* Compression: Low compression in one or more cylinders prevents the fuel-air mixture from being compressed enough for proper ignition. A compression test is crucial to rule this out. Worn piston rings, valve problems, or a blown head gasket are common causes of low compression.
* Timing: The ignition timing might be severely off. The spark plugs are firing at the wrong time relative to the piston position. This could be due to a faulty crankshaft position sensor (CKP sensor), camshaft position sensor (CMP sensor), or a problem within the distributor (if it has one; some Aleros used coil-on-plug ignition).
* Camshaft Position Sensor (CMP Sensor) or Crankshaft Position Sensor (CKP Sensor): These sensors tell the engine's computer where the crankshaft and camshaft are in their rotation. If faulty, the computer won't know when to send the spark signal, even if the ignition system is otherwise working.
* Fuel Injector Issues (despite having fuel): While you say it has fuel, it's possible the injectors aren't *delivering* the fuel properly. They might be clogged, electrically faulty, or have low fuel pressure. A fuel pressure test is essential here.
* Engine Control Module (ECM): The computer that controls the engine's functions could be faulty. A bad ECM can cause a variety of starting problems, even with spark and fuel.
* Security System: The Alero's anti-theft system (if equipped) could be preventing the engine from starting. This is less likely if you have a key that normally starts the car.
* Vacuum Leaks: Significant vacuum leaks can disrupt the air/fuel mixture and prevent proper combustion.
Troubleshooting Steps:
1. Compression Test: This is the first and most important test. Low compression points to a major internal engine problem.
2. Fuel Pressure Test: Measure the fuel pressure to ensure the injectors are receiving adequate fuel.
3. Check for Codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the ECM. These codes can pinpoint the problem.
4. Inspect Sensors: Visually inspect the CKP and CMP sensors for damage or loose connections.
5. Timing Check (if applicable): If you have the knowledge and tools, you can check the ignition timing. This is more advanced and potentially risky if not done correctly.
Without further diagnostics, it's impossible to say definitively what's wrong. A mechanic with diagnostic tools and experience with these vehicles will be needed to accurately diagnose and repair the problem.
