do you have a hand held tuner to watch engine sensors?as noted fuel trims and o2 sensors are readable to help diagnostics...here is the fueltrim story...section 1/2,,,2003 PCED OBD
SECTION 1: Description and Operation
Procedure revision date: 08/28/2003
Fuel System Monitor
The Fuel System Monitor is an on-board strategy designed to monitor the fuel trim system. The fuel control system uses fuel trim tables stored in the PCM's Keep Alive Memory (KAM) to compensate for variability in fuel system components due to normal wear and aging. Fuel trim tables are based on engine rpm and engine load. During closed-loop fuel control, the fuel trim strategy learns the corrections needed to correct a "biased" rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim has two means of adapting; Long Term Fuel Trim and a Short Term Fuel Trim. Both are described in greater detail in this section under Powertrain Control Software, Fuel Trim. Long Term relies on the fuel trim tables and Short Term refers to the desired air/fuel ratio parameter "LAMBSE". LAMBSE is calculated by the PCM from HO2S inputs and helps maintain a 14.7:1 air/fuel ratio during closed-loop operation. Short Term Fuel Trim and Long Term Fuel Trim work together. If the HO2S indicates the engine is running rich, the PCM will correct the rich condition by moving Short Term Fuel Trim in the negative range (less fuel to correct for a rich combustion). If after a certain amount of time the Short Term Fuel Trim is still compensating for a rich condition, the PCM "learns" this and moves the Long Term Fuel Trim into the negative range to compensate and allows Short Term Fuel Trim to return to a value near 0%. Input from the ECT or CHT, IAT, and MAF sensors is required to activate the fuel trim system, which in turn activates the Fuel System Monitor. Once activated, the Fuel System Monitor looks for the fuel trim tables to reach the adaptive clip (adaptive limit) and LAMBSE to exceed a calibrated limit. The Fuel System Monitor will store the appropriate DTC when a fault is detected as described below.
- The heated oxygen sensor (HO2S) detects the presence of oxygen in the exhaust and provides the PCM with feedback indicating air/fuel ratio.
- A correction factor is added to the fuel injector pulsewidth calculation and/or mass air flow calculation, according to the Long and Short Term Fuel Trims as needed to compensate for variations in the fuel system.
- When deviation in the parameter LAMBSE increases, air/fuel control suffers and emissions increase. When LAMBSE exceeds a calibrated limit and the fuel trim table has clipped, the Fuel System Monitor sets a Diagnostic Trouble Code (DTC) as follows:
The DTCs associated with the monitor detecting a lean shift in fuel system operation are DTCs P0171 and P0174.
The DTCs associated with the monitor detecting a rich shift in fuel system operation are DTCs P0172 and P0175.
- The MIL is activated after a fault is detected on two consecutive drive cycles.
Typical Fuel System Monitor Entry Conditions:
- RPM range between Idle and 4,000 rpm.
- Air Mass Rang greater than 0.75 lb/min.
- Purge duty cycly of 0%.
Typical Fuel Monitor Malfunction Thresholds:
- Lean Malfunction: LTFT > 25%, STFT > 5%.
- Rich Malfunction: LTFT < 25%, STFT < 10%
[IMG]file:///C:/TSO/tsocache/HOUSE-62B5A491B_180/V32~us~en~file=A0013918.gif~gen~ref.gif[/IMG]
Figure 10: Fuel System Monitor
2003 PCED OBD
SECTION 2: Diagnostic Methods
Procedure revision date: 12/19/2002
Adaptive Fuel DTCs Diagnostic Techniques
Adaptive Fuel DTCs Diagnostic Techniques help isolate the root cause of the adaptive fuel concern. Before proceeding, attempt to verify if any driveability concerns are present. These diagnostic aids are meant as a supplement to the pinpoint test steps in Section 5. For a description of fuel trim, refer to
Section 1 , Powertrain Control Software, Fuel Trim.
Obtain Freeze Frame Data
Freeze Frame Data can be helpful in duplicating and diagnosing adaptive fuel concerns. This data (a snapshot of certain PID values, recorded at the time the DTC was stored in Continuous Memory) is helpful to determine how the vehicle was being driven when the fault occurred, and can be especially useful on intermittent concerns. Freeze Frame Data, in many cases, can help to isolate possible areas of concern as well as rule out others. Refer to Freeze Frame Data in this section for a more detailed description of this data.
Using the LONGFT1 and LONGFT2 (dual bank engines) PIDs
The LONGFT1/2 PIDs can be useful for diagnosing fuel trim concerns. A negative PID value indicates that fuel is being reduced to compensate for a rich condition, while a positive PID value indicates that fuel is being increased to compensate for a lean condition. It is important to know that there is a separate LONGFT value that is used for each rpm/load point of engine operation. When viewing the LONGFT1/2 PIDs, the values may change a great deal as the engine is operated at different rpm and load points. This is because the fuel system may have learned corrections for fuel delivery concerns that can change as a function of engine rpm and load. The LONGFT1/2 PIDs will display the fuel trim currently being used at that rpm and load point. Observing these changes in LONGFT1/2 can help when diagnosing fuel system concerns. For example:
- A contaminated MAF sensor would result in matching LONGFT1/2 correction values that are negative at idle (reducing fuel), but positive (adding fuel) at higher rpm and loads.
- LONGFT1 values that differ greatly from LONGFT2 values would rule out concerns that are common for both banks (for example, fuel pressure concerns, MAF sensor, etc. could be ruled out).
- Vacuum leaks would result in large rich corrections (positive LONGFT1/2 value) at idle, but little or no correction at higher rpm and loads.
