Engine Controls - Diagnostic Methods (except Diesel & Hybrid) Lincoln Mark LT I

Overview

The Diagnostic Method article provides information on routine diagnostic tasks.

When following powertrain diagnostics on vehicles with on board diagnostics (OBD), the system may be checked by an off-board tester referred to as a scan tool. This article contains information for carrying out diagnostics with a scan tool. A scan tool has certain generic capabilities that are standard across the automotive industry in the United States and Canada. All functions are selected from a menu. Refer to the instruction manual provided by the tool manufacturer.

Diagnostic Tools

Below is an equipment list with corresponding part numbers

REQUIRED EQUIPMENT

1. Vehicle Communication Module (VCM) and Integrated Diagnostic System (IDS) software with appropriate hardware, or equivalent scan tool with functionality described under Scan Tool Setup and Functionality.
2. Rotunda Smoke Machine, Fuel Evaporative Emission System Tester 218-00001 (522) or equivalent.

RECOMMENDED EQUIPMENT

1. Rotunda Vacuum/Pressure Tester 164-R0253 or equivalent. Range 0-101.3 kPa (0-30 in-Hg.) Resolution 3.4 kPa (1 in-Hg.)
2. Rotunda Vacuum Tester 014-R1054 or equivalent. Range 0-101.3 kPa (0-30 in-Hg.)
3. Rotunda 73III Automotive Meter 105-R0057 or equivalent. Input impedance 10 Megaohm minimum.
4. Spark Tester D81P-6666-A (303-D037) or equivalent.
5. Non-powered test lamp.

OPTIONAL EQUIPMENT

1. Rotunda Fuel (Gasoline) pressure test kit 134-R0087 or equivalent. (Use tool manufacturer's instructions.)

Scan Tool Setup and Functionality

Connect the scan tool to the data link connector (DLC) for communication with the vehicle.

The DLC is located in the driver side compartment under the steering column. It is attached to the instrument panel and accessible from the driver seat.

The DLC is rectangular in design and capable of accommodating up to 16 terminals. The connector has keying features to allow easy connection.

The required scan tool functions are described below

1. monitor, record, and playback of parameter identification (PIDs)
2. freeze frame PID data
3. diagnostic test modes; self-test, clear diagnostic trouble codes (DTCs)
4. output test mode
5. resetting keep alive memory (KAM)
6. diagnostic monitoring test results (mode 6) for on board diagnostic (OBD) on board monitors
7. on-board system readiness (OBD monitor completion status)

Some of these functions are described. Refer to the scan tool manufacturer's instruction manual for specific information on scan tool setup and operation.

Vehicle Check/Preparation

Before using the scan tool to carry out any test, refer to the important Safety Notice located at the beginning of this manual and the necessary visual checks listed below.

Visual Checks

1. Inspect the air cleaner and inlet duct.
2. Check all engine vacuum hoses for damage, leaks, cracks, kinks, and proper routing.
3. Check the electronic engine control (EEC) system wiring harness for proper connections, bent or broken pins, corrosion, loose wires, and proper routing.
4. Check the powertrain control module (PCM), sensors, and actuators for physical damage.
5. Check the engine coolant for proper level and mixture.
6. Check the transmission fluid level and quality.
7. Make all necessary repairs before continuing with the quick test. Refer to «QUICK TEST»(/lincoln/mark-lt/i-2005-2009/remont/testing-diagnostics/#engine-controls-diagnostic-methods-except-diesel-hybrid) .

Vehicle Preparation

1. Carry out all safety steps required to start and run vehicle tests. Apply the parking brake, place the gear selector lever firmly into the PARK position on automatic transmission vehicles or NEUTRAL on manual transmission vehicles, and block the drive wheels.
2. Turn off all electrical loads such as radios, lamps, A/C, blower, and fans.
3. Start the engine (if the engine runs) and bring it up to the normal operating temperature before running the quick test.

