1. Heat-Related Issues:
* Fuel Pump Relay: Heat can cause the fuel pump relay to fail intermittently. When the engine is idling for a prolonged period, the relay might overheat and cut power to the fuel pump. This is especially likely if the relay is in a poorly ventilated location.
* Fuel Pump: An aging fuel pump might struggle to maintain sufficient pressure at low RPM. Heat can exacerbate this, leading to fuel starvation.
* Distributor Issues (if using a distributor): While less common in 91's (many had opti-sparks), if still using a distributor, the points or electronic ignition components might be failing when hot.
* Throttle Position Sensor (TPS): A failing TPS can provide inaccurate readings to the computer, leading to incorrect fuel mixture and potential stalling, especially at low idle. Heat can worsen a failing TPS.
2. Fuel Delivery Problems:
* Clogged Fuel Filter: A clogged filter restricts fuel flow, more noticeable at low demand (idle).
* Fuel Pressure Regulator: A faulty regulator could cause erratic fuel pressure, especially under low-load conditions.
* Fuel Lines: Kinks or restrictions in the fuel lines can reduce fuel delivery.
3. Ignition System Problems:
* Ignition Coil: An aging coil might struggle to generate sufficient spark at low RPM when the engine is hot.
* Crankshaft Position Sensor (CKP) or Camshaft Position Sensor (CMP) (if applicable): These sensors are critical for ignition timing. A failing sensor can cause intermittent stalling, particularly at low speeds.
* Wiring Issues: Poor connections or damaged wiring in the ignition system can cause intermittent failures.
4. Other Possibilities:
* IAC (Idle Air Control) Valve: A dirty or malfunctioning IAC valve might not be able to maintain a stable idle, leading to stalling.
* Vacuum Leaks: A vacuum leak can disrupt the engine's idle, causing it to stall. Heat can sometimes make a small leak worse.
* Alternator: A failing alternator might not be charging the battery adequately, leading to a voltage drop that causes the engine to stall. However, this is less likely since it restarts easily.
Troubleshooting Steps:
1. Check the obvious: Inspect all connections, vacuum lines, and fuel lines for damage or leaks.
2. Check for codes: Use an OBD-II scanner (or appropriate scanner for your car's system) to check for diagnostic trouble codes (DTCs).
3. Test the fuel pressure: Measure the fuel pressure at the fuel rail. It should be within the manufacturer's specifications.
4. Inspect the fuel pump relay: Check for overheating or signs of damage.
5. Test the fuel pump: Listen for it to prime when the ignition is turned on.
6. Check the ignition system: Inspect the coil, wires, distributor (if applicable), and sensors for damage or wear.
7. Clean the IAC valve: This is a relatively easy and inexpensive thing to try.
Because the problem is intermittent and heat-related, focus your attention on the heat-sensitive components listed above. A professional mechanic with experience in classic muscle cars could diagnose this effectively with diagnostic tools and a systematic approach. Don't randomly replace parts; systematic diagnosis is key to saving money and fixing the problem correctly.