- A plugged fuel filter will result in no correction at idle, but large rich corrections (positive LONGFT1/2 value) at high rpm and load.
Resetting Long Term Fuel Trims
Long term fuel trim corrections can be reset by resetting the PCM Keep Alive Memory (KAM). Refer to Resetting Keep Alive Memory in this section to reset KAM. After making a fuel system repair, KAM must be reset. For example, if dirty/plugged injectors cause the engine to run lean and generate rich long term corrections, replacing the injectors and not resetting KAM will now make the engine run very rich. The rich correction will eventually be "learned out" during closed loop operation, but the vehicle may have poor driveability and have high CO emissions while it is learning.
P0171/P0174 System Too Lean Diagnostic Aids Note: If the system is lean at certain conditions, then the LONGFT PID would be a positive value at those conditions, indicating that increased fuel is needed.
The ability to identify the type of lean condition causing the concern can be crucial to a correct diagnosis.
Air Measurement System:
With this condition, the engine may actually run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the Powertrain Control Module (PCM) is not able to compensate enough to correct for the condition. One possibility is that the mass of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. For example, with a contaminated MAF sensor, the engine would run lean at higher rpm because the PCM would deliver fuel for less air than is actually entering the engine.
Examples: MAF sensor measurement inaccurate (corroded connector, contamination/dirty (a contaminated MAF sensor will typically result in a rich system at low airflows (PCM will reduce fuel) and a lean system at high airflows (PCM will increase fuel), etc).
Vacuum Leaks/Unmetered Air:
With this condition, the engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio) if the Powertrain Control Module (PCM) is not able to compensate enough to correct for the condition. This condition can be caused by unmetered air entering the engine, or due to a MAF malfunction. In this situation, the volume of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. Vacuum leaks will normally be most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates that the fault occurred at idle, a check for vacuum leaks/unmetered air might be the best starting point.
Examples: Loose, leaking or disconnected vacuum lines, intake manifold gaskets or o-rings, throttle body gaskets, brake booster, air inlet tube, stuck/frozen/aftermarket PCV valve, unseated engine oil dipstick, etc.
Insufficient Fueling:
With this condition, the engine may actually run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition can be caused by a fuel delivery system concern that restricts or limits the amount of fuel being delivered to the engine. This condition will normally be most apparent when the engine is under a heavy load and at high rpm, when a higher volume of fuel is required. If freeze frame data indicates that the fault occurred under a heavy load and at higher rpm, a check of the fuel delivery system (checking fuel pressure with engine under a load) might be the best starting point.
Examples: low fuel pressure (fuel pump, fuel filter, fuel leaks, restricted fuel supply lines), fuel injector concerns, etc.
Exhaust System Leaks:
In this type of condition, the engine may actually be running rich of stoichiometry (14.7:1 air / fuel ratio) because the fuel control system is adding fuel to compensate for a perceived (not actual) lean condition. This condition is caused by oxygen (air) entering the exhaust system from an external source. The HO2S will react to this exhaust leak by increasing fuel delivery. This condition will cause the exhaust gas mixture from the cylinder to be rich.
Examples: Exhaust system leaks upstream or near HO2S, poorly welded/leaking HO2S boss, malfunctioning Secondary Air Injection system, etc.
P0172/P0175 System Too Rich Diagnostic Aids Note: If the system is rich at certain conditions, then the LONGFT PID would be a negative value at that airflow, indicating that decreased fuel is needed.
System rich concerns are usually caused by fuel system concerns, although the MAF sensor, and base engine (for example, engine oil contaminated with fuel) should also be checked.
Air Measurement System:
With this condition, the engine may actually run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the Powertrain Control Module (PCM) is not able to compensate enough to correct for the condition. One possibility is that the mass of air entering the engine is actually less than what the MAF sensor is indicating to the PCM. For example, with a contaminated MAF sensor, the engine would run rich at idle because the PCM would deliver fuel for more air than is actually entering the engine.
Examples: MAF sensor measurement inaccurate (corroded connector, contamination/dirty (a contaminated MAF sensor will typically result in a rich system at low airflows (PCM will reduce fuel) and a lean system at high airflows (PCM will increase fuel), etc.).
Fuel System:
With this condition, the engine may actually run rich of stoichiometry (14.7:1 air/fuel ratio) if the Powertrain Control Module (PCM) is not able to compensate enough to correct for the condition. This situation can be caused by a fuel delivery system that is delivering excessive fuel to the engine.
Examples:
- Fuel pressure regulator causes excessive fuel pressure (system rich at all airflows)(fuel pressure can be intermittent, going to pump deadhead pressure, then returning to normal after engine is turned off then restarted).
- Fuel pressure regulator vacuum hose off (causes excessive fuel pressure at idle, system rich at idle airflows).
- Fuel pressure regulator diaphragm ruptured (fuel leaking into intake manifold, system rich at lower airflows).
- Fuel return line crimped/damaged (fuel pressure high, system rich at lower airflows).
- Fuel injector leaks (injector delivers extra fuel).
- EVAP canister purge valve leak (if canister is full of vapors, introduces extra fuel).
- Fuel rail pressure sensor (electronic returnless fuel systems) concern causes sensor to indicate lower pressure than actual. PCM commands higher pressure to the fuel pump driver module (FPDM), causing high fuel pressure (system rich at all airflows).
Base Engine
Engine oil contaminated with fuel can contribute to a rich running engine.
'96 Cobra BLINKING CHECK ENGINE light!! 
