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Discussion Starter #1
96 Bronco XLT 5.8

Error Codes: P0171 & P0174
System Too Lean Bank 1 & 2

To me all I can think is hey, this mean either my air intake is messed, or my fuel system is jacked.

So I'm going to start with cheap easy fixes:

1. Replace fuel filter.
2. Fuel injector cleaner.
3. Clean Mass Airflow Sensor.

Anyone have any other easy things I can do, or ways to check?
How does a truck go from running just fine to all of a sudden running lean, hmmm.
 

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Mine did the same thing. Check the MAF sensor and clean that. I even had to replace it. You can see corrosion on the wire sometimes and that means you should probably replace it .
 

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02 sensor after the cat... if my memory is correct.
 

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Discussion Starter #8
It fixed mine. I tried everything else and that ended up being it.
I hope some MAF Sensor cleaner will do the trick b4 i gotta buy a new one. Im still gonna try the other shit tho too incase my situation isnt the MAF, and prolly just good for me to replace the fuel filter its been awhile.
 

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Since both banks are reading lean it's probably not an oxygen sensor imo.
the 96 has 2 before the cat (L/R) and one after, could also be a leak in the exhaust system. mine would trip every now and then until i replaced the rear 02 sensor. i'm not saying its the issue, but needs to be checked along with fuel pressure and vacuum/exhaust leaks.
 

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yo,
see my reply @ http://www.fullsizebronco.com/forum/showthread.php?t=196661

and;
http://repairpal.com/OBD-II-Code-P0171-and-P0174

and;
Check PCV/ line for;
PCV hose or elbow collapsed PCV valve Info & pic, 1 port vs. 2 port for 5.0; "...That's a genuine Ford PCV valve. It's stock two port top (top of pic); makes it difficult to fit under the upper intake on an EFI 5.0 engine. Buy an aftermarket single port(lower in pic) and use the top on the Ford PCV..." miesk5 Note, often the rubber cap on the top port pops off

Source: by Sixlitre (Malcolm H, Eddie Bauer)


and;
Vacuum leak test Acronyms & gauge pic http://fullsizebronco.com/forum/showthread.php?t=206824&highlight=Vacuum+Leak+Test;+idling
see post #11
 

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the 96 has 2 before the cat (L/R) and one after, could also be a leak in the exhaust system. mine would trip every now and then until i replaced the rear 02 sensor. i'm not saying its the issue, but needs to be checked along with fuel pressure and vacuum/exhaust leaks.
That's correct, the codes posted by OP would be thrown by the two upstream sensors which monitor A/F ratio. So, if an oxygen sensor was at fault here, it would be the two upstream sensors. Both of them.

I'm casting my vote for unmetered air getting in via a vacuum leak or intake manifold leak.
 

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I had that error on my 96 EB and it ended up that the MAF was dirty. I sprayed it out liberally with MAF cleaner and the error has not come back.
I think I also had that code come up when my DPFE sensor went bad.
 

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I know I had this a long time ago on my previous 96. I just cant for the life of me remember what cause it. may have been my EVAP system, may have been my EGR stuck, may have been my PCV valve, may have been my brake booster leaking, may have been my vacuum line to my A/C switch, may have been my vacuum reservoir was damaged. I just cant remember but those are some things to check that you don't normally suspect
 

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yo,
Here is the text of the hot link I posted in my previous reply from 96 Bronco PCED/EVTM;
DTC P0171 - System to Lean (Bank 1); "... The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit.
For lean and rich DTCs:
Fuel system Excessive fuel pressure. ( FUEL PRESSURE SPECIFICATIONS; Engine Running 30-45 psi; Key ON Engine OFF 35-45 psi)
Leaking/contaminated fuel injectors.
Leaking fuel pressure regulator. (also, pull vacuum line from FPR; replace if aroma of gas or gas is in line; indicating diaphram is ruptured.)
Low fuel pressure or running out of fuel.
Vapor recovery system.
Induction system:
Air leaks after the MAF. Check MAF for contamination; see below)
Vacuum Leaks. (96 Bronco Normal Engine Vacuum at Idle (15-22 in-Hg)
PCV system.
Improperly seated engine oil dipstick.
EGR system: Leaking gasket. Stuck EGR valve. Leaking diaphragm or EVR. SEE BELOW for TESTING
Base Engine: Oil overfill. Cam timing. Cylinder compression. Exhaust leaks before or near the HO2Ss

