High-Probability Causes:
* Crankshaft Position Sensor (CKP): This sensor is crucial for determining engine position and triggering the spark. A faulty CKP sensor will prevent the engine from firing. This is a prime suspect given your description.
* Camshaft Position Sensor (CMP): Similar to the CKP, the CMP sensor tells the computer the position of the camshaft. A bad CMP can also lead to no spark.
* Ignition Control Module (ICM): This module controls the ignition system. A failing ICM can cause intermittent or complete loss of spark. It's often located near the distributor (if your truck has one – some 4.7L's did, some didn't).
* Wiring Issues: Check all wiring harnesses related to the ignition system, especially those going to the CKP, CMP, ICM, and coil packs. Look for corrosion, broken wires, or loose connections. Pay close attention to the harness near the engine, which is subject to a lot of heat and vibration.
* Powertrain Control Module (PCM) / ECM: While you've already changed a sensor, a faulty PCM is a possibility. However, this is less likely than the other options since you're getting *some* initial firing. PCM issues often result in no start at all.
Less-Likely Causes:
* Coil Packs: While less likely to cause *complete* lack of spark in all cylinders, a faulty coil pack could be contributing to the problem, particularly if only one or a few cylinders are truly dead.
* Distributor (if equipped): If your 4.7L has a distributor, issues within it (rotor, cap, etc.) could be causing problems, but this is less common in later-model 4.7L engines.
* Fuel Pump Relay: While the description points to a spark problem, a faulty relay could prevent the fuel pump from starting, which would cause the engine to stall shortly after startup. This is worth checking, as a short fuel supply could mimic a spark issue.
* Security System: In some cases, a faulty security system can prevent the engine from starting.
Troubleshooting Steps:
1. Check for Codes: Use an OBD-II scanner to retrieve any diagnostic trouble codes (DTCs). This is the first and most important step. Codes will give you valuable clues about the problem's source.
2. Visual Inspection: Carefully inspect all wiring harnesses and connectors associated with the ignition system. Look for any signs of damage, corrosion, or loose connections.
3. CKP Sensor Test: Test the CKP sensor for proper resistance and signal. You can find testing procedures online specific to your year and engine. This is a crucial step.
4. CMP Sensor Test (If applicable): Similar to the CKP test, test the CMP sensor for proper operation.
5. Coil Pack Testing: Test the resistance and voltage output of the coil packs.
6. Ignition System Voltage Checks: Check voltage at the various components (CKP, CMP, coils) to make sure they're getting power.
Important Note: Working on automotive electrical systems can be dangerous. If you're not comfortable working with electricity and car electronics, it's highly recommended to take your truck to a qualified mechanic. Attempting repairs without proper knowledge and safety precautions can lead to injury or further damage.
This systematic approach should help you pinpoint the cause of the no-spark issue. Remember to check those DTCs first! Good luck!
