1999 Ford Expedition check engine light on


Engine Performance problem
1999 Ford Expedition V8 Four Wheel Drive Automatic 100000 miles

codes P0171 & P0174 keep coming back

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Thursday, December 4th, 2008 AT 4:07 PM

2 Replies


A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (Use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (Try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)

* TIP: On many Fords, a P0171 and/or P0174 Lean Code may sometimes appear because of a bad Differential Pressure Sensor (DPFE). This sensor monitors EGR flow, and is located on the engine near the EGR valve. There are two hoses that connect the sensor to the tube that runs from the exhaust manifold to the EGR valve. The sensor misreads EGR flow and the computer increases EGR which has a leaning effect on the fuel mixture. The fix is to replace the DPFE sensor.

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Thursday, December 4th, 2008 AT 4:20 PM
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Basically after you have had the MAF sensor replaced you should disconnect battery for five minutes. So the vehicle loses it's KAM (keep alive memory) then complete the drive cycle relearn.

Reference Number(s): 98-23-10, Date of Issue: November 23, 1998

Related Ref Number(s): 98-23-10
Model(s): Ford: 1990-97 Thunderbird
1990-99 Mustang, Taurus SHO
1991-99 Crown Victoria, Escort,
1992-94 Tempo
1993-97 Probe
1995-99 Contour
Lincoln-Mercury: 1990-97 Cougar
1991-99 Continental, Grand Marquis,
Sable, Town Car, Tracer
1992-94 Topaz
1993-98 Mark VIII
1995-99 Mystique
Light Truck: 1990 Bronco II
1990-97 Aerostar
1990-99 Ranger
1991-99 Explorer
1994-96 Bronco
1994-97 F Super Duty, F-250 HD
1994-99 Econoline, F-150. F-250 LD,
1995-99 Windstar
1997-99 Expedition, Mountaineer
1998-99 Navigator
1999 F-250 HD,
Super Duty F Series
Bulletin No: 98-23-10
Date: November 23, 1998
This TSB article is a diagnostic procedure to address vehicles that exhibit lean driveability symptoms and may or may not have any Diagnostic Trouble Codes (DTCs) stored in memory.
Follow the diagnostic procedures described in the following SERVICE TIP. The revised diagnostic procedure is a more accurate means of diagnosing the symptoms.
MAF sensors can get contaminated from a variety of sources: dirt, oil, silicon, spider webs, potting compound from the sensor itself, etc. When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher air flows.
If vehicle is driven at Wide Open Throttle (WOT) or high loads, the fuel system normally goes open loop rich to provide maximum power. If the MAF sensor is contaminated, the fuel system will actually be lean because of under-estimated air flow. During open loop fuel operation, the vehicle applies Long Term Fuel Trim corrections that have been learned during closed loop operation. These corrections are often lean corrections learned at lower air flows. This combination of under-estimated air flow and lean fuel trim corrections can result in spark knock/detonation and lack of power concerns at WOT and high loads.
One of the indicators for diagnosing this condition is barometric pressure. Barometric pressure (BARO) is inferred by the Powertrain Control Module (PCM) software at part throttle and WOT (there is no actual BARO sensor on MAF-equipped vehicles, except for the 3.8L Supercharged engine). At high air flows, a contaminated MAF sensor will under-estimate air flow coming into the engine, hence the PCM infers that the vehicle is operating at a higher altitude. The BARO reading is stored in Keep Alive Memory (KAM) after it is updated. Other indicators are Long Term Fuel Trim and MAF voltage at idle.
NOTE:The following procedure may also be used to diagnose vehicles that do not have fuel system/HO2S sensor DTCs.

Lack of Power
Spark Knock/Detonation
Hesitation/Surge on Acceleration
Malfunction Indicator Lamp (MIL) Illuminated DTCs P0171, P0172, P0174, P0175 may be stored in memory
P0171, P0174 (Fuel system lean, Bank 1 or 2)
P0172, P0175 (Fuel system rich, Bank 1 or 2)
P1130, P1131, P1132, (HO2S11 lack of switching, Bank 1)
P1150, P1151, P1152, (HO2S21 lack of switching, Bank 2)
181, 189 (Fuel system lean, Bank 1 or 2)
179, 188 (Fuel system rich, Bank 1 or 2)
171, 172, 173 (HO2S11 lack of switching, Bank 1)
175, 176, 177 (HO2S21 lack of switching, Bank 2)
184, 185 (MAF higher/lower than expected)
186, 187 (Injector pulse width higher/lower than expected)
NOTE:Do not disconnect the battery. It will erase Keep Alive Memory and reset Long Term Fuel Trim and BARO to their starting/base values. The BARO Parameter Identification Display (PID) is used for this diagnostic procedure. All OBDII applications have this PID available. There are some OBDI vehicles that do not have the BARO PID, for these vehicles omit the BARO check and refer only to steps 2, 3, and 4 in the diagnostic procedure.

