Owner's Problem Description
The owner reports that their 1991 Chevy S-10 with a 4.3L engine starts normally when cold but fails to start when hot. They noted that fuel is not injecting into the throttle body and have attempted to bypass the fuel pump relay without success. The owner checked the ECT sensor resistance, which was within range. They are considering whether low oil pressure could be a contributing factor, as there is good oil pressure when the engine runs. To start the vehicle, they need to spray fuel into the throttle body while cranking. The owner is hesitant to replace the ICM without further diagnostics.
Professional Technical Analysis
The 1991 Chevy S-10 is equipped with a 4.3L V6 engine and experiences a no-start condition when hot, which is often linked to fuel delivery issues. The vehicle has an estimated mileage of over 150,000 miles, and environmental factors such as heat may exacerbate the problem. The symptoms include the engine starting normally when cold but failing to inject fuel into the throttle body when hot. The owner has bypassed the fuel pump relay, indicating a potential issue with the fuel pump operation, which requires at least 4 PSI of oil pressure to activate the oil pressure switch that powers the fuel pump. This situation suggests that if the oil pressure switch is faulty, it may not allow the fuel pump to operate correctly when the engine is hot, leading to stalling. Additionally, the ignition control module (ICM) can fail thermally, which is a common issue in this era of GM vehicles. The ICM should be tested for proper operation when the engine is at operating temperature. Warning lights may illuminate due to the no-start condition, and the ECM has been replaced, indicating prior troubleshooting efforts. Assessing the oil pressure switch using a manual gauge is necessary to confirm its functionality, as it could be preventing fuel delivery by not closing the circuit.
Possible Causes
Most common causes (ordered by frequency):
- Faulty Oil Pressure Switch: The oil pressure switch may not be closing due to internal failure, preventing the fuel pump from activating when the engine is hot. This switch requires a minimum of 4 PSI to close and provide power to the fuel pump. Testing the switch with a manual gauge can confirm its operational status. If the switch fails to close, it will lead to fuel delivery issues, especially when hot, resulting in a no-start condition. - Engine runs when fuel is sprayed into throttle body, indicating fuel delivery issue. Oil pressure should be checked to ensure it closes the switch.
- Thermal Failure of Ignition Control Module (ICM): The ICM may fail when hot, causing no spark to the injectors. This is a common issue in older GM models, where the ICM components can break down under heat stress. Testing ICM operation at elevated temperatures can reveal intermittent failure that would not occur when cold. - Check engine light may illuminate; no-start condition when hot. Normal operation when cold suggests intermittent failure. Possible stored codes related to ICM.
- Fuel Pump Relay or Circuit Issue: If the fuel pump relay or circuit has a fault, it may not supply power to the fuel pump when needed. Bypassing the relay indicates possible wiring or connection issues that need to be investigated. Continuity testing of the relay and circuit is essential to ensure that power reaches the pump. - Fuel pump does not prime when hot; continuity tests needed to verify relay and circuit integrity.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Scan: Start with a thorough scan using GeekOBD APP to retrieve any stored trouble codes and review freeze frame data. This will help identify if the ECM is detecting any issues related to the fuel system or ignition system.
- STEP 2 - Test Oil Pressure Switch: Using a manual gauge, check the oil pressure to ensure it achieves at least 4 PSI when the engine is running. If the pressure is sufficient but the switch does not close, the switch may need to be replaced. This step is crucial to confirm fuel pump activation.
- STEP 3 - ICM Thermal Testing: Monitor the ICM while the engine is running at operating temperature. If the vehicle fails to start when hot, consider testing the ICM for proper operation, as thermal failures in the module can lead to a no-spark condition.
- STEP 4 - Relay and Circuit Continuity Testing: Perform continuity tests on the fuel pump relay and associated wiring. Ensure that power is being delivered to the fuel pump when the ignition is in the ON position and the engine is cranking.
