* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's rotational position. A faulty CKP prevents the engine from firing. It's a relatively inexpensive part, and a common culprit when everything else seems fine.
* Camshaft Position Sensor (CMP): Similar to the CKP, a faulty CMP will prevent proper ignition timing and starting.
* Fuel Pump Relay: Even with good fuel pressure, a failing relay might not be supplying power consistently to the pump. Try swapping the relay with another (like the horn relay, for example, checking for a matching amperage), to see if that solves it.
* Computer (ECM/PCM): While less likely, a faulty Engine Control Module could be the issue. This is a more expensive repair. A scan tool might reveal codes pointing to the ECM, though often it requires more advanced diagnostics.
* Security System: Does your truck have a factory or aftermarket anti-theft system? A problem with the system could be preventing the engine from starting.
* Wiring Harness Issues: A corroded, damaged, or loose wire in the main harness, particularly around the ignition switch or engine computer, can cause intermittent or complete failure to start. This requires careful visual inspection, potentially with a multimeter to test continuity.
* Neutral Safety Switch: The automatic transmission needs to be in park or neutral for the engine to start. A faulty switch can prevent starting even if it's in the correct gear.
Recommendations:
1. Check the CKP and CMP sensors: Have your mechanic test these sensors, or replace them if they show any sign of wear or fault. These are relatively inexpensive parts to replace and are high probability areas given your description.
2. Test the Fuel Pump Relay: The easy swap test mentioned above is worth the few minutes it takes.
3. Inspect the Wiring Harness: Look carefully for any damaged, frayed, or corroded wires, especially near the engine and firewall.
4. Obtain a Diagnostic Code Scan: Even if your mechanic ran a scan, it is worth getting a professional-level scan to check for any hidden trouble codes that might not have been captured by a basic code reader.
5. Professional Diagnostic: If the above steps don't yield results, consider taking the truck to a specialist who works on older GM vehicles. They might have more experience with these types of intermittent no-start issues.
Remember, working on a vehicle's electrical system can be dangerous. If you're not comfortable performing these checks yourself, it's always best to have a qualified mechanic do the work.
