Engine Controls - Diagnostic Methods - Cng, Flex-Fuel & Gasoline Ford Crown Victoria II

Overview

The Diagnostic Methods article provides information on routine diagnostic tasks.

When following powertrain diagnostics on OBD II vehicles, the system may be checked by an off-board tester referred to as a scan tool. This article contains information for performing diagnostics with a scan tool. A scan tool has certain generic capabilities that are standard across the automotive industry in the United States. 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. Rotunda Worldwide Diagnostic System (WDS) 418-F224 or scan tool with functionality described under Scan Tool Set-up and Functionality.
2. Rotunda Evaporative Emission System Tester 310-F007 (134-00056) or equivalent.

RECOMMENDED EQUIPMENT

1. Rotunda EEC-IV type 60-Pin Breakout Box 418-005 (014-00322) or equivalent.
2. Rotunda EEC-V 104-Pin Breakout Box 418-049 (014-00950) or equivalent.
3. Rotunda Vacuum/Pressure Tester 164-R0253 or equivalent. Range 0 -101.3 kPa (0-30 in-Hg.) Resolution 3.4 kPa (1 in -Hg.)
4. Rotunda Vacuum Tester 014-R1054 or equivalent. Range 0 -101.3 kPa (0-30 in-Hg.)
5. Rotunda Engine EAR Amplifier 107-R2100 or equivalent.
6. Rotunda 73 III Automotive Meter 105-R0057 or equivalent. Input impedance 10 Megaohm minimum.
7. Rotunda Electronic Ignition (EI) System Tester (Distributor/less Ignition System Tester) 418-F024 (007-00075) or equivalent.
8. Dist Integrated EDIS Cable Option 418-F039 (007-00110) or equivalent.
9. Series 100 Engine/Ignition Analyzer 010-01060 or equivalent.
10. Spark Tester D81P-6666-A or equivalent.
11. Non-powered Test Lamp.

OPTIONAL EQUIPMENT

1. Rotunda Auxiliary Adapter 418-F007 (007-00023) or equivalent.
2. Rotunda Pressure/Vacuum Adapter 418-F006 (007-00022) or equivalent.
3. Rotunda Fuel (Gasoline) pressure test kit 134-R0087 or equivalent (Use tool manufacturer's instructions.).
4. Rotunda Fuel Composition Test Kit 014-00770.
5. Fuel Injector Tester/Cleaner 164-R3759 or equivalent.
6. Rotunda NG Tool Kit 134-00114 or equivalent.

Scan Tool Set-up and Functionality

The scan tool must be connected to the data link connector (DLC) for communication with the vehicle.

The DLC is located in the passenger compartment. It is attached to the instrument panel and accessible from the driver's 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 vehicle connector and the test equipment connector have latching features that ensure the test equipment connector will remain mated when properly connected.

The required scan tool functions are described below

1. Monitor, record and playback of 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. Digital measurement system (multimeter functionality)
7. Diagnostic monitoring test results (for OBD II on-board monitors)
8. On board system readiness (OBD II monitor completion status)

Some of these functions are described in this article. Refer to the scan tool manufacturer's manual for specific information on scan tool set-up and operation.

Vehicle Check/Preparation

Before using the scan tool to perform any test, refer to any important Safety Notices 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 Electronic EC 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 Quick Test.

Vehicle Preparation

1. Perform ALL safety steps required to start and run vehicle tests. Apply parking brake, place shift lever firmly into PARK position (NEUTRAL on manual transmission) and block drive wheels.
2. Turn off ALL electrical loads: radios, lamps, A/C, blower, and fans.
3. Start engine and bring up to normal operating temperature before running Quick Test.

