C4 Corvette Start Sequence
C4 Corvette Start Sequence for L-98 Engines
When starting an L-98 Corvette engine, a sequence of events take place that precedes the engine running. Knowing these sequences will help you troubleshoot no start issues with your C4 Corvette.
- Fuel Rail Pressurization
- C4 Corvette Start Sequence begins when you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Schrader valve on the passenger side fuel rail near the rear of the engine and proper fuel pressure after the pump runs should be between 40-42 pounds of pressure. The reading will drop to 38-40 pounds after the engine is running.
- Initial C4 Corvette Start Action: Once rotating the key to the start position if the anti-theft system has not disabled the starter the engine will rotate.
- Once the oil pressure has reaches 4 PSI, the oil pressure switch contacts will close allowing the fuel pump to run. Note that you should have a black color oil pressure switch/sender. This sender is mounted behind the distributor on the driver’s side. If its not replace it with one that's only black. If the car fires then stalls replace this switch.
- The distributor outputs rotational frequencies to the ECM (Engine Control Module) as a result of the engine being rotated by the starter. This frequency continues as long as the engine turns (both starting and running) if no frequency output is present the engine will not run.
- ECM Outputs : Once the ECM sees oil pressure greater than 4 PSI and the reference frequency from the distributor the ECM will command the injector drivers which will begin pulsing the injectors open for 4 ms (milliseconds) periods. (The L98 injectors all fire on one side of the engine at the same time followed by all injectors on the other side firing at the same time. The LT-1 fires the injectors individually per the firing order).
- Brainiac Fact for C4 Corvette Start Sequence! The ECM also pulls the fuel pump relay which makes the contacts and parallel's it electrically with the oil pressure switch. This way if the fuel pump relay fails you can still normally get the car to run. This will only happen if you make at least 4 PSI oil pressure. This “limp home mode” feature was put in place to allow for a fuel pump relay failure.
- The ECM requires an input from the TPS (Throttle Position Sensor which is mounted on the throttle body assembly) required voltage is .54 volts DC. Greater than 0.54 volts the ECM will assume the engine is flooded. This logic is triggered by the driver when pressing the accelerator pedal to the floor. The flooded condition would clear as a result of restricted fuel flow. The output of . 54 volts during start and idle from the TPS is very important in both starting and run performance. Its important to have a good voltmeter to troubleshoot something with such low voltage. Poor quality voltmeters can be inaccurate up to .2 volts.
- Assuming the ignition module has spark of sufficient intensity to ignite the air-fuel mixture the engine will start.
- C4 Corvette Start Sequence of Engine "Initial Start" for 1985 through 1989 L-98: When the engine initially starts, the MAF sends a signal to the ECM reporting that air is flowing at "XXXX" hertz frequency through it and into the intake manifold. The ECM refers to its Airflow Table for ECT conditions of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms this supplies a proper air/fuel mixture for combustion. For information on the 1990 and 1991 L-98 see below.
- Note 1990 and 1991 L-98 : The MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.
- GM used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system calculates air flow indirectly. Errors and vacuum leaks can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.
- Initial engine idle speed of should be around 900-1100 RPM and then slowly decay to around 600-700 RPM unless the air conditioner is on at which case it will run at around 800-1000 RPM depending on pressure altitude. If this does not happen, the Idle Air Mixture Valve or IAC may be plugged or faulty. You could also have an intake manifold leak or another vacuum leak. Check for vacuum leaks.
- ECM Open Loop Mode: In "Open Loop" mode the ECM is controlling the air/fuel mixture by referencing generic values stored in the ECM Flash memory. These values do not benefit from from the O2 sensor monitoring and lean rich correcting factors.
- After the C4 Corvette Start Sequence is finished the Oxygen Sensor (mounted in the exhaust pipe) indirectly is considered to have reached operating temperature of several hundred degrees when the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) is 160 degrees. This would be a Boolean logic type of result. In simple terms X+X+X= Output "Y" The computer then switches to "closed loop" mode meaning the Oxygen sensor’s output is being referenced and being used along with the MAT and ECT outputs for the ECM to adjust the injector pulse widths (more “ms longer” or less “ms shorter”) to maintain a 14.7:1 air/fuel mixture.
C4 Corvette Start Problem Summary
If you have a no start condition or if your C4 Corvette starts and then dies, check the above items in sequence to see if all the events are occurring as stated.
Having the proper tools make this job much easier and are highly recommended for troubleshooting these sorts of problems.
Check the Corvette Forum for more help