Quick Test

The quick test is divided into 3 specialized tests

1. Key On Engine Off (KOEO) On-Demand Self-Test
2. Key On Engine Running (KOER) On-Demand Self-Test
3. Continuous Memory Self-Test

The quick test checks the integrity and function of the electronic engine control (EEC) system and outputs the test results when requested by the scan tool. The quick test also provides a quick check of the powertrain control system, and is usually carried out at the start of each diagnostic procedure with all accessories off. The quick test is also carried out at the end of most pinpoint tests for verification of the repair and to make sure no other concerns are incurred while repairing a previous concern. A system pass is displayed when no diagnostic trouble codes (DTCs) are output and a scan tool communication error does not exist. System pass means that hardware monitored by the powertrain control module (PCM) is functioning within the normal operating limits. Only a system pass, a DTC, or an incomplete on board diagnostic (OBD) drive cycle (P1000) is displayed.

For applications that use a stand-alone transmission control module (TCM) the PCM does not output TCM DTCs. For TCM self-test and diagnostics, refer to the appropriate AUTOMATIC TRANSMISSION/TRANSAXLE article.

Key On Engine Off (KOEO) On-Demand Self-Test

The KOEO on-demand self-test is a functional test of the PCM carried out on-demand with the key on and the engine off. This test carries out checks on certain sensor and actuator circuits. A concern must be present at the time of testing for the KOEO self-test to detect the concern. When a concern is detected, a DTC is output on the data link at the end of the test as requested by the scan tool.

Key On Engine Running (KOER) On-Demand Self-Test

The KOER on-demand self-test is a functional test of the PCM carried out on-demand with the key on, the engine running and the vehicle stopped. A check of certain inputs and outputs is made during operating conditions and at a normal operating temperature. The brake pedal position, transmission control, and the power steering tests are part of the KOER on-demand self-test and must be carried out during this operation if applicable. These are described below. A concern must be present at the time of testing for the KOER on-demand self-test to detect the concern. When a concern is detected, a DTC is output on the data link at the end of the test as requested by the scan tool.

Brake Pedal Position (BPP) Test

The BPP test checks the ability of the EEC system to detect a change of state in the BPP switch. The brake pedal is briefly applied and released on all vehicles equipped with a BPP input. This is done during a KOER on-demand self-test.

Power Steering Pressure (PSP) Test

The PSP test checks the ability of the EEC system to detect a change in the power steering system fluid pressure. The steering wheel is briefly turned at least 1/4 of a revolution on vehicles equipped with a PSP switch or sensor. This is done during a KOER on-demand self-test.

Transmission Control Switch (TCS) Test

The TCS test checks the ability of the EEC system to detect a change of state in the TCS. The switch is briefly cycled on all vehicles equipped with a TCS input. This is done during a KOER on-demand self-test.

Continuous Memory Self-Test

The continuous memory self-test is a functional test of the PCM carried out under any condition (engine running or off) with the key on. Unlike the KOEO and KOER self-tests, which can only be activated on-demand, the continuous self-test is always active. A concern does not need to be present when accessing continuous memory self-test DTCs, making the test valuable when diagnosing intermittent concerns. The vehicle may need to be driven or the on board diagnostic (OBD) drive cycle completed to allow the PCM to detect a concern. Refer to ON BOARD DIAGNOSTIC (OBD) DRIVE CYCLE for more information. When a concern is stored in memory, a DTC is output on the data link when requested by the scan tool.

There are two types of continuous DTCs. The first type is an emission-related code which illuminates the malfunction indicator lamp (MIL) in the instrument cluster. The second is a non-emission related, non-MIL code which does not illuminate the cluster indicator.

For emission-related MIL codes, the PCM stores the DTC in continuous memory when a concern is detected for the first time. At this point the DTC does not illuminate the MIL and is now considered a pending code. The purpose of pending codes is to assist in repair verification by reporting a pending DTC after one drive cycle. If the same concern is detected after the next ignition start-run cycle, the emission-related MIL code illuminates the MIL. The MIL remains on even if the concern is intermittent. The MIL is extinguished if the concern is not present through 3 consecutive drive cycles or if the concern is fixed and the DTCs are cleared. Also, an emission-related pending MIL and any non-emission related, non-MIL DTCs are erased after approximately 40 vehicle warm-up cycles or if the DTCs are cleared.

Any scan tool that meets OBD requirements can access the continuous memory to retrieve emission-related MIL DTCs. However, not all scan tools access pending and non-emission related, non-MIL DTCs in the same way.

During most diagnostic procedures, it is required that all DTCs be retrieved and cleared. Consult the instruction manual from the tool manufacturer for specific instructions.