Quick Test

Quick Test is divided into three 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

Quick Test checks the integrity and function of the Electronic EC System and outputs the test results when requested by a scan tool. Quick Test also provides a quick end check of the powertrain control system and is usually performed at the start of each diagnostic procedure with all accessories off. Quick Test is also performed at the end of most pinpoint tests for verification of repair and to make sure no other faults were incurred while repairing a previous fault. A system pass will be displayed when no DTCs are output and a scan tool communication error does not exist. System pass means that hardware monitored by the PCM is functioning within normal operating limits. Only a system pass, DTC or an incomplete OBDII drive cycle (P1000) will be displayed.

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

Key On Engine Off (KOEO) On-Demand Self-Test is a functional test of the powertrain control module performed on demand with the key on and the engine off. This test will perform checks on certain sensor and actuator circuits. A fault must be present at the time of testing for the KOEO Self-Test to detect the fault. When a fault is detected, a Diagnostic Trouble Code (DTC) will be output on the data link at the end of the test when requested by a scan tool.

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

Key On Engine Running (KOER) On-Demand Self-Test is a functional test of the powertrain control module performed on demand with the key on engine running and vehicle stopped. A check of certain inputs and outputs is made during operating conditions and at normal temperature. The brake pedal position, transmission control and power steering switch tests are part of KOER On-Demand Self-Test and MUST be performed during this operation if applicable. These are described below. A fault must be present at the time of testing KOER On-Demand Self-Test to detect the fault. When a fault is detected, a Diagnostic Trouble Code (DTC) will be output on the data link at the end of the test when requested by a scan tool.

Brake Pedal Position Test

This tests the ability of the Electronic EC system to detect a change of state in the stoplight switch. Brake pedal MUST briefly be applied and released on all vehicles equipped with Brake Pedal Position input. This is done during KOER On-Demand Self-Test.

Transmission Control Switch Test

This tests the ability of the Electronic EC system to detect a change of state in the transmission control switch (TCS). Switch MUST briefly be cycled on all vehicles equipped with TCS input. This is done during KOER On-Demand Self-Test.

Power Steering Pressure Test

This tests the ability of the Electronic EC system to detect a change in power steering system fluid pressure. The steering wheel MUST briefly be turned at least 1/4 of a revolution on vehicles equipped with a power steering pressure (PSP) switch or sensor. This is done during KOER On-Demand Self-Test.

Continuous Memory Self-Test

Testing for Continuous Memory DTC's is a functional test of the powertrain control module performed 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 fault does not need to be present at the time of testing for Continuous DTC's and is therefore, especially valuable when diagnosing intermittent faults. This test will detect failures contributing to driveability or emission concerns. The vehicle may need to be driven or the OBDII Drive Cycle completed to allow the PCM to detect a fault. Refer to DRIVE CYCLES for more information. When a fault is stored in memory, a Diagnostic Trouble Code (DTC) will be output on the data link at the end of the test when requested by a scan tool.

There are two types of Continuous DTC's. The first type is an emission related malfunction indicator lamp (MIL) code which will illuminate the CHECK ENGINE or SERVICE ENGINE SOON indicator in the instrument cluster. The second is a non emission related non-MIL code which will never illuminate the cluster indicator.

For emission related MIL codes, the PCM will store the DTC in continuous memory when a fault is detected for the first time. At this point the DTC will 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 fault is detected after the next ignition start-run cycle, the emission related MIL code will illuminate the MIL. The MIL will remain on even if the fault is intermittent. The MIL will be extinguished if the fault is not present through three consecutive drive cycles or the DTCs are cleared. Also, an emission related pending MIL and non emission related (non-MIL) code will be erased after approximately 40 vehicle warm up cycles or the DTCs are cleared.

Any scan tool that meets OBDII requirements can access Continuous Memory to retrieve emission related MIL DTC's. However, not all scan tools access pending and non emission related (non-MIL) DTC's in the same way.

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

Description

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 system status. There are two types of PID lists available and both are used throughout this service information. The first is the Generic (J1979) OBDII PID list. This is a standard set of PIDs for all manufacturers all scan tools must be able to access. The second is a Ford specific (J2190) list which can be accessed by an adequate scan tool. When accessing any of these PIDs, the values will be 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) J2205 document.