Test & Overview, W/Differential Pressure Feedback (DPFE) Sensor
by Mike N at smpcorp.com via web.archive.org
;
MIESK5 NOTE: only 95 5.8L California models & all 96 have the DPFE Sensor instead of EGR Valve Position Sensor (EVP) & Trucks never used a Pressure Feedback Exhaust (PFE) Sensor
"...The first step is to perform a thorough visual inspection of the system. If the inspection reveals nothing obvious, then some diagnostics are in order. Install a vacuum gauge in the vacuum line between the EGR valve and the control solenoid. Next, run the Key On-Engine Running self test. At one point during the self test, the PCM will check the EGR system by applying vacuum to the EGR valve. It accomplishes this by grounding the EGR regulating solenoid at a duty cycle of approximately 30%. If you see that the vacuum gauge indicates a vacuum reading of a few inches or more, you can be sure that the PCM, wiring, vacuum lines, and the regulating solenoid are functional. If not, then you must investigate where the source of the problem lies. If you do get a vacuum signal, then, exit the self test. With the engine at idle, apply vacuum to the EGR valve with a vacuum pump. When you apply vacuum, you should notice a change in engine RPM, with the possibility that the engine may stall. If the engine RPM changes, then we know the EGR is working. If there are no changes, then either the EGR valve is not functioning, or, the EGR passages are blocked with carbon. If engine RPM does change, then our next step is to examine the PFE (Pressure Feedback) or DPFE (Differential Pressure Feedback) sensor. Some early systems use a single hose sensor called the PFE sensor, while others use the DPFE sensor. Both sensors have three wires. The three wires consist of a 5 volt reference (which is shared with other sensors), a sensor ground (which is also shared), and a dedicated signal or output wire which is connected to the PCM. The PFE sensor measures exhaust gas pressure, while the DPFE sensor measures the difference between the upstream pressure (exhaust side), and the downstream pressure (intake side). Both sides are separated by an orifice assembly which is a calibrated opening that exhaust gas flows through when the EGR valve opens (refer to diagram #1). The DPFE sensor should measure equal pressure on both sides with the EGR valve closed. This means that we can measure and compare the signal voltage at idle with the signal voltage at key on, engine not running. Under these two conditions, if we see a difference in voltage output, then the EGR valve must be open (normally it should not be open at idle, or, with the engine not running). When the EGR valve opens under normal conditions, the upstream pressure should be greater than the downstream pressure. If not, then one of the hoses of the sensor is either off or plugged, or the orifice is blocked. The signal of the DPFE sensor at idle ranges from .20 to .90 volts DC. The actual voltage will vary depending upon the vehicle. Any voltage reading that is out of that range at idle will usually set a fault code. A fault code will also set if the PCM does not see a sufficient voltage change when EGR operation is commanded. Testing of the DPFE sensor can be done by monitoring its output voltage while applying vacuum to the EGR valve. If you notice an RPM change, or, the engine stalls, and the output voltage does not change, then the sensor is suspect. This assumes that the reference voltage and ground circuits are good. Also, tap on the sensor LIGHTLY to see if the output voltage changes. If the output changes, then the sensor is suspect. Typical output voltage on a PFE sensor with "Key ON, Engine OFF", or, with no EGR command is 3.20 to 3.30 volts DC. The voltage signal should increase when vacuum is applied, and decrease when pressure is applied. The DPFE’s output voltage with "Key On, Engine Off" is between .30 and .60 volts DC. This figure is applicable to the sensor with the aluminum housing. Look for an output voltage that’s about .2 to .3 volts higher for the sensor with the black plastic housing. If you apply vacuum to the EGR valve with the engine at idle, you should notice a change in the sensor’s output voltage, as well as some change in the idle RPM. The common causes of sensor failure are contamination, and excessive exhaust backpressure. Contamination can be caused by exhaust by-products as well other problems such as coolant from a failed gasket. Moisture from condensation can also present problems. One final note about all EEC V OBD II systems: the EGR system will not set any codes as long as the ambient temperature sensor "sees" the temperature below 32°F. Under these conditions, it is likely that if condensation has occurred, it will freeze and create some type of problem..."

....
for the hand-held vacuum pump/gauge, if you don't have or can borrow one from pals, ck out AZ's or other stores' LOAN-A-TOOL® PROGRAM
Vacuum Pump, Gauge reads 0 to 30 inches of mercury. All aluminum construction with reverse pistol grip for easy one-handed operation. One 24 inch piece of clear plastic tubing included with pump. OEM27010.


MAF Contamination TSB 98-23-10 for 94-96 Source: by Ford via miesk5 at cc
ISSUE: 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.
ACTION: Follow the diagnostic procedures described in the following Service Tip. The revised diagnostic procedure is a more accurate means of diagnosing the symptoms.
SERVICE TIP MASS AIR FLOW (MAF) DISCUSSION
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.
Symptoms
Lack of Power
Spark Knock/Detonation
Buck/Jerk
Hesitation/Surge on Acceleration
Malfunction Indicator Lamp (MIL) Illuminated - DTCs P0171, P0172, P0174, P0175 may be stored in memory
OBDII DTCs
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)
OBDI DTCs
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.
1. Look at the BARO PID. Refer to the Barometric Pressure Reference Chart 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.
2. 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.
3. 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.
4. 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.
OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS: INFORMATION ONLY
OASIS CODES: 206000, 610000, 610500, 610600, 610700, 611000, 611500, 612000, 612500, 614000, 614500, 614600, 698298
-------------------------

Long shot is a bad DPFE Sensor
Testing in a 96 5.0; "...DPFE comes in as .15 VDC during KOEO test between REF & Ground. TOMCO says it should read 0.45 +/- 0.25 volts for the .55 volt for this original Aluminum type sensor My Ford PCED/EVTM is frozen, so I can't view Ford's +/- values. &, when pulling REF hose off, fine particles fell out of DPFE. So, figuring DPFE Sensor is bad according to Ref voltage and particles... Yes, your voltage of 0.15V on the DPFE line is a typical sign of a bad one. Try to obtain a Motorcraft replacement, as the retail part store pieces don't last..."
Source: by SigEpBlue (Steve) & miesk5 at FSB
 

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I got lean codes when the vacuum line for the vents broke. The other symptom was that the vents would switch to defrost on their own.
 
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