Description

All on board diagnostics (OBD) scan tools support the clearing of continuous DTCs and resetting of emission monitors information in the PCM.

The clearing of the continuous DTCs allows the scan tool to command the PCM to clear/reset all emission-related diagnostic information. While carrying out this operation DTC P1000 is stored in the PCM until all the OBD system monitors or components have been tested to satisfy a drive cycle without any other concerns occurring. For more information about a drive cycle, refer to ON BOARD DIAGNOSTIC (OBD) DRIVE CYCLE .

The following events occur when the continuous DTCs and the emission monitors information is cleared from the PCM

1. the number of DTCs and the DTCs
2. the freeze frame data
3. the diagnostic monitoring test results
4. the status of the OBD system monitors is reset
5. DTC P1000 is set

Resetting the KAM returns the powertrain control module (PCM) memory to its default setting. Adaptive learning contents such as adaptive airflow, idle speed, refueling event, and fuel trim are included. Clear the continuous diagnostic trouble codes (DTCs) in the PCM and reset the emission monitors information, is part of a KAM reset. Refer to CLEAR THE CONTINUOUS DIAGNOSTIC TROUBLE CODES (DTCS) AND RESET THE EMISSION MONITORS INFORMATION IN THE POWERTRAIN CONTROL MODULE (PCM) . Both can be useful in post-repair testing.

After the KAM has been reset, the vehicle may exhibit certain driveability concerns. It is necessary to allow the engine to idle at normal operating temperature with the air conditioning (A/C) OFF for 2 minutes. Then drive the vehicle to allow the PCM to learn the values for optimum driveability and performance.

This function may not be supported by all scan tools. Refer to the scan tool manufacturer's instruction manual.

If an error message is received or the scan tool does not support this function, disconnecting the battery ground cable for a minimum of 5 minutes may be used as an alternative procedure.

All on board diagnostic (OBD) scan tools display the on-board system readiness (OSR) test. The OSR displays the supported monitors on the vehicle and the status of all monitors (complete or not complete) at that time. Fuel, misfire, and comprehensive component monitors run continuously and always display YES status. Clearing the continuous diagnostic trouble codes (DTCs) and resetting the emission monitors information in the powertrain control module (PCM), or resetting the keep alive memory (KAM) causes the non-continuous monitors to change to a NO status.

A detailed description of completing the OBD monitors is found. Refer to ON BOARD DIAGNOSTIC (OBD) DRIVE CYCLE .

The OSC aids in diagnosing output actuators associated with the powertrain control module (PCM) for the engine. This mode allows the technician to command the individual actuator state. For example: the output can be enabled or disabled, the duty cycle or the angle of the output can be increased or decreased. The OSC is used to help test the electrical, hydraulic or mechanical components of the vehicle. This function is supported by the vehicle strategy but may not be present on all vehicles or available on all scan tools.

Retrieve the continuous codes and carry out a key on, engine off (KOEO) and key on, engine running (KOER) on-demand self-test before using any OSC. Any diagnostic trouble codes (DTCs) related to the transmission range (TR) sensor, output shaft sensor (OSS) or the vehicle speed sensor (VSS) must be fixed or the PCM does not allow the OSC to operate.

The OSC has 2 options for operation, the Bench Mode and the Drive Mode. The Bench Mode is functional only when the vehicle gear selector is in the PARK or NEUTRAL position. The Bench Mode may be used when the engine is on (running) or off (not running).

Each OSC function has a unique set of vehicle operating requirements that the technician is required to meet before operating the OSC. If the vehicle requirements are not met while commanding the OSC value, an error message appears. When the error message is received, OSC is canceled.

To confirm that the scan tool sent the OSC value and the PCM has accepted the OSC substitution, a corresponding parameter identification (PID) for each OSC parameter must be monitored.

One Touch Integrated Start System

Some vehicles are equipped with one touch integrated start system. It may be necessary to disable the one touch integrated start system to carry out diagnostic procedures that require extended cranking. Connect the scan tool, access the PCM and select the one touch integrated start system control PID to disable the system.

The OTM aids in diagnosing output actuators associated with the powertrain control module (PCM). This mode allows the technician to energize and de-energize most of the system output actuators on command. When entering OTM, the outputs can be turned off and on without activating the fan control. The low and high speed fan controls may be turned on separately without energizing the other outputs. This function is supported by each vehicle strategy and may not be available on all scan tools.

