* Timing: The spark might be there, but it's not happening at the right time. The timing chain could be stretched, the distributor could be out of adjustment, or the timing mechanism itself could be faulty. A timing light is needed to check this.
* Compression: Low compression in one or more cylinders prevents the spark from igniting the air/fuel mixture effectively. A compression test is crucial here. Worn piston rings, valve problems, or a blown head gasket are possibilities.
* Fuel Delivery Issues (despite seeming okay): While you say you're getting gas, the issue might be more nuanced:
* Fuel Pressure: The fuel pressure might be too low, even if fuel is reaching the engine. A fuel pressure gauge is needed to verify this.
* Fuel Injectors (if equipped): Clogged or faulty injectors could prevent proper fuel delivery, even with sufficient pressure. This would require more advanced diagnostics. (A 1986 model *might* still have a carburetor, but fuel injection was becoming more common then).
* Fuel Pump: A weak fuel pump might provide insufficient pressure, especially under cranking load.
* Crankshaft Position Sensor (CKP Sensor): If your truck has electronic ignition, this sensor tells the computer the crankshaft's position, allowing for accurate timing. A faulty CKP sensor would prevent starting.
* Computer/ECM Issues (if fuel injection): The engine control module (ECM) is the "brain" of a fuel-injected engine. If it's malfunctioning, it could prevent starting despite other components working.
Troubleshooting Steps:
1. Compression Test: This is the most important first step. Low compression points to a major internal engine problem.
2. Fuel Pressure Test: Check the fuel pressure to rule out low pressure.
3. Timing Check: Use a timing light to verify the ignition timing.
4. Spark Quality: While you say you have spark, ensure it's a strong, consistent spark at each plug. A weak spark might appear present but be insufficient for ignition.
5. Visual Inspection: Carefully inspect all components related to ignition, fuel delivery, and the intake system. Look for loose wires, vacuum leaks, damaged components, etc.
6. Consider a Professional: If you're not comfortable performing these tests, it's best to take the vehicle to a qualified mechanic. Improper diagnostics can lead to unnecessary repairs and expense.
Addressing these possibilities systematically will help pinpoint the exact cause of the starting problem. Remember safety first – work in a well-ventilated area, disconnect the battery negative terminal when working on electrical components, and use appropriate safety equipment.
