1. Heat Soak Issues:
* IPM (Injection Pump Module): This is a frequent suspect. The IPM controls the injection pump and can overheat and fail, especially in older 6.5Ls. Once it cools down, it might restart temporarily. This is often characterized by the engine running fine until it reaches operating temperature, then shutting down.
* Fuel Pump: The lift pump might be weak, only able to supply sufficient fuel for a short period before failing under heat. The engine dies because it's starved of fuel.
* Fuel Filter: A clogged fuel filter restricts fuel flow. The restriction might be slight enough to allow initial running but insufficient once the engine reaches temperature and demands more fuel.
2. Electrical Problems (less likely given the initial run time):
* Wiring Harness: Look for damaged or corroded wiring, especially near the engine where it's exposed to heat and vibration. A failing connection could interrupt power to crucial components after the engine warms up.
* Sensors: Various sensors (like the crankshaft position sensor or coolant temperature sensor) might fail when they heat up. A bad reading can cause the engine to shut down.
3. Other Possibilities:
* Glow Plugs (less likely): While more of a cold start issue, a weak glow plug system might contribute to an inability to restart after the engine cools down slightly.
* Air in the Fuel System: Air in the fuel lines can disrupt fuel flow, although this usually causes erratic running rather than a clean shut down after a short period.
* Fuel Pressure Regulator: A failing fuel pressure regulator might not maintain adequate pressure once the engine heats up.
Troubleshooting Steps:
1. Check the obvious: Inspect fuel level, look for fuel leaks, and examine the air filter.
2. Listen Carefully: When attempting to restart, listen for the fuel pump priming. If you don't hear it, suspect a fuel pump or wiring issue.
3. Test the IPM: This requires some specialized tools, but a mechanic can diagnose IPM failure relatively easily. This is often the most likely cause.
4. Check Fuel Pressure: If you have a fuel pressure gauge, measure the fuel pressure at the injection pump. Low pressure indicates a problem with the fuel pump, filter, or regulator.
5. Inspect Wiring Harnesses: Carefully examine all wiring harnesses near the engine for any signs of damage, corrosion, or loose connections.
6. Test Sensors: This might require an OBD-II scanner capable of reading GM 6.5L codes.
Important Note: Diagnosing this problem requires a methodical approach. Since the engine runs initially, it's likely *not* a major mechanical failure within the engine itself. Focusing on fuel delivery and the IPM is the best starting point. It's highly recommended to consult a mechanic experienced with GM 6.5L diesel engines, as improper diagnosis and repair can lead to further damage.
