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Ron, ASE Certified Technician
Category: Ford
Satisfied Customers: 35614
Experience:  23 years with Ford specializing in drivability and electrical and AC. Ford certs and ASE Certs
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please explain short and long term fuel trim. I have a 1997

Resolved Question:

please explain short and long term fuel trim. I have a 1997 Explorer with a 4.0L vin X -- Long term 25 % @ Idle/ short term between 2% and -2% @ Idle (both banks) Has code P0171 -- Has new intake gaskets/MAS .
Submitted: 8 years ago.
Category: Ford
Expert:  Ron replied 8 years ago.




Thank you for asking your question on JustAnswer.

The code P0171 is a lean code, if you sure both lower and upper intakes are sealing and you have no other leaks. I would suspect eaither a bad HO2S (bank 1) or a fuel pressure issue.


Long and Short Term Fuel Trim Short Term fuel trim is a PCM parameter identification (PID) used to indicate Short Term fuel adjustments. This parameter is expressed as a percentage and its range of authority is from -10% to +10%. Once the engine enters closed loop, if the PCM receives a HO2S signal that indicates the A/F mixture is richer than desired, it moves the SHRTFT command into a more negative range to correct for the rich condition.If the PCM detects the SHRTFT is adjusting for a rich condition for too long a time, the PCM will "learn" this fact, and move LONGFT into a negative range to compensate so that SHRTFT can return to a value close to 0%. Once a change occurs to LONGFT or SHRTFT, the PCM adds a correction factor to the injector pulsewidth calculation to adjust for any variations. If the change is too large, the PCM will detect a fault.
Note: If a fuel injector or fuel pressure regulator is replaced, do a PCM Reset and then drive the vehicle through the Fuel System Monitor drive pattern to reset the Fuel Control table in the PCM.
Help for Fuel Trim Trouble Codes (P0171, P0172, P0174 & P0175) The Fuel Trim readings on a Scan Tool do not always indicate a problem by themselves. However, Fuel Trim readings can be a real asset when attempting to determine the cause of one of these trouble codes, and to determine "where to start testing" to find the cause of an Air/Fuel mixture problem or Fuel System control related fault (i.e., related to a fuel control sensor or solenoid that may have failed).One of the first steps is to determine if the PCM is in "control" of the fuel delivery system. There are many different methods that can be used to make this determination. If the PCM is not in "control" of the fuel delivery system, the repair table below can help find the problem.
Fuel Trim Repair Table

HO2S SignalTailpipe EmissionsFuel Trim ValueLogical First Step
Rich (high O2S voltage)Rich A/F MixturePCM adding fuel(+%)Check PCM power and ground circuits for faults
Rich (high O2S voltage)Rich A/F MixturePCM subtracting fuel(-%)High fuel pressure, leaking injector(s), intake is restricted, plugged exhaust (MAP sensor)
Rich (high O2S voltage)Lean A/F MixturePCM adding fuel(+%)Irrational result - more than 1 fault is present!
Rich (high O2S voltage)Lean A/F MixturePCM subtracting fuel(-%)HO2S is contaminated, or the HO2S Heater power circuit is shorted to the HO2S signal
Lean (low O2S voltage)Rich A/F MixturePCM adding fuel(+%)Exhaust leaks in the manifold or pipes, air injection leaks in front of the HO2S or the HO2S is contaminated or has failed
Lean (low O2S voltage)Rich A/F MixturePCM adding fuel (+%)Irrational result - more than 1 fault is present!
Lean (Low O2S voltage)Lean A/F MixturePCM subtracting fuel (-%)Low fuel pressure, clogged fuel injectors, large vacuum leak, MAF sensor is contaminated
Lean (Low O2S voltage)Lean A/F MixturePCM subtracting fuel(-%)Check PCM power and ground circuits for faults
Lean or Rich (low or high O2S volts)Lean or Rich A/F Mixture0% Short Term Fuel Trim readingEngine operating in Open Loop mode or Limp-In mode - check for any stored trouble codes
MIL Operation, How to Clear Memory Codes This Monitor is a two-trip monitor that runs continuously whenever the engine is running in closed loop. The first time the PCM detects a Fuel Trim fault; the PCM sets a "pending" code and records the current vehicle conditions in Freeze Frame. If the same fault is detected on consecutive trips, the PCM turns on the MIL and sets a diagnostic trouble code.This OBD II Monitor uses a unique set of rules to turn off the MIL. The engine must be running with the engine speed and load in a similar range to when the trouble code first set before the PCM will retest the same fault. If this Monitor passes for (3) consecutive trips, the MIL is turned off. If it passes for 40 warmup cycles under "similar conditions", the PCM will clear the code. If the engine does not run under these "similar conditions", the code will be cleared after 80 warmup cycles if the same fault does not reappear.Fuel System Monitor Repair Verification An example of how to drive a vehicle to verify the repair of a trouble code related to the Fuel System Monitor on this vehicle application is shown in the Graphic below.

Expert:  Ron replied 8 years ago.



Thank you for asking your question on JustAnswer.


Here is a Ford TSB that deals with the code P0171 which is a bad Bank 1 HO2S.


Download here


Customer: replied 8 years ago.
what are the normal fuel trims at idel
Expert:  Ron replied 8 years ago.
Short fuel trim will run in the low negatives and long fuel trim will vary . If you long fuel trim is running 25 percent , this would indicate you hav a rich condition. In most case on the 1997 this is caused by leaking intake gaskets.
Customer: replied 8 years ago.

it has new intake gaskets/Mas meter/fuel pressure in specs/responces to forced lean and forced rich/O2 cross counts good/ Have you ever nad to replace a upper manifold?

thank you


Expert:  Ron replied 8 years ago.
No in most case replacing the lower intake o rings does the trick. This sounds like your gasket is leaking.This engine typically leaks during cold engine operation only. Once the engine has reached normal operating temperature, the gaskets swell and the leak is gone.
Expert:  Ron replied 8 years ago.



Thank you for asking your question on JustAnswer.

Check the Barometric Pressure (BARO) reading on the scan tool. A typical reading at sea level elevations should indicate 155-159 Hz and decrease 3 Hz for every 1000 feet gained in altitude.

If the BARO reading is low, clean or replace the Mass Air Flow (MAF) sensing element and drain PCM adaptive memory. Then recheck the BARO reading when turning on the ignition switch before starting the engine to verify it is indicating a normal value.

Road test the vehicle and perform a few brief Wide Open Throttle (WOT) accelerations to determine if the BARO reading drops or stays high indicating if there is still an issue present.

If the BARO reading shows a normal value, monitor the Short Term Fuel Trim (SFT) readings, during COLD engine operation after closed loop is achieved, to verify the engine is running lean. Nurse the engine fuel around the intake gaskets to see if SFT readings can be driven down. Normal SFT readings will indicate single digit numbers and have a tendency to stay close to 0% bouncing from a negative to a positive value.

If a leak is present, the SFT value will increase to positive double digit values on both banks at the same time. When the trim number is XXXXX attempt to isolate the leak by nursing added fuel around the intake gasket surfaces and lines of the intake manifold. At any point the SFT value severely decreases potentially to a negative reading, the leak has been found.



Customer: replied 8 years ago.
ok we will try this/this one is kicking our butts/we have done the pcm readapt/but the LTFT goes to +25% with in a mineut and stays there/lowers to 0 off idle/STFT goes up off idle
Expert:  Ron replied 8 years ago.
Ok let me know how it goes.
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