* Fuel delivery issues: Heat can affect fuel pump performance. The lift pump might be weakening with age and heat, failing to provide sufficient fuel pressure to the injection pump. The injection pump itself can also be affected by heat, leading to poor fuel metering. This is a very likely suspect.
* Glow plug relay/glow plug issues (even though it's warm): While glow plugs are primarily for cold starts, a malfunctioning glow plug relay or a weak glow plug can cause starting problems even when the engine is warm. The increased heat from a warm engine might exacerbate an existing weakness, preventing the engine from firing. This is less likely but possible.
* Ignition system problems (though less common in diesels): While diesel engines don't use spark plugs, the ignition system (specifically the components related to the engine's electronics or the starting motor itself) could be affected by heat. Worn wires or a failing component could overheat and malfunction when the engine is hot. This is less likely than the fuel system issues.
* Low compression: While less likely to be directly caused *by* heat, low compression in one or more cylinders can make it harder to start a diesel engine, especially when warm. The heat might further hinder the already weakened compression.
* Intake air leaks: A leak in the intake manifold or related components can cause the engine to run poorly and be harder to start, especially when hot. This is less probable but worth considering.
* Water in the fuel: Water in the fuel can cause problems that are exacerbated by heat, leading to poor fuel delivery.
Troubleshooting Steps:
1. Check the fuel system: This is the most likely area. Start by checking the fuel filter – is it clogged? Inspect the fuel lines for leaks or cracks. Listen carefully for the lift pump – is it working strongly? A pressure gauge is ideal to measure fuel pressure. If possible, have a mechanic test the injection pump; they are complex and often need specialized tools.
2. Test the glow plug relay and glow plugs: Even though it's a warm-engine problem, check to see if there are any signs of failure here. A multimeter can help test the relay and glow plugs.
3. Check for intake air leaks: Carefully inspect all hoses and connections related to the intake system.
4. Compression test: While not a primary suspect, a compression test will reveal if low compression is a contributing factor.
Recommendation: Given the age and potential complexity of the issue, I strongly recommend taking your truck to a mechanic specializing in diesel engines. The 6.9L IDI is not a simple engine to diagnose, and incorrect procedures could exacerbate the problem. They have the tools and experience to accurately diagnose and fix the problem efficiently and safely.
