* Compression: A lack of sufficient compression in the cylinders prevents the fuel-air mixture from igniting properly. This could be due to worn piston rings, blown head gasket, valve problems (burnt valves, sticking valves), or other internal engine damage. A compression test is needed to diagnose this.
* Timing: The engine's timing could be off. If the spark plugs are firing at the wrong time relative to the piston position, the engine won't start. This requires checking the timing belt or chain and potentially the crankshaft position sensor.
* Fuel Delivery Issues (despite presence): While you say fuel is present, the *delivery* might be insufficient. Check fuel pressure using a fuel pressure gauge. A weak fuel pump, clogged fuel filter, or problems with the fuel injectors could be at fault, even if some fuel is reaching the engine.
* Crank Position Sensor (CKP): This sensor tells the engine computer the crankshaft's position. A faulty CKP sensor can prevent the engine from starting, even with fuel and spark.
* Cam Position Sensor (CMP): Similar to the CKP sensor, a faulty CMP sensor affects the timing and can prevent starting.
* Air Intake: A severely clogged air filter can restrict airflow enough to prevent starting.
Troubleshooting Steps:
1. Check for obvious issues: Look for loose or disconnected wires, vacuum leaks, and anything obviously out of place.
2. Compression Test: This is crucial to rule out internal engine problems.
3. Fuel Pressure Test: Measure fuel pressure to ensure the injectors are receiving adequate fuel.
4. Spark Plug Inspection: While you say spark is present, check the spark plugs visually – are they wet with fuel? This can indicate a problem with the air/fuel mixture. Soot buildup might point to a rich mixture, while dryness points to a lean mixture.
5. Timing Belt/Chain Inspection: Visually inspect for damage or looseness. (This often requires more mechanical skill.)
6. Scan for Codes: Use an OBD-II scanner to check for diagnostic trouble codes (DTCs) stored in the engine control unit (ECU). These codes can pinpoint the problem.
If you're not comfortable performing these checks, it's best to take your vehicle to a qualified mechanic for diagnosis and repair. Attempting repairs without the proper knowledge and tools can lead to further damage.
