Part 1 of 2
INJECTION SYSTEMAll engines used have a sequential Multi-Port Electronic Fuel Injection system. The MPI system is computer regulated and provides precise air/fuel ratios for all driving conditions. The Powertrain Control Module (PCM) operates the fuel injection system.
The PCM regulates:
- Ignition timing
- Air/fuel ratio
- Emission control devices
- Cooling fan
- Charging system
- Idle speed
- Vehicle speed control
Various sensors provide the inputs necessary for the PCM to correctly operate these systems. In addition to the sensors, various switches also provide inputs to the PCM.
The PCM can adapt its programming to meet changing operating conditions.
Fuel is injected into the intake port above the intake valve in precise metered amounts through electrically operated injectors. The PCM fires the injectors in a specific sequence. Under most operating conditions, the PCM maintains an air fuel ratio of 14.7 parts air to 1 part fuel by constantly adjusting injector pulse width. Injector pulse width is the length of time the injector is open.
The PCM adjusts injector pulse width by opening and closing the ground path to the injector. Engine RPM (speed) and manifold absolute pressure (air density) are the primary inputs that determine injector pulse width.
Modes Of Operation
As input signals to the PCM change, the PCM adjusts its response to output devices. For example, the PCM must calculate a different injector pulse width and ignition timing for idle than it does for Wide Open Throttle (WOT). There are several different modes of operation that determine how the PCM responds to the various input signals.
There are two different areas of operation, OPEN LOOP and CLOSED LOOP.
During OPEN LOOP modes the PCM receives input signals and responds according to preset PCM programming. Inputs from the upstream and downstream heated oxygen sensors are not monitored during OPEN LOOP modes, except for heated oxygen sensor diagnostics (they are checked for shorted conditions at all times).
During CLOSED LOOP modes the PCM monitors the inputs from the upstream and downstream heated oxygen sensors. The upstream heated oxygen sensor input tells the PCM if the calculated injector pulse width resulted in the ideal air-fuel ratio of 14.7 to one. By monitoring the exhaust oxygen content through the upstream heated oxygen sensor, the PCM can fine tune injector pulse width. Fine tuning injector pulse width allows the PCM to achieve optimum fuel economy combined with low emissions.
For the PCM to enter CLOSED LOOP operation, the following must occur:
1. Engine coolant temperature must be over 35 °F.
- If the coolant is over 35 °F the PCM will wait 44 seconds.
- If the coolant is over 50 °F the PCM will wait 38 seconds.
- If the coolant is over 167 °F the PCM will wait 11 seconds.
2. For other temperatures the PCM will interpolate the correct waiting time.
3. O2 sensor must read either greater than 0.745 volts or less than 0.1 volt.
4. The multi-port fuel injection systems has the following modes of operation:
- Ignition switch ON (Zero RPM)
- Engine start-up
- Engine warm-up
- Cruise
- Idle
- Acceleration
- Deceleration
- Wide Open Throttle
- Ignition switch OFF
5. The engine start-up (crank), engine warm-up, deceleration with fuel shutoff and wide open throttle modes are OPEN LOOP modes. Under most operating conditions, the acceleration, deceleration (with A/C ON), idle and cruise modes, with the engine at operating temperature are CLOSED LOOP modes.
Ignition Switch ON (Zero Rpm) Mode
When the ignition switch activates the fuel injection system, the following actions occur:
- The PCM monitors the engine coolant temperature sensor and throttle position sensor input. The PCM determines basic fuel injector pulse width from this input.
- The PCM determines atmospheric air pressure from the MAP sensor input to modify injector pulse width.
When the key is in the ON position and the engine is not running (zero rpm), the Auto Shutdown (ASD) and fuel pump relays de-energize after approximately 1 second. Therefore, battery voltage is not supplied to the fuel pump, ignition coil, fuel injectors and heated oxygen sensors.
Engine Start-up Mode
This is an OPEN LOOP mode. If the vehicle is in park or neutral (automatic transaxles) or the clutch pedal is depressed (manual transaxles) the ignition switch energizes the starter relay. The following actions occur when the starter motor is engaged.
If the PCM receives the camshaft position sensor and crankshaft position sensor signals, it energizes the Auto Shutdown (ASD) relay and fuel pump relay. If the PCM does not receive both signals within approximately one second, it will not energize the ASD relay and fuel pump relay. The ASD and fuel pump relays supply battery voltage to the fuel pump, fuel injectors, ignition coil and heated oxygen sensors.
- The PCM energizes the injectors (on the 69 ° degree falling edge) for a calculated pulse width until it determines crankshaft position from the camshaft position sensor and crankshaft position sensor signals. The PCM determines crankshaft position within 1 engine revolution.