As a safety precaution, OTM defaults to the off state after 10 minutes, and the fuel pump off state after approximately 7-10 seconds. OTM also turns off after the vehicle is started or after cycling the key off then on.

The parameter identification (PID) mode allows access to powertrain control module (PCM) information. This includes analog and digital signal inputs and outputs along with calculated values and the system status. There are 2 types of PID lists available . The first is the generic (J1979) OBD PID list. This is a standard set of PIDs that all scan tools must be able to access. The second is a Ford-specific (J2190) list which can be accessed by an appropriate scan tool. When accessing any of these PIDs, the values are continuously updated. The generic or Ford PID list provides definitions and values in appropriate units. For more information, refer to the Society of Automotive Engineers (SAE) document J2205.

Generic OBD PID List

CL = Closed loop using HO2S(s) as feedback for fuel control.

OL DRIVE = Open loop due to driving conditions (heavy acceleration).

OL FAULT = Open loop due to fault with all upstream HO2S sensors.

CL FAULT = Closed loop fuel control, but fault with one upstream HO2S sensor on dual bank vehicles.

Ford PID List

Note. This is not a complete list of Ford PIDs available.

Freeze frame data allows access to emission-related values from specific generic parameter identification (PIDs). These values are stored when an emission-related diagnostic trouble code (DTC) is stored in continuous memory. This provides a snapshot of the conditions that were present when the DTC was stored. Once one set of freeze frame data is stored, this data remains in memory even if another emission-related DTC is stored, with the exception of misfire or fuel system DTCs. Once freeze frame data for a misfire or fuel system DTC is stored, it overwrites any previous data, and freeze frame data is no longer overwritten. When a DTC associated with the freeze frame data is erased or the DTCs are cleared, new freeze frame data can be stored again. In the event of multiple emission-related DTCs in memory, always note the DTC for the freeze frame data.

FREEZE FRAME DATA TABLE

Some unique PIDs are stored in the keep alive memory (KAM) of the powertrain control module (PCM) to help in diagnosing the root cause of misfires. These PIDs are collectively called misfire freeze frame (MFF) data. These parameters are separate from the generic freeze frame data that is stored for every MIL code. They are used for misfire diagnosis only. The MFF data could be more useful for misfire diagnosis than the generic freeze frame data. It is captured at the time of the highest misfire rate, and not when the DTC is stored at the end of a 1,000 or 200 revolution block. (Generic freeze frame data for misfire can be stored minutes after the misfire actually occurred.)

Note. MFF PIDs are supported on all vehicles, but may not be available on all scan tools because enhanced PID access may vary by scan tool manufacturer.

MISFIRE FREEZE-FRAME PIDS

The EEPROM is contained in an integrated circuit (IC) internal to the powertrain control module (PCM). The EEPROM contains the vehicle strategy including calibration information specific to the vehicle, and is capable of being programmed or flashed repeatedly.

As part of the calibration there is an area referred to as the vehicle identification (VID) block. The VID block is programmed when installing a new PCM as described under Programming the VID Block for a Replacement PCM. Failure to carry out this procedure may generate DTCs P1635 or P1639. The VID block in an existing PCM can also be tailored to accommodate various hardware or parameter changes made to the vehicle since production. Failure to carry out this procedure properly may generate DTC P1635, Tire/Axle Ratio out of Acceptable Range. An incorrect tire/axle ratio is one of the main causes for DTC P1639. This is described under Making Changes to the VID Block and also under Making Changes to the PCM Calibration. The VID block contains many items used by the strategy for a variety of functions. Some of these items include the vehicle identification number (VIN), octane adjust, fuel octane, fuel type, vehicle speed limit, tire size, axle ratio, the presence of speed control, and 4-wheel drive electronic shift-on-the-fly (ESOF) versus manual shift-on-the-fly (MSOF). Only items applicable to the vehicle hardware and supported by the VID block is displayed on the scan tool.

When changing items in the VID block, the strategy places range limits on certain items such as tire and axle ratio. The number of times the VID block may be reconfigured is limited. When this limit is reached, the scan tool displays a message indicating the need to flash the PCM again to reset the VID block.

