* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can cause the computer to enrich the fuel mixture (more fuel) only when the pedal is pressed. If it's failing, it might be sending incorrect signals, leading to insufficient fuel at idle. The rough running and incomplete turnover are likely due to a very lean fuel mixture (not enough fuel) when the pedal isn't pressed.
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the engine's crankshaft is in its rotation. A bad CKP sensor will cause erratic or no spark, leading to a rough start or no start at all. The symptoms are consistent with a problem triggering the ignition system. It needs a correct signal from the CKP sensor to ignite the fuel even when there is adequate fuel pressure.
* Fuel Pump Relay: This relay controls power to the fuel pump. It may be failing intermittently, only supplying power when the pedal is pressed and thus building fuel pressure. A clicking sound from the relay might indicate this issue.
* Fuel Pump: The fuel pump itself might be weak or failing. This would result in insufficient fuel pressure under normal starting conditions, but pressing the accelerator may momentarily increase pressure enough to start.
* Mass Airflow Sensor (MAF) Sensor: If it's reading incorrectly (likely a low reading) the computer may reduce the fuel injection pulse leading to very lean mixtures that make starting difficult. This however wouldn't directly explain why the pedal has to be pressed, although a very faulty MAF sensor could give unusual readings
* Ignition System Problems (Distributor, Coil, Wires, Spark Plugs): While the TPS/CKP/Fuel pump are most likely culprits because of the pedal interaction, don't entirely rule out weak ignition components. Weak spark will cause rough running and difficulty starting, but usually, it doesn't have the accelerator pedal relationship unless another sensor has failed and is masking the ignition problem
Troubleshooting Steps:
1. Check for Codes: Use a code reader to see if the car's onboard diagnostic system has stored any trouble codes. This will often point directly to the faulty sensor.
2. Fuel Pressure Test: This requires a fuel pressure gauge. Check the fuel pressure at the fuel rail both with the engine off and while cranking. Low pressure points to a fuel pump, relay, or regulator problem.
3. Inspect Fuel Pump Relay: Listen for clicking when the key is turned. A weak or faulty relay will make a faint clicking sound.
4. Test the TPS and CKP: These sensors can often be tested with a multimeter or a dedicated sensor tester. Refer to a repair manual for specific testing procedures.
5. Inspect Ignition Components: Visually inspect the distributor cap, rotor, spark plug wires, and spark plugs for wear or damage.
6. Check the MAF sensor: With the engine running, disconnect the MAF sensor and see if the engine runs differently. A faulty MAF sensor will usually cause a difference in idle or running state.
It's crucial to consult a repair manual specific to your 1991 Chevy Corsica. These manuals provide detailed diagrams and troubleshooting procedures. Working on fuel and ignition systems can be dangerous due to potential fire hazards, so if you're not comfortable performing these tests and repairs yourself, take it to a qualified mechanic.