Generic OBD II PID List

GENERIC OBD II PID LIST

Ford PID List

FORD PID LIST

All OBD II scan tools display the On-Board System Readiness (OSR) Test. The OSR will display 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 will 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.

Freeze Frame Data allows access to emission-related values from specific generic PIDs. These values are stored when an emission-related 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 will remain in memory even if another emission-related DTC is stored, with the exception of Misfire or Fuel System DTCs. Once freeze frame data for Misfire or Fuel System DTC is stored, it will overwrite any previous data, and freeze frame will not be further overwritten. When a DTC associated with the freeze frame is erased or clearing the continuous diagnostic trouble codes (DTCs) and resetting the emission monitors information in the powertrain control module (PCM) is performed, 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

Some unique parameters (PIDs) are stored in the Keep Alive Memory of the 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 is more useful for misfire diagnosis than the normal diagnosis only. It is captured at the time of the highest misfire rate and not when the DTC is stored at the end of a 1000 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 Output Test Mode (OTM) aids in servicing output actuators associated with the 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 control(s) 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, Output Test Mode will default to the off state after 10 minutes and fuel pump off after approximately 7-10 seconds. OTM will also turn off after the vehicle is started or after cycling the key off then on.

Note. Clear the continuous diagnostic trouble codes (DTCs) and reset the emission monitors information in the powertrain control module (PCM) was previously called PCM reset.

All 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 a DTC P1000 will be stored in the PCM until all the OBD system monitors or components have been tested to satisfy a drive cycle without any other faults occurring. For more information about a drive cycle, refer to DRIVE CYCLES in this article.

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

1. Clears the number of DTCs.
2. Clears the DTCs.
3. Clears the freeze frame data.
4. Clears the diagnostic monitoring test results.
5. Resets the status of the OBD system monitors.
6. Sets DTC P1000.

Resetting the KAM returns the PCM memory to its default setting. Adaptive learning contents such as idle speed, refueling event, and fuel trim are included. To clear the continuous DTCs in the PCM and have it reset the emissions monitors information, is also 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) in this article. Both can be useful in post repair retest.

After the KAM has been reset, the vehicle may exhibit certain driveability concerns. It is necessary to 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.

Flash Electrically Erasable Programmable Read Only Memory (EEPROM) is contained in an Integrated Circuit (IC) internal to the PCM. The EEPROM contains the vehicle strategy including calibration information specific to the vehicle and is capable of being reprogrammed or reflashed repeatedly.

As part of the calibration there is an area referred to as the Vehicle Identification (VID) block. The VID block must be programmed when replacing the PCM as described under Programming the VID Block for a Replacement PCM.

Failure to perform this procedure may generate fault codes: P1635, P1639, VID Block not programmed or is corrupt. The VID block in an existing PCM can also be tailored to accommodate various hardware/parameter changes made to the vehicle since production. Failure to perform this procedure properly may generate fault code: P1635, Tire/Axle Ratio out of Acceptable Range is one of the main causes for code: 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 VIN number, octane adjust, fuel octane, fuel type, vehicle speed limit, tire size, axle ratio, the presence of speed control and four wheel drive electronic shift on the fly versus manual shift on the fly. Only items applicable to vehicle hardware and supported by the VID block will be displayed on the scan tool.

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

Each of the procedures described below use the Worldwide Diagnostic System (WDS). Reprogramming can be performed by a local Ford dealer for any non Ford facility. There are other Enhanced Scan Tools that may have reprogramming capabilities available. Refer to the manufacturers users manual for details.

Programming the VID Block for a Replacement PCM

A new PCM will contain the latest strategy and calibration level for a particular vehicle. However, the VID block will be blank and will need programming. There are two 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 will be performed if the old PCM is capable of communicating. This is done by the use of a scan tool to retrieve data from the old PCM before removing it from the vehicle. The stored data can now be downloaded to the new PCM after it has been replaced.