Programming can be carried out by a local Ford dealer or any non-Ford facility. Refer to the scan tool manufacturer's instruction manual for details.

Programming the VID Block for a Replacement PCM

A new PCM contains the latest strategy and calibration level for a particular vehicle. However, the VID block is blank and needs programming. There are 2 procedures available. The first is an automatic data transfer from the old PCM to the new PCM, and the second is manual data entry into the new PCM.

Automatic data transfer is carried out if the old PCM is capable of communicating. This is done by using a scan tool to retrieve data from the old PCM before removing it from the vehicle. The stored data can then be downloaded to the new PCM after it has been installed.

Carry out manual data entry if the old module is damaged or incapable of communicating. Remove and install a new PCM. Using a compatible scan tool, select and carry out the module/parameter programming, referring to the scan tool manufacturer's instruction manual. Make certain that all parameters are included. Failure to properly program tire size in revolutions per mile, (rev/mile equals 63,360 divided by the tire circumference in inches), axle ratio, 4x4/4x2, and/or MSOF/ESOF may result in DTCs P1635 and P1639. You may be instructed to contact the As-Built Data Center for the information needed to manually update the VID block with the scan tool. Contact the center only if the old PCM cannot be used or the data is corrupt. For Ford and Lincoln Mercury technicians, contact your National Hotline or the Professional Technician Society (PTS) website for As-Built data listed under the Service Publications Index. Non-Ford technicians use the Motorcraft® website at www.motorcraft.com. From the Motorcraft® homepage, use the search function to find the Module Programming or As-Built Data.

For Ford and Lincoln Mercury technicians, check the Programmable Module Installation link on the PTS website for quick Programmable Module data information by vehicle.

Making Changes to the VID Block

A PCM which is programmed may require changes to be made to certain VID information to accommodate the vehicle hardware. Refer to Module Reprogramming on the scan tool.

Making Changes to the PCM Calibration

At certain times, the entire EEPROM needs to be completely reprogrammed. This is due to changes made to the strategy or calibration after production, or the need to reset the VID block because it has reached its limit. Refer to Module Reprogramming on the scan tool.

Diagnostic Monitoring Test Results - Mode 6

Mode 6 allows access to the results of on board diagnostic (OBD) monitor diagnostic test results. The test values are stored at the time of the particular monitor completion. Refer to mode 6 on the scan tool for test information.

Description of On Board Diagnostic (OBD) Drive Cycle

The following procedure is designed to execute and complete the OBD monitors and to clear the Ford P1000, I/M readiness code. To complete a specific monitor for repair verification, follow steps 1 through 4, then continue with the step described by the appropriate monitor found under the OBD Monitor Exercised column. For the EVAP/secondary AIR monitor to run, the ambient air temperature must be between 4.4 to 37.8°C (40 to 100°F), and the altitude below 2,438 meters (8,000 feet). If the P1000 code must be cleared in these conditions, the powertrain control module (PCM) must detect them once (twice on some applications) before the EVAP/secondary AIR monitor can be bypassed and diagnostic trouble code (DTC) P1000 is cleared. The EVAP/secondary AIR bypassing procedure is described in the following drive cycle.

The OBD drive cycle is carried out using a scan tool. Refer to the manufacturer's instruction manual for each described function.

A detailed description for clearing the DTCs is found. Refer to CLEAR THE CONTINUOUS DIAGNOSTIC TROUBLE CODES (DTCS) AND RESET THE EMISSION MONITORS INFORMATION IN THE POWERTRAIN CONTROL MODULE (PCM) .

Drive Cycle Recommendations

1. Most OBD monitors complete more readily using a steady foot driving style during cruise or acceleration modes. Operating the throttle in a smooth fashion minimizes the time required for monitor completion.
2. The fuel tank level should be between 1/2 and 3/4 full with 3/4 full being the most desirable.
3. The evaporative monitor can operate only during the first 30 minutes of engine operation. When executing the procedure for this monitor, stay in part throttle mode and drive in a smooth fashion to minimize fuel slosh.
4. When bypassing the EVAP/secondary AIR engine soak times, the PCM must remain powered (key ON) after clearing the continuous DTCs and relearning emission diagnostic information.