Look at the BARO PID. Refer to the BAROMETRIC PRESSURE REFERENCE TABLE in this article. At sea level, BARO should read about 159 Hz (29.91 in. Hg). As a reference, Denver, Colorado at 1524 meters (5000 ft.) Altitude should be about 144 Hz (24.88 in. Hg.). Normal learned BARO variability is up to +/-6 Hz (+/-2 in. Hg.). If BARO indicates a higher altitude than you are at (7 or more Hz lower than expected), you may have MAF contamination. If available, Service Bay Diagnostic System (SBDS) has a Manifold Absolute Pressure (MAP) sensor that can be used as a barometric pressure reference. Use "MAP/BARO" test under "Powertrain, " "Testers and Meters." Ignore the hookup screen. Connect GP2 to the reference MAP on the following screen.
NOTE:Remember that most weather services report a local barometric pressure that has been corrected to sea level. The BARO PID, on the other hand, reports the actual barometric pressure for the altitude the vehicle is being operated in. Local weather conditions (high and low pressure areas) will change the local barometric pressure by several inches of mercury (+/-3 Hz, +/-1 in. Hg.).

NOTE:BARO is updated only when the vehicle is at high throttle openings. Therefore, a vehicle which is driven down from a higher altitude may not have had an opportunity to update the BARO value in KAM. If you are not confident that BARO has been updated, perform three or four heavy, sustained accelerations at greater than half-throttle to allow BARO to update.

Barometric Pressure (in. Hg.)Barometric Pressure (kPa)BARO/MAP PID (Hz)Altitude Above Sea Level (ft)
30101.3158.90 (sea level

On a fully warmed up engine, look at Long Term Fuel Trim at idle, in Neutral, A/C off, (LONGFT1 and/or LONGFT2 PIDs). If it is more negative than -12%, the fuel system has learned lean corrections which may be due to the MAF sensor over-estimating air flow at idle. Note that both Banks 1 and 2 will exhibit negative corrections for 2-bank system. If only one bank of a 2-bank system has negative corrections, the MAF sensor is probably not contaminated.
On a fully warmed up engine, look at MAF voltage at idle, in Neutral, A/C off (MAF V PID). If it's 30% greater than the nominal MAF V voltage listed in the Powertrain Control/Emissions Diagnosis (PC/ED) Diagnostic Value Reference Charts for your vehicle, or greater than 1.1 volts as a rough guide, the MAF sensor is over-estimating air flow at idle.
If at least two of the previous three steps are true, proceed to disconnect the MAF sensor connector. This puts the vehicle into Failure Mode and Effects Management (FMEM). In FMEM mode, air flow is inferred by using rpm and throttle position instead of reading the MAF sensor. (In addition, the BARO value is reset to a base/unlearned value.) If the lean driveability symptoms go away, the MAF sensor is probably contaminated and should be replaced. If the lean driveability symptoms do not go away, go to the PC/ED Service Manual for the appropriate diagnostics.
NOTE:Due to increasingly stringent emission/OBDII requirements, it is possible for some vehicles with MAF sensor contamination to set fuel system DTCs and illuminate the MIL with no driveability concerns. Disconnecting the MAF on these vehicles will, therefore, produce no improvements in driveability. In these cases, if the BARO, LONGFT1, LONGFT2, and MAF V PIDs indicate that the maf is contaminated, proceed to replace the MAF sensor.

After replacing the MAF sensor, disconnect the vehicle battery (5 minutes, minimum) to reset KAM, or on newer vehicles, use the "KAM Reset" feature on the New Generation Star (NGS) Tester and verify that the lean driveability symptoms are gone.
WARRANTY STATUS: Information Only.
OASIS CODES: 206000, 610000, 610500, 610600, 610700, 611000,
611500, 612000, 612500, 614000, 614500, 614600.

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Thursday, December 4th, 2008 AT 4:20 PM

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