Manual data entry must be performed if the old module is damaged and/or incapable of communicating. Remove and replace the old PCM. Using a compatible Scan Tool select and execute Module/Parameter reprogramming referring to the manufacturers users manual. Important, make certain that all parameters are included. Failure to properly program Tire Size in revolutions per mile, (rev/mile = 63,360 divided by the tire circumference in inches) Axle Ratio, 4x4/4x2, and/or Manual/Electronic shift on the fly (MSOF/ESOF) may result in codes: P1635, 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 L-M technician's, contact your National Hotline or the Professional Technician Society (PTS) web sight for "AS Built" data. Non Ford technicians, use the Fed World website at "fedworld.gov". Select Auto Service Information and search for "Calibrations" or "Vehicle Calibrations" then specify vehicle manufacturer, model name and model year as required.

For Ford L -M technician's, check out the "Programmable Module Installation" link on the Professional Technician Society (PTS) web sight for quick Programmable Module data information by vehicle when using WDS or NGS.

Making Changes to the VID Block

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

Making Changes to the PCM Calibration

At certain times, the entire EEPROM will need 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 PCM/Module Reprogramming on the Scan Tool.

Diagnostic Monitoring Test Results

The purpose of this test mode is to allow access to the results of OBD II monitor diagnostic test results. The test values that are stored at the time of the particular monitor completion are displayed when the particular test identification is requested. Refer to the following table for test information.

DIAGNOSTIC MONITORING TEST RESULTS

The conversion is done as follows

If the value is > 32767 then complement (change 0's to 1's and 1's to 0's), add 1 and a negative sign.

Example

Description of OBD II Drive Cycle

The following procedure is designed to execute and complete the OBDII 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 "OBDII Monitor Exercised" column. When the ambient air temperature is outside 4.4 to 37.8°C (40 to 100 °F), or the altitude is above 2438 meters (8000 feet), the EVAP monitor will not run. If the P1000 code must be cleared in these conditions, the PCM must detect them once (twice on some applications) before the EVAP monitor can be "bypassed" and the P1000 cleared. The EVAP "bypassing" procedure is described in the following drive cycle.

The OBDII Drive Cycle will be performed using a scan tool. Consult the instruction manual for each described function.

Note. A detailed description for clearing the DTCs is found in this article. 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 OBDII monitors will complete more readily using a "steady foot" driving style during cruise or acceleration modes. Operating the throttle in a "smooth" fashion will minimize the time required for monitor completion.
2. Fuel tank level should be between 1/2 and 3/4 fill with 3/4 fill being the most desirable.
3. The Evaporative Monitor can only operate 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".

For best result, 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 faults associated with the Electronic Engine Control System. The information is organized to help find the fault and perform the repair. The process of finding and isolating an intermittent starts with recreating a fault symptom, accumulating PCM data and comparing that data to typical values, then analyzing the results. Refer to the scan tool users manual for functions described below.

Before proceeding, be sure that

1. Customary mechanical system tests and inspections do not reveal a concern. (Remember, 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 fault, and the symptom is still present.

Recreating the Fault

Recreating the fault is the first step in isolating the cause of the intermittent symptom. A thorough investigation should start with the customer information worksheet located in the Introduction. 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 fault

Accumulating PCM Data

PCM data can be accumulated in a number of ways. This includes circuit measurements with a DVOM or scan tool 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 fault is occurring to prevent improper diagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent fault. Compare this data with the known good data values located in REFERENCE VALUES - CNG, FLEX-FUEL & GASOLINE in the Typical Diagnostic Reference Values. This will require recording data in four conditions for comparison: 1) KOEO, 2) HOT IDLE, 3) 48 km/h (30 mph), and 4) 89 km/h (55 mph).

