean code. This is a very common cause.
* Vacuum leaks: Leaks in the intake manifold, vacuum lines, or elsewhere in the intake system allow unmetered air to enter the engine, throwing off the air/fuel ratio. This is a major contributor to lean running.
* Dirty air filter: A severely restricted air filter reduces airflow but the MAF sensor doesn't necessarily compensate for it fully, resulting in a lean condition.
Engine Management System (EMS) Problems:
* Oxygen sensor (O2 sensor): A faulty O2 sensor provides inaccurate feedback to the computer, causing it to adjust the fuel mixture incorrectly. A slow-responding or inaccurate O2 sensor is a common reason for lean running.
* Computer (ECM/PCM): While less common, a malfunctioning engine control module can cause incorrect fuel delivery calculations. This is generally diagnosed *after* all other components have been checked.
* Throttle Position Sensor (TPS): An inaccurate TPS reading can cause the computer to miscalculate the fuel delivery.
Other Possibilities:
* EGR valve issues: A malfunctioning EGR valve can contribute to lean conditions, especially at idle or low RPM.
* Intake manifold gasket leaks: Similar to vacuum leaks, these allow unmetered air to enter.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any trouble codes stored in the engine's computer. These codes can pinpoint the problem area.
2. Inspect fuel pressure: Measure the fuel pressure using a fuel pressure gauge.
3. Check the MAF sensor: Inspect the sensor for cleanliness and damage. A replacement might be necessary.
4. Check for vacuum leaks: Inspect all vacuum lines and the intake manifold for cracks or loose connections. A smoke test is very effective for detecting these leaks.
5. Inspect the air filter: Replace if dirty.
6. Check the O2 sensors: These sensors can be tested with a multimeter or by comparing the readings from different sensors.
A lean condition can cause serious engine damage if not addressed. Overheating and even a catastrophic engine failure are possibilities. It's best to diagnose and repair the problem promptly. If you're not comfortable performing these diagnostics yourself, taking your vehicle to a qualified mechanic is strongly advised.