For best results, follow each of the following steps as accurately as possible

Intermittent Diagnostic Techniques

Intermittent diagnostic techniques help find and isolate the root cause of intermittent concerns associated with the electronic engine control (EEC) system. The information is organized to help find the concern and carry out the repair. The process of finding and isolating an intermittent concern starts with recreating a fault symptom, accumulating powertrain control module (PCM) data, and comparing that data to typical values, then analyzing the results. Refer to the scan tool manufacturer's instruction manual for the functions described below.

Before proceeding, be sure that

1. Customary mechanical system tests and inspections do not reveal a concern. NOTE: Mechanical component conditions can make a PCM system react abnormally.
2. Technical Service Bulletins (TSBs), if available, are reviewed.
3. Quick Test and associated diagnostic subroutines have been completed without finding a concern, and the symptom is still present.

Recreating the Fault

Recreating the concern is the first step in isolating the cause of the intermittent symptom. A thorough investigation should start with the customer information worksheet . If freeze frame data is available, it may help in recreating the conditions at the time of a malfunction indicator lamp diagnostic trouble code (MIL DTC). Listed below are some of the conditions for recreating the concern

CONDITIONS TO RECREATE FAULT

Accumulating PCM Data

PCM data can be accumulated in a number of ways. This includes circuit measurements with a digital multimeter (DMM) or scan tool parameter identification (PID) data. Acquisition of PCM PID data using a scan tool is one of the easiest ways to gather information. Gather as much data as possible when the concern is occurring to prevent improper diagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent concern. Compare this data with the known good data values located TYPICAL DIAGNOSTIC REFERENCE VALUES . This requires recording data in 4 conditions for comparison: 1) KOEO, 2) Hot Idle, 3) 48 km/h (30 mph), and 4) 89 km/h (55 mph).

Comparing PCM Data

After the PCM values are acquired, it is necessary to determine the concern area. This typically requires the comparison of the actual values from the vehicle to the typical values from the TYPICAL DIAGNOSTIC REFERENCE VALUES . The charts apply to different vehicle applications (engine, model, transmission).

Analyzing PCM Data

Look for abnormal events or values that are clearly incorrect. Inspect the signals for abrupt or unexpected changes. For example, during a steady cruise most of the sensor values should be relatively stable. Sensors such as throttle position (TP), mass air flow (MAF), and RPM that change abruptly when the vehicle is traveling at a constant speed are clues to a possible concern area.

Look for an agreement in related signals. For example, if the APP1, APP2, or APP3, is changed during acceleration, a corresponding change should occur in idle air control (IAC), RPM, and SPARK ADV PID.

Make sure the signals act in proper sequence. An increase in RPM after the TP1 and TP2 is increased is expected. However, if the RPM increases without a TP1 and TP2 change, a concern may exist.

Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values.

Adaptive Fuel DTC Diagnostic Techniques

The Adaptive Fuel Diagnostic Trouble Codes (DTC) 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. For a description of fuel trim, refer to POWERTRAIN CONTROL SOFTWARE , Fuel Trim.

Obtain Freeze Frame Data

Freeze frame data is helpful in duplicating and diagnosing adaptive fuel concerns. The data (a snapshot of certain parameter identification (PID) values recorded at the time the DTC is stored in Continuous Memory) is helpful to determine how the vehicle was being driven when the concern occurred, and is especially useful on intermittent concerns. Freeze frame data, in many cases, helps to isolate possible areas of concern as well as rule out others. Refer to FREEZE FRAME DATA for a more detailed description of this data.

Using the LONGFT1 and LONGFT2 (Dual Bank Engines) PIDs

The LONGFT1/2 PIDs are useful for diagnosing fuel trim concerns. A negative PID value indicates that fuel is being reduced to compensate for a rich condition. 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 operating 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 display the fuel trim currently being used at that RPM and load point. Observing the changes in LONGFT1/2 can help when diagnosing fuel system concerns. For example

1. A contaminated mass air flow (MAF) sensor results in matching LONGFT1/2 correction values that are negative at idle (reducing fuel), but positive (adding fuel) at higher RPM and loads.
2. LONGFT1 values that differ greatly from LONGFT2 values rule out concerns that are common for both banks (for example, fuel pressure concerns, MAF sensor, etc. can be ruled out).
3. Vacuum leaks result in large rich corrections (positive LONGFT1/2 value) at idle, but little or no correction at higher RPM and loads.
4. A plugged fuel filter results 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 are reset by resetting the keep alive memory (KAM). Refer to RESETTING THE KEEP ALIVE MEMORY (KAM) . After making a fuel system repair, the KAM must be reset. For example, if dirty/plugged injectors cause the engine to run lean and generate rich long term corrections, installing new injectors and not resetting the KAM causes the engine to run very rich. The rich correction eventually leans out during closed loop operation, but the vehicle may have poor driveability and high CO emissions while it is learning.