Analyzing Data From Playback of Stored PIDs

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 TP, MAF and RPM that change abruptly when the vehicle is traveling at a constant speed are clues to a possible fault area.

Look for agreement in related signals. For example, if TP is changed during acceleration, a corresponding change should occur in IAC, RPM and SPARK ADV PID.

Make sure the signals act in proper sequence. An increase in rpm after the TP is increased is expected. However, if rpm increases without a TP change, then a fault may exist.

Table Format (Scheme 141): Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values. (Refer to the following TP example in (Scheme 141) ). Notice the major jump in the TP voltage while scrolling through the information. This example would require a smooth and progressive accelerator pedal travel during a key on and engine off mode.

Graph Format (Scheme 142): Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the linear lines showing the transformation of values to the line graph. This example would require smooth progressive accelerator pedal pressure with the key on and the engine off.

Scheme 141

Scheme 142

Peripheral Inputs

Some signals may require certain peripherals or auxiliary tools for diagnosis. These tools include the Auxiliary Adapter and Pressure/Vacuum Adapter. In some cases, these devices can be inserted into the measurement jacks of the scan tool or multimeter. For example, connecting an electronic fuel pressure gauge to monitor and record the fuel pressure voltage reading and capturing the data would help find the fault.

Comparing PCM Data

After the PCM values have been acquired, it is necessary to determine the fault area. Typically, it will require 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 (i.e., model, engine, transmission, etc.).

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 POWERTRAIN DTC CHARTS & DESCRIPTIONS - CNG, FLEX-FUEL & GASOLINE . For a description of fuel trim, refer to 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 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

1. 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.
2. 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).
3. Vacuum leaks would 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 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 are reset by resetting the keep alive memory (KAM). Refer to RESETTING THE KEEP ALIVE MEMORY (KAM) 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.

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

1. 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).
2. Fuel pressure regulator vacuum hose off (causes excessive fuel pressure at idle, system rich at idle airflows).
3. Fuel pressure regulator diaphragm ruptured (fuel leaking into intake manifold, system rich at lower airflows).
4. Fuel return line crimped/damaged (fuel pressure high, system rich at lower airflows).
5. Fuel injector leaks (injector delivers extra fuel).
6. EVAP canister purge valve leak (if canister is full of vapors, introduces extra fuel).
7. 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.

Basic circuit checks help to minimize pinpoint test steps by providing a procedure to diagnose harness faults associated with the Electronic Engine Control (EC) System. The following techniques provide helpful reminders for diagnosing open circuits (continuity), shorts to ground and shorts to power.

Note. The suspect circuit must be isolated before testing. When disconnecting any harness connector, always inspect for damaged or pushed out pins, corrosion and loose wires. Repair as necessary. The digital multimeter must be set to the correct scale. The techniques do not apply in all situations, therefore, it is necessary to follow each pinpoint test step accurately and completely. General resistance and voltage values are specified below. Always use the pinpoint test values if they differ. Always turn the key to the OFF position unless directed otherwise by the pinpoint test.

Each of the following procedures will require the powertrain control module (PCM) and component to be disconnected to isolate the harness.

Open Circuit (Continuity)

Disconnect PCM. Measure the harness resistance between the suspect circuit at the harness connector and the appropriate PCM harness connector pin or PCM breakout box (if available). The resistance must be less than 5.0 ohms.

Shorts to Ground

Measure the harness resistance between the suspect circuit at the harness connector and a reliable ground (B-, chassis gnd or PWR GND at the PCM breakout box, if available). The resistance must be greater than 10,000 ohms.

Shorts to Power

Key ON to power up circuit. Measure voltage between the suspect circuit at the harness connector and a reliable ground. The voltage must be less than 1.0 volt.

See also:
• REFERENCE VALUES - CNG, FLEX-FUEL & GASOLINE
• POWERTRAIN DTC CHARTS & DESCRIPTIONS - CNG, FLEX-FUEL & GASOLINE
• FUEL TRIM
• DRIVE CYCLES