Fuel System Monitor

BACKGROUND
To control the level of undesirable emissions, the fuel system must be able to maintain strict control of the air/fuel ratio. The stoichiometric air/fuel ratio (14.7:1) is optimum. At that ratio, the best balance between the production of HC's and CO's (which drop as the mixture becomes leaner) and NOx (which increases as the air/fuel mix becomes leaner) can be found. This is also the point where the catalytic converter is most efficient at converting all three gases to less harmful compounds. The goal of the Powertrain Control Module (PCM) is to examine input information and control outputs to produce a constant stoichiometric ratio.

OPERATION
The PCM varies the pulse width of the fuel injectors to provide precise control of the air/fuel mixture. Wider pulse widths increase the volume of fuel delivered to the cylinders. The PCM uses the input from a number of sensors in its attempt to reach and maintain this air/fuel ratio. Manifold Absolute Pressure (MAP) and the O2 sensor have the greatest influence (authority) over injector pulse width. Other inputs such as the Throttle Position Sensor (TPS), Engine RPM, Engine Coolant Temperature (ECT) Sensor, Intake Air Temperature (IAT) Sensor, Vehicle Speed Sensor (VSS), and battery voltage all have varying levels of influence on pulse width, depending on the circumstances.





As noted earlier, this system uses two oxygen sensors, both of which monitor the oxygen content of the combustion by-products on their way out of the engine as exhaust. Only the upstream sensor has authority over fuel injector pulse width.

If a large amount of oxygen remains following the combustion process, the upstream O2 sensor produces a low voltage. This indicates a lean condition caused by an air/fuel ratio greater than stoichiometric. Little oxygen in the exhaust allows the sensor to produce a higher voltage, indicating a rich condition where the air/fuel ratio is less than stoichiometric

Upstream O2 sensor feedback to the PCM is used to fine tune injector pulse width to maintain stoichiometry and meet emission standards. It can increase or decrease injector pulse width by as much as 50%. Pulse width is calculated from the data supplied from the MAP, TPS, ECT, IAT, upstream O2 sensor, battery voltage, RPM, and VSS.

To control air/fuel ratio feedback, the PCM uses short term correction and long term memory. Before the PCM can alter the programmed injector pulse width, it must enter closed loop operation. The requirements for closed loop operation are listed below:

^ Engine temperature exceeds 35° F
^ O2 sensor is in the ready mode
^ All timers have timed out following the Start to run transfer. (The length of these timers varies with engine temperature)

35° F - 41 sec.
50° F - 36 sec.
70° F - 19 sec.
167° F - 11 sec.

Once in closed loop control, the feedback systems begin to operate. Short term correction works with long term correction, which is broken down into 14 different cells. Two of these cells (12 and 13) are used only during idle. Each cell represents a manifold pressure and RPM range, and can be accessed with the DRB III diagnostic scan tool.

Purge free cells are cells that parallel purge normal cells.





On a Talon with an automatic transaxle, the purge free cells are as follows:

1. Idle purge free cell = Cell 12
2. Purge free cell 2 = Cell 4
3. Purge free cell 3 = Cell 3






The purge free cells for the Talon with a manual transaxle are as follows:

1. Idle purge free cell = Cell 13
2. Purge free cell 2 = Cell 4
3. Purge free cell 3 = Cell 6

There are a total of 17 long term memory cells. The purge free cells parallel the same structure as the purge normal cells. Purge is allowed to function normally in the purge normal cells, where as in the purge free cells, information is not corrupted by the evaporative canister.

EXAMPLE:

In order for the PCM to update cell 3, the MAP sensor must be indicating that the voltage is between 1.38 and 2.0 volts, and engine RPM is greater than 2048.





If, during vehicle operation, the oxygen sensor registers a rich or lean condition in this cell, the cell will require updating to aid in fuel control. The short term correction Is used first. It starts increasing pulse width quickly (kick), then ramps up slowly. Each control is in inverse relation to the signal sent from the O2 sensor.

EXAMPLE:

The O2 sensor switches lean to rich. Short term compensation kicks-in lean, then ramps lean until the O2 sensor switches lean. At this point short term compensation reverses the process.

