* Ignition System: This is the most likely culprit. Even though it's turning over, the spark plugs might not be firing. Check these things:
* Ignition Coil: Test the coil's output voltage. A weak or faulty coil won't provide sufficient spark.
* Distributor (if equipped): Older 7.3Ls had distributors; newer ones used an electronic system. If it has a distributor, check the cap, rotor, and ignition module for wear or damage. Cracks in the cap or rotor can prevent proper spark.
* Wiring: Inspect all the wires in the ignition system for damage, corrosion, or loose connections. A broken wire can interrupt the spark.
* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's position so it can time the ignition. A bad CKP sensor will prevent starting.
* Cam Position Sensor (CMP): Similar to the CKP, but for the camshaft. A faulty CMP can lead to no start.
* Fuel Delivery Issues (despite what you think): Even if you *think* it's getting fuel, there might be a problem:
* Fuel Pressure: It's crucial to measure fuel pressure. Low pressure, even if some fuel is reaching the injectors, won't allow proper combustion. A fuel pressure gauge is essential for diagnosing this.
* Injector Problems: While it's less likely if it's turning over, a clogged or failing injector could prevent enough fuel from reaching the combustion chamber for starting. A contribution test can help identify bad injectors.
* Glow Plugs (for cold starts): If the engine is cold, faulty glow plugs can prevent the engine from starting. This is more of a concern in colder weather.
* Low Compression: While less likely if it's turning over easily, very low compression in one or more cylinders could prevent starting. A compression test is needed to rule this out.
Troubleshooting Steps:
1. Check for Spark: The easiest initial test is to pull a spark plug wire, attach a spark plug to it (grounding it), and crank the engine. Look for a strong, blue spark. If no spark, focus on the ignition system.
2. Check Fuel Pressure: This is a critical step. Rent or borrow a fuel pressure gauge to check the pressure. Your owner's manual or a reliable online source should show the correct pressure specs.
3. Check Glow Plugs (if applicable): Check the glow plugs for continuity if the engine is cold.
4. Listen Carefully: While cranking, listen for unusual noises. Unusual sounds might point to a problem with the starter, or something internal to the engine.
If you're not comfortable working on your vehicle's ignition and fuel system, it's best to take it to a qualified mechanic. Improperly diagnosing and working on these systems can be dangerous. A diagnostic scan tool might also be helpful to get codes from the engine control module (ECM).