DTCs 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 is crucial to a correct diagnosis.

Air Measurement System

With this condition, the engine runs 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 runs lean at higher RPM because the PCM delivers fuel for less air than is actually entering the engine. Examples

1. The MAF sensor measurement is inaccurate due to a corroded connector, contaminated or dirty connector. A contaminated MAF sensor typically results in a rich system at low airflows (PCM reduces fuel) and a lean system at high airflows (PCM increases fuel).

Vacuum Leaks/Unmetered Air

With this condition, the engine runs 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 is caused by unmetered air entering the engine, or due to a MAF concern. 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 are normally most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates that the concern occurred at idle, a check for vacuum leaks/unmetered air is the best starting point. Examples

1. loose, leaking, or disconnected vacuum lines
2. intake manifold gaskets, or O-rings
3. throttle body gaskets
4. brake booster
5. air inlet tube
6. stuck/frozen/aftermarket positive crankcase valve (PCV)
7. unseated engine oil dipstick.

Insufficient Fueling

With this condition, the engine runs 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 is caused by a fuel delivery system concern that restricts or limits the amount of fuel being delivered to the engine. This condition is normally apparent as the engine is under a heavy load and at high RPM, when a higher volume of fuel is required. If the freeze frame data indicates that the concern occurs under a heavy load and at higher RPM, a check of the fuel delivery system (checking fuel pressure with engine under a load) is the best starting point. Examples

1. low fuel pressure (fuel pump, fuel filter, fuel leaks, restricted fuel supply lines)
2. fuel injector concerns

Exhaust System Leaks

In this type of condition, the engine runs 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 react to this exhaust leak by increasing fuel delivery. This condition causes the exhaust gas mixture from the cylinder to be rich. Examples

1. exhaust system leaks upstream or near the HO2S
2. cracked/leaking HO2S boss
3. inoperative secondary air injection system

DTCs 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 caused by fuel system concerns, although the MAF sensor and base engine (for example, engine oil contaminated with fuel) should also be checked.

With this condition, the engine runs rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the 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 runs rich at idle because the PCM delivers fuel for more air than is actually entering the engine. Examples

1. MAF sensor measurement inaccurate due to a corroded connector, contamination/dirt. A contaminated MAF sensor typically results in a rich system at low airflows (PCM reduces fuel) and a lean system at high airflows (PCM increases fuel).

Fuel System

With this condition, the engine runs rich of stoichiometry (14.7:1 air/fuel ratio), if the PCM is not able to compensate enough to correct for the condition. This situation causes a fuel delivery system that is delivering excessive fuel to the engine.

Examples

1. fuel pressure regulator causes excessive fuel pressure (system rich at all airflows), fuel pressure is intermittent, going to pump deadhead pressure, then returning to normal after the engine is turned off and restarted.
2. fuel pulse dampener diaphragm ruptured (fuel leaking into the intake manifold, system rich at lower airflows).
3. fuel injector leaks (injector delivers extra fuel).
4. EVAP canister purge valve leak (if the canister is full of vapors, introduces extra fuel).
5. fuel rail pressure (FRP) sensor (electronic returnless fuel systems) concern causes the sensor to indicate a lower pressure than actual. The PCM commands a higher duty cycle to the fuel pump driver module (FPDM), causing high fuel pressure (system rich at all airflows).

Air Inlet System

A restriction within any of the following components may be significant enough to affect the ability of the PCM adaptive fuel control.

1. air inlet tube
2. air cleaner element
3. air cleaner assembly
4. resonators
5. clean air tube

Base Engine

Engine oil contaminated with fuel can contribute to a rich-running engine.

See also:
• TYPICAL DIAGNOSTIC REFERENCE VALUES
• POWERTRAIN CONTROL SOFTWARE
• QUICK TEST