If the oxygen sensor shows lean, the short term compensation goes rich and multiplies the pulse width from long term memory in that cell by an amount greater than 1. If the sensor shows rich, the short term compensation drives the pulse width narrower by multiplying by a number less than 1 (perhaps 0.97). The short term compensation can multiply pulse width by as much as 1.25 or as little as 0.75 to compensate for lean or rich conditions. In that way, the short term compensation can increase pulse width by up to 25% (by multiplying by 1.25), or decrease pulse width by up to 25% (by multiplying by 0.75).

EXAMPLE:

Pulse width 0.05 x 1.25 = 0.0625 (an increase of 25%)
Pulse width 0.05 x 0.75 = 0.0375 (a decrease of 25%)

Long term memory also has control over pulse width by being able to increase or decrease the pulse width stored in the cell by up to 25%. Long term memory is retained by the battery in the PCM, while short term correction is lost whenever the ignition is turned off.





The long term memory works to bring the short term correction to the point where the average percent of pulse width compensation it provides in this memory cell is 0%. The long term memory returns to this level of pulse width compensation the next time the PCM enters this cell. It is in this way that the PCM is continually relearning the most appropriate level of control, even as the vehicle ages, internal engine components wear, and operating conditions change.

There are several "purge free" cells. These cells contain information on how much effect the canister has on the air/fuel ratio. The purge solenoid is turned off to shut off the purge flow, and the cell is allowed to register any purge corruption. The monitor looks at the combination of short and long term fuel control values to see if the system is in control.

ENABLING CONDITIONS
The following conditions must be met before the fuel system monitor will run:

^ Closed loop operation

PENDING
The fuel system monitor does not run if the MIL lamp is illuminated due to one of the following conditions:

^ Misfire DTC
^ Purge monitor DTC
^ Upstream O2 sensor heater DTC
^ EGR monitor DTC
^ Vehicle is in limp-in mode due to MAP, TPS, or engine temperature DTC
^ Camshaft/crankshaft sensor failures
^ EGR solenoid DTC
^ Purge solenoid DTC
^ Upstream O2 sensor rationality DTC

CONFLICT
The fuel system monitor does not run if any of the following are present:

^ One trip misfire maturing code
^ One trip purge monitor maturing code
^ One trip upstream O2 sensor heater maturing code One trip EGR monitor maturing code

SUSPEND
There are no suspend conditions for the fuel system monitor. However, the monitor may be disabled if the fuel level reaches 0.60 gallon or less.

Talons equipped with automatic transaxies (OBD I diagnostics), continuously monitor the fuel system once in closed loop. The test fails if the fuel control system reduces injection pulse width by 25% from long term memory and 25% from short term compensation due to a rich condition, or increase injection pulse width by 25% from long term memory and 25% from short term compensation due to a lean condition. Furthermore, both the rich or lean condition must remain at these values for a total of eight minutes while in closed loop.

For all Eagle Talons equipped with manual transaxles (OBD II diagnostics), the fuel system is continuously monitored during each trip once the enabling conditions have been met. The test fails if the fuel control system reduces pulse width by 25% long term memory and 7% short term compensation due to a rich condition, or increases pulse width by 25% long term memory and 12% short term compensation due to a lean condition. The MIL illuminates and a DTC is set when the adaptive fuel system exceeds these limits for two consecutive trips. The message on the scan tool will appear as follows:

Fuel System Rich
Flash Code - 52 J 2012 Code - P 0172

Fuel System Lean
Flash Code - 51 J 2012 Code - P 0171

When the PCM stores a maturing code, it also stores a freeze frame of the vehicle operating data at the time the fault was set. This information is accessible with the DRB III diagnostic scan tool. The MIL extinguishes if the malfunction causing it to illuminate is not repeated for three consecutive trips monitored within 10% of the load value and 375 RPM of the original freeze-frame conditions.

Malfunction of the fuel monitor can possibly be caused by problems with any of the following components:

^ Catalytic converter
^ Intake air temperature sensor
^ Ignition coil
^ EGR valve assembly
^ PCM
^ Valves (worn)
^ Piston rings (worn)
^ Head gasket
^ Head (cracked)
^ Exhaust manifold
^ Fuel pump
^ Fuel filter
^ Ignition secondary wires
^ Injectors
^ Map sensor
^ O2 sensor
^ Fuel pressure regulator
^ Fuel pump relay
^ Spark plugs
^ Wiring harness/connectors