- After determining crankshaft position, the PCM begins energizing the injectors in sequence. It adjusts injector pulse width and controls injector synchronization by turning the individual ground paths to the injectors ON and OFF.
- When the engine idles within ±64 RPM of its target RPM, the PCM compares current MAP sensor value with the atmospheric pressure value received during the Ignition Switch ON (zero RPM) mode.
Once the ASD and fuel pump relays have been energized, the PCM determines injector pulse width based on the following:
- Battery voltage
- Engine coolant temperature
- Engine RPM
- Inlet/Intake air temperature (IAT)
- MAP
- Throttle position
- The number of engine revolutions since cranking was initiated
During Start-up the PCM maintains ignition timing at 9 ° BTDC.
Engine Warm-up Mode
This is an OPEN LOOP mode. The following inputs are received by the PCM:
- Engine coolant temperature
- Manifold Absolute Pressure (MAP)
- Inlet/Intake air temperature (IAT)
- Crankshaft position (engine speed)
- Camshaft position
- Knock sensor
- Throttle position
- A/C switch
- Battery voltage
- Vehicle speed
- Speed control
- O2 sensors
The PCM adjusts injector pulse width and controls injector synchronization by turning the individual ground paths to the injectors ON and OFF.
Cruise Or Idle Mode
When the engine is at operating temperature this is a CLOSED LOOP mode. During cruising or idle the following inputs are received by the PCM:
- Inlet/intake air temperature
- Engine coolant temperature
- Manifold absolute pressure
- Crankshaft position (engine speed)
- Camshaft position
- Knock sensor
- Throttle position
- Exhaust gas oxygen content
- A/C control positions
- Battery voltage
- Vehicle speed
The PCM adjusts injector pulse width and controls injector synchronization by turning the individual ground paths to the injectors ON and OFF.
The PCM adjusts engine idle speed and ignition timing. The PCM adjusts the air/fuel ratio according to the oxygen content in the exhaust gas (measured by the upstream and downstream heated oxygen sensor).
The PCM monitors for engine misfire. During active misfire and depending on the severity, the PCM either continuously illuminates or flashes the malfunction indicator lamp (Check Engine light on instrument panel). Also, the PCM stores an engine misfire DTC in memory.
The PCM performs several diagnostic routines. They include:
- Oxygen sensor monitor
- Downstream heated oxygen sensor diagnostics during open loop operation (except for shorted)
- Fuel system monitor
- EGR monitor
- Purge system monitor
- All inputs monitored for proper voltage range.
- All monitored components (refer to the Emission for On-Board Diagnostics).
The PCM compares the upstream and downstream heated oxygen sensor inputs to measure catalytic convertor efficiency. If the catalyst efficiency drops below the minimum acceptable percentage, the PCM stores a diagnostic trouble code in memory.
During certain idle conditions, the PCM may enter a variable idle speed strategy. During variable idle speed strategy the PCM adjusts engine speed based on the following inputs.
- A/C sense
- Battery voltage
- Battery temperature
- Engine coolant temperature
- Engine run time
- Inlet/intake air temperature
- Vehicle mileage
Acceleration Mode
This is a CLOSED LOOP mode. The PCM recognizes an abrupt increase in Throttle Position sensor output voltage or MAP sensor output voltage as a demand for increased engine output and vehicle acceleration. The PCM increases injector pulse width in response to increased fuel demand.
Deceleration Mode
This is a CLOSED LOOP mode. During deceleration the following inputs are received by the PCM:
- A/C sense
- Battery voltage
- Inlet/intake air temperature
- Engine coolant temperature
- Crankshaft position (engine speed)
- Exhaust gas oxygen content (upstream heated oxygen sensor)
- Knock sensor
- Manifold absolute pressure
- Throttle position
- IAC motor control changes in response to MAP sensor feedback
The PCM may receive a closed throttle input from the Throttle Position Sensor (TPS) when it senses an abrupt decrease in manifold pressure. This indicates a hard deceleration. In response, the PCM may momentarily turn OFF the injectors. This helps improve fuel economy, emissions and engine braking.
Wide-open-throttle Mode
This is an OPEN LOOP mode. During wide-open- throttle operation, the following inputs are used by the PCM:
- Inlet/intake air temperature
- Engine coolant temperature
- Engine speed
- Knock sensor
- Manifold absolute pressure
- Throttle position
When the PCM senses a wide-open-throttle condition through the Throttle Position Sensor (TPS) it deenergizes the A/C compressor clutch relay. This disables the air conditioning system.
