1. Fuel Delivery Problems (Most Likely):
* Fuel Pump Relay: Heat can cause relays to fail intermittently. When the engine warms up, the relay might be failing to power the fuel pump. Try tapping the relay with a screwdriver handle while the engine is warm and see if it restarts. If so, replace the relay.
* Fuel Pump: The fuel pump itself could be weakening with heat. It might be delivering enough fuel when cold but failing to keep up when the engine is hot and demanding more. A fuel pressure gauge is crucial here to measure pressure at various engine temperatures. Low pressure when hot is a strong indicator of a failing pump.
* Fuel Filter: A clogged fuel filter restricts fuel flow, and this restriction will be more noticeable when the engine is hot and demands more fuel. Replace it as a relatively inexpensive preventative measure.
* Throttle Position Sensor (TPS): A faulty TPS can cause erratic fuel delivery. A failing TPS often acts up when hot. This needs testing with a multimeter, following the specifications in your vehicle's repair manual.
* Mass Airflow Sensor (MAF): Similar to the TPS, a malfunctioning MAF sensor can lead to incorrect fuel mixture calculation, exacerbated by heat. Again, this requires testing with a multimeter.
* Fuel Injectors: While less likely to be the sole cause, sticking or weak injectors can cause problems, especially when hot. This is a more involved diagnosis requiring specialized tools or professional assessment.
2. Ignition System Problems:
* Ignition Coil: Heat can cause the coil to break down, leading to weak or no spark when hot. A visual inspection for cracks or signs of overheating is a good starting point. Testing the coil's output with a spark tester is recommended.
* Ignition Module (ECM): This controls the ignition timing and spark. Heat can cause failure. This is a more advanced diagnosis and often requires professional testing.
* Distributor Cap and Rotor (If applicable): Cracks or carbon buildup can cause poor spark distribution, particularly when things expand with heat. Inspect for wear and tear and replace if necessary. (Note: some 305s in 1989 might have electronic ignition without a distributor).
* Wires: Heat can degrade wires causing intermittent shorts or open circuits. Inspect for any signs of damage or wear.
Troubleshooting Steps:
1. Check for obvious things: Look for loose or corroded connections, especially around the fuel system and ignition components.
2. Listen to the fuel pump: When you turn the key to "on" (not start), you should hear the fuel pump briefly prime. If you don't hear it, there's a problem with the pump or relay.
3. Check the fuel pressure: This is the most important step for fuel-related issues. You'll need a fuel pressure gauge to connect to the fuel rail.
4. Check for spark: Use a spark tester or even a spare spark plug to check for spark at the distributor cap (if applicable) or coil wire while cranking.
5. Consult a repair manual: A Haynes or Chilton repair manual for your specific vehicle will have detailed diagrams, specifications, and troubleshooting guides.
Important Note: Working on fuel systems involves working with flammable materials. Always disconnect the battery negative terminal before starting any work. If you're not comfortable working with automotive electrical or fuel systems, it's best to take your van to a qualified mechanic. The full throttle start is a dangerous situation and should be addressed quickly.
