I continually see people saying that when the engine is in WOT/PE that the PCM is in OPEN LOOP,
Not true, here is the several modes PCM operates in :
PCM Modes of Operation
The engine operating modes controlled by the PCM are:
Starting
Fuel Cut-Off
Clear Flood
Selective Fuel Cut-Off
Run
Fuel Backup
Acceleration Enrichment
Battery Voltage Correction
Deceleration Enleanment
Starting Mode
When the ignition is first turned "ON," the PCM powers up and sends 12 volts to the fuel pump relay for 2 seconds. Simultaneously, the fuel pump runs to build up pressure in the fuel system. Unless the engine is cranked within this 2-second period, providing ignition reference signals to the PCM, the fuel pump relay is de-energized to shut "OFF" the pump.
Before engine cranking, the PCM is receiving readings for coolant temperature, intake air temperature, atmospheric pressure (MAP/BARO) and throttle position to determine the initial air/fuel ratio. During cranking, the PCM delivers one injector pulse for each rpm reference pulse. The lower the coolant temperature, the longer the pulse width (injector on-time) and the richer the air/fuel ratio becomes. As coolant temperature goes up, the pulse width becomes shorter and the air/fuel ratio becomes leaner. Cranking mode air/fuel ratios determined by the PCM may range from 1.5:1 at -36°C (-38°F) to 14.7:1 at 94°C (202°F).
— IMPORTANT —
Normal Starting Mode fuel delivery follows the description above as long as the throttle angle is 0%.
If the throttle is opened, even a little, the air/fuel ratio changes.
Clear Flood Mode
If the engine becomes flooded, the driver can depress the accelerator pedal to the 80% or greater open position to activate the Clear Flood Mode. To ensure that the 80% position is attained, most instructions simply require pressing the accelerator pedal to the floor (wide open throttle).
With the throttle wide open and cranking rpm below 600, the PCM issues injector pulses at a rate that would be equal to an air/fuel ratio of 20:1. In some applications, fuel is cut off completely, with no injector pulses from the PCM.
The Run Mode has two conditions: Open Loop and Closed Loop.
Open Loop
When the engine is first started, the system is in open loop. In open loop, the PCM does not use the oxygen sensor (O2S) signal. Instead, it calculates the air/fuel mixture ratio from the TP, ECT, MAP/MAF, IAT, and CKP sensors (Figure 2-20).
The system will stay in open loop until the following conditions are met:
The oxygen sensor output voltage is varying, meaning that it is hot enough (600°F or higher) to operate properly and is sending a valid signal to the PCM.
The engine coolant sensor is above a specified temperature, which varies with individual engine application.
A specific amount of time has elapsed after start-up, which also varies with engine application. The specific values for these conditions are stored in the PCM’s Memory Calibration, which is calibrated to individual vehicle specifications.
Closed Loop
When O2S signal, coolant temperature sensor and time conditions are met, the system goes into closed loop. Closed loop means that the PCM corrects the air/fuel mixture ratio based on varying voltage signals from the O2S. An O2S signal below 450 mV causes the PCM to increase injector pulse width. When the O2S signal rises above 450 mV in response to the richer mixture, the PCM reduces injector pulse width. It is important to note that closed loop control still relies on all of its other inputs, in addition to the O2 sensor.
By constantly sensing the oxygen content of the exhaust, the PCM can maintain the air/fuel ratio at very close to the ideal 14.7:1 (stoichiometric) ratio. This is the point where the catalytic converter is most efficient.
Semi-Closed Loop: To improve fuel economy on some models, a sub-mode of closed loop is used. This sub-mode, called semi-closed loop, occurs during highway driving and light engine load. The PCM corrects the fuel leaner than 14.7:1 while continuing to monitor the oxygen sensor.
Converter Protection Mode: The PCM constantly monitors the engine operation through inputs such as the oxygen sensor, and estimates conditions that could cause the catalytic converter to reach excessive temperatures. If the PCM determines that a converter overheat condition may occur, the system returns to open loop, and enriches the fuel mixture which, in turn, cools the converter.
Acceleration Enrichment Mode
When the throttle is opened rapidly, or the vehicle is accelerated, the opening of the throttle causes a simultaneous increase in manifold absolute pressure (MAP) and throttle angle. The amount of fuel must be increased to compensate for the extra air. In response to the changes in TP and MAP signals, the PCM supplies longer injector pulses and additional pulses timed between the base synchronous pulses. Because the extra pulses are not synchronized to crankshaft position, they are called asynchronous pulses. This provides the extra fuel needed to prevent the engine from stumbling due to a mixture that is too lean.
Deceleration Enleanment
When the engine is required to decelerate, a leaner air/fuel mixture is required to reduce the emission of hydrocarbons (HC) and carbon monoxide (CO) and prevent deceleration backfire. To adjust injector on-time, the PCM uses the decrease in MAP and the decrease in throttle position to calculate a decrease in pulse width.
Deceleration may be either partial or full. That is, the driver may have simply backed off the throttle, or may have allowed the throttle valve to return all the way to the idle position. It is important to make this distinction because the PCM reacts differently to each of these conditions.
Under light-throttle deceleration, the PCM reduces fuel flow by shortening injector on-time (deceleration enleanment).
Under closed throttle deceleration, the PCM senses that the driver intends the engine to return to idle speed. Fuel delivery may be cut off entirely (deceleration fuel cut-off). As desired idle speed is approached, fuel delivery and idle air control come into play to maintain the desired idle.
Fuel Cut-Off Mode
One purpose of fuel cut-off is to remove fuel from the engine during extreme deceleration conditions.
The PCM may also be programmed to shut off fuel flow for safety reasons when the vehicle reaches a predetermined maximum speed. Fuel flow is also shut off on some engines if rpm reaches a predetermined maximum. These values, maximum mph and maximum rpm, vary from vehicle to vehicle.
Fuel cut-off also occurs when the ignition is turned "OFF." Without operating voltage and without ignition reference pulses, the PCM does not trigger the injectors, and no fuel is delivered. This prevents dieseling or run-on.
Selective Fuel Cut-Off
Selective fuel cut-off is used on some applications for engine torque management and engine protection. On these applications, the PCM can disable up to four injectors (on a V8 application) under the following conditions:
Torque management enabled - used to reduce torque during transmission/transaxle shifts.
Traction control enabled - used to reduce torque in conjunction with the brakes applying.
Low coolant condition - protects the engine from overheating. When no fuel is delivered to selected cylinders, less heat is generated which reduces coolant temperatures.
Fuel Backup Mode (Some Applications)
In this mode, the computer runs on the internal calibrations that allows it to run the engine with only rpm, throttle position, and coolant temperature inputs to change fuel and timing calculations. Backup is used only when the computer cannot operate normally and is sometimes referred to as the limp-home mode. The computer goes into Fuel Backup Mode if any one, or a combination of the following conditions, exists:
Computer voltage is below 9 volts.
Cranking voltage is below 9 volts.
PROM is missing or not functioning.
Circuits fails to issue Computer Operation Pulses (COP).
Battery Voltage Correction
Battery voltage correction compensates for variations in battery voltage to the fuel pump and injectors. The PCM modifies the pulse width by a programmed-in correction factor. When battery voltage drops, the fuel pump slows and fuel volume decreases. To compensate, the PCM increases injector pulse width. Battery voltage correction can take place in any operating mode, if required.
The PCM may also compensate for low battery voltage by increasing the idle speed. On Electronic Ignition (EI) systems, the PCM compensates for a low primary ignition current by increasing dwell (coil saturation) time. This ensures adequate spark.
