* Crank Position Sensor (CPS): This is a very common cause of no-start issues, especially when the engine gets hot. The CPS tells the computer the engine's rotational position, and if it fails, the computer can't properly time the fuel injection and ignition. Heat can exacerbate a failing CPS.
* Cam Position Sensor (CMP): Similar to the CPS, a failing CMP will prevent proper ignition timing. It too is susceptible to heat-related failures.
* Ignition Coil(s): Heat can cause an ignition coil to fail or weaken, leading to misfires or no spark. This is more likely to result in a rough running condition *before* complete failure, but complete failure is possible, especially after an hour of running.
* Fuel Pump Relay or Fuel Pump: While less likely to cause a sudden failure *only* after running, a failing fuel pump may struggle once it heats up. The fuel pump relay could also be failing due to heat.
* Mass Airflow Sensor (MAF): While unlikely to cause a complete no-start, a malfunctioning MAF sensor can lead to poor fuel mixture and rough running, potentially culminating in a no-start condition, especially under heat stress.
* Distributor (if equipped): While less common on 1999 Tahoes, a faulty distributor cap, rotor, or ignition module could also cause this problem. Again, heat is a significant factor in their potential failure.
* Wiring Issues: Heat can damage or cause loose connections in wiring harnesses. This can affect various components, leading to intermittent starting problems. Look for any signs of melted insulation or corrosion.
* Engine Control Module (ECM): While less common, an ECM failure is possible, though less likely to be triggered solely by heat unless there's a related heat-sink issue.
Troubleshooting Steps:
1. Check for Spark: Use an inline spark tester on a spark plug wire to see if you're getting spark. No spark points to ignition-related issues (coil, CPS, CMP, distributor if applicable).
2. Check for Fuel Pressure: You'll need a fuel pressure gauge to check if the fuel pump is delivering sufficient pressure. Low pressure points to the pump, relay, or fuel filter.
3. Listen for the fuel pump: When you turn the key to the "on" position (not starting), you should hear the fuel pump prime for a few seconds. If you don't hear it, suspect the pump or its relay.
4. Check engine codes: Use an OBD-II scanner to retrieve any diagnostic trouble codes (DTCs) stored in the ECM. This can provide valuable clues.
5. Inspect wiring: Look for any signs of damage, corrosion, or loose connections in the engine bay, particularly around heat-sensitive components.
Given the sudden nature of the problem after the engine has run, focus on heat-sensitive components first (CPS, CMP, coils, fuel pump). A qualified mechanic with experience in older vehicles is likely the best resource for accurate diagnosis and repair.