The PCM does not monitor the heated oxygen sensor inputs during wide-open-throttle operation except for downstream heated oxygen sensor and both shorted diagnostics. The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.
Ignition Switch OFF Mode
When the operator turns the ignition switch to the OFF position, the following occurs:
- All outputs are turned off; unless O2 Heater Monitor test is being run. Refer to the Emission for On-Board Diagnostics.
- No inputs are monitored except for the heated oxygen sensors. The PCM monitors the heating elements in the oxygen sensors and then shuts down.
Typical Adaptive Memory Fuel Cells:
FUEL CORRECTION or ADAPTIVE MEMORIES
In Open Loop, the PCM changes pulse width without feedback from the O2 Sensors. Once the engine warms up to approximately 30 to 35 °F, the PCM goes into closed loop Short Term Correction and utilizes feedback from the O2 Sensors. Closed loop Long Term Adaptive Memory is maintained above 170 ° to 190 °F unless the PCM senses wide open throttle. At that time the PCM returns to Open Loop operation.
Short Term
The first fuel correction program that begins functioning is the short term fuel correction. This system corrects fuel delivery in direct proportion to the readings from the Upstream O2 Sensor.
The PCM monitors the air/fuel ratio by using the input voltage from the O2 Sensor. When the voltage reaches its preset high or low limit, the PCM begins to add or remove fuel until the sensor reaches its switch point. The short term corrections then begin.
The PCM makes a series of quick changes in the injector pulse-width until the O2 Sensor reaches its opposite preset limit or switch point. The process then repeats itself in the opposite direction.
Short term fuel correction will keep increasing or decreasing injector pulse-width based upon the upstream O2 Sensor input. The maximum range of authority for short term memory is 25% (±) of base pulse-width.
Long Term
The second fuel correction program is the long term adaptive memory. In order to maintain correct emission throughout all operating ranges of the engine, a cell structure based on engine rpm and load (MAP) is used.
The number of cells varies upon the driving conditions. Two cells are used only during idle, based upon TPS and Park/Neutral switch inputs. There may be two other cells used for deceleration, based on TPS, engine rpm, and vehicle speed. The other twelve cells represent a manifold pressure and an rpm range. Six of the cells are high rpm and the other six are low rpm. Each of these cells is a specific MAP voltage range
As the engine enters one of these cells the PCM looks at the amount of short term correction being used. Because the goal is to keep short term at 0 (O2 Sensor switching at 0.5 volt), long term will update in the same direction as short term correction was moving to bring the short term back to 0. Once short term is back at 0, this long term correction factor is stored in memory.
The values stored in long term adaptive memory are used for all operating conditions, including open loop. However, the updating of the long term memory occurs after the engine has exceeded approximately 17 °F, with fuel control in closed loop and two minutes of engine run time. This is done to prevent any transitional temperature or start-up compensations from corrupting long term fuel correction.
Long term adaptive memory can change the pulse-width by as much as 25%, which means it can correct for all of short term. It is possible to have a problem that would drive long term to 25% and short term to another 25% for a total change of 50% away from base pulse-width calculation.
Fuel Correction Diagnostics
There are two fuel correction diagnostic routines:
- Fuel System Rich
- Fuel System Lean
A DTC is set and the MIL is illuminated if the PCM detects either of these conditions.
PROGRAMMABLE COMMUNICATIONS INTERFACE (PCI) BUS
The Programmable Communication Interface Multiplex system (PCI Bus) consist of a single wire. The Body Control Module (BCM) acts as a splice to connect each module and the Data Link Connector (DLC) together. Each module is wired in parallel to the data bus through its PCI chip set and uses its ground as the bus reference. The wiring is a minimum 20 gauge wire.
Various modules exchange information through a communications port called the PCI Bus. The Powertrain Control Module (PCM) transmits the Malfunction Indicator Lamp (Check Engine) ON/OFF signal and engine RPM on the PCI Bus. The PCM receives the Air Conditioning select input, transaxle gear position inputs over the PCI Bus. The PCM also receives the air conditioning evaporator temperature signal from the PCI Bus.
The following components access or send information on the PCI Bus.
- Instrument Panel
- Body Control Module
- Air Bag System Diagnostic Module
- Full ATC Display Head
- ABS Module
- Transmission Control Module
- Powertrain Control Module
- Overhead Travel Module
SYSTEM DIAGNOSIS
The PCM can test many of its own input and output circuits. If the PCM senses a fault in a major system, the PCM stores a Diagnostic Trouble Code (DTC) in memory.
For DTC information see On-Board Diagnostics.