1. Timing Issues:
* Timing Belt/Chain: A broken or severely slipped timing belt or chain will prevent the engine from firing properly, even with fuel and spark. This is a serious issue requiring professional attention. Listen closely during cranking; a "clanking" sound might indicate this.
* Timing Mark Alignment: Incorrect timing mark alignment will lead to improper ignition timing. This is less likely if the engine ran recently before the problem started, but still possible if any timing-related work was performed.
2. Compression Issues:
* Low Compression: Worn piston rings, valve problems (burnt, bent, or leaking), or head gasket failure can lead to insufficient compression to ignite the air/fuel mixture. A compression test is crucial to diagnose this.
* Cylinder Leakage: A leak in the cylinder head gasket, intake manifold gasket, or cracked block allows compression to escape before combustion.
3. Fuel Delivery Problems (Despite "Good Fuel"):
* Fuel Injectors: Even with fuel in the tank, clogged or malfunctioning fuel injectors may not be delivering fuel properly to the cylinders. A fuel pressure test is needed.
* Fuel Pressure Regulator: This regulates the fuel pressure in the fuel rail; a faulty one can cause low or inconsistent fuel pressure.
* Fuel Pump: While you say you have "good fuel," a weak fuel pump might not be supplying enough pressure for proper atomization. Listen for the fuel pump whirring during cranking; a faint or absent sound might point to a failing pump.
4. Ignition System Problems (Despite "Good Spark"):
* Spark Strength/Timing: While you have spark, it might be weak or mistimed (incorrect timing). A mechanic can check spark strength and timing.
* Crankshaft Position Sensor (CKP) or Camshaft Position Sensor (CMP): These sensors tell the engine's computer the position of the crankshaft and camshaft; if faulty, the computer won't know when to fire the spark plugs.
* Ignition Coil: A weak or failing ignition coil may not provide sufficient spark to all cylinders consistently.
5. Other Potential Issues:
* Security System: An immobilizer or alarm system could be preventing the engine from starting.
* Neutral Safety Switch (Automatic Transmission): This switch prevents starting unless the vehicle is in park or neutral.
* Starter Motor Issues (Less Likely): Although it's cranking, a weak starter might not be providing sufficient power to reliably ignite the engine.
Troubleshooting Steps:
1. Check for Obvious Issues: Inspect for any loose wires, vacuum leaks, or obvious damage.
2. Listen Carefully During Cranking: Pay attention to any unusual sounds.
3. Fuel Pressure Test: This is crucial to rule out fuel delivery problems.
4. Compression Test: This is essential for diagnosing compression issues.
5. Spark Test: Verify spark strength at each plug.
6. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any error codes stored in the vehicle's computer.
Recommendation: Given the complexity of the situation, and the potential for serious mechanical damage (like a broken timing belt), it's best to have a qualified mechanic diagnose and repair the problem. Attempting some repairs yourself without proper knowledge and tools can lead to further damage and increased costs.
