Short Term Trims Not Agreeing---Advance Tech Needed
06-08-2007, 10:59 PM
#11
3rd Gear Member
Join Date: Mar 2004
Location: Gettysburg,PA
Posts: 582
RE: Short Term Trims Not Agreeing---Advance Tech Needed
I forgot to add the possible causes of this are a fault in a coil,a spark plug,an injector,a circuitry problem(either to the injector or the coil),and sometimes an intake leak.Has your check engine light ever came one while its been doing this?
06-08-2007, 11:00 PM
#12
3rd Gear Member
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Join Date: Nov 2006
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Posts: 641
RE: Short Term Trims Not Agreeing---Advance Tech Needed
ORIGINAL: 99mustanggt
Although you've already ruled out the 02's,I just wanted to add that during open loop your 02 sensors are'nt even being looked at by the pcm,thats why when you get into closed loop your stft's even out because the pcm is looking at your 02's and realizing the problem.
Although you've already ruled out the 02's,I just wanted to add that during open loop your 02 sensors are'nt even being looked at by the pcm,thats why when you get into closed loop your stft's even out because the pcm is looking at your 02's and realizing the problem.
06-09-2007, 04:33 AM
#13
TECH SAVANT
Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
RE: Short Term Trims Not Agreeing---Advance Tech Needed
Here's the whole blurb from the shop manual re: fuel trim. The rear sensors are not an input to this system, their only function is to monitor catalyst efficientcy...[/align][/align]
[/align]Fuel Trim [/align]Short Term Fuel Trim
If the oxygen sensors are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7 to 1 for gasoline), the PCM goes into closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy must constantly adjust the desired air/fuel ratio rich and lean to get the oxygen sensor to "switch"around the stoichiometric point.
If the time between switches are the same, then the system is actually operating at stoichiometry. The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometry is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%. Some calibrations will have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometry. This practice is referred to as using "bias". For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce NOx.
Values for SHRTFT1 and 2 may change a great deal on a scan tool when the engine is operated at different rpm and load points. This is because SHRTFT1 and 2 will react to fuel delivery variability that can change as a function of engine rpm and load. Short term fuel trim values are not retained after the engine is turned off.
Long Term Fuel Trim
While the engine is operating in closed loop fuel, the short term fuel trim corrections can be "learned" by the PCM as long term fuel trim (LONGFT1 and 2) corrections. These corrections are stored in Keep Alive Memory (KAM) in tables that are referenced by engine speed and load (and by bank for engines with two HO2S sensors forward of the catalyst).
Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:
[*]Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.[*]Long term fuel trim corrections can be used both while in open loop and closed loop modes. [/ul]
Long term fuel trim is represented as a percentage, just like short term fuel trim, however it is not a single parameter. There is a separate long term fuel trim value that is used for each rpm/load point of engine operation. Long term fuel trim corrections may change depending on the operating conditions of the engine (rpm and load), ambient air temperature and fuel quality (% alcohol, oxygenates, etc.).
When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different rpm and load points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at that rpm/load point.
If the oxygen sensors are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7 to 1 for gasoline), the PCM goes into closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy must constantly adjust the desired air/fuel ratio rich and lean to get the oxygen sensor to "switch"around the stoichiometric point.
If the time between switches are the same, then the system is actually operating at stoichiometry. The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometry is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%. Some calibrations will have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometry. This practice is referred to as using "bias". For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce NOx.
Values for SHRTFT1 and 2 may change a great deal on a scan tool when the engine is operated at different rpm and load points. This is because SHRTFT1 and 2 will react to fuel delivery variability that can change as a function of engine rpm and load. Short term fuel trim values are not retained after the engine is turned off.
Long Term Fuel Trim
While the engine is operating in closed loop fuel, the short term fuel trim corrections can be "learned" by the PCM as long term fuel trim (LONGFT1 and 2) corrections. These corrections are stored in Keep Alive Memory (KAM) in tables that are referenced by engine speed and load (and by bank for engines with two HO2S sensors forward of the catalyst).
Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:
[*]Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.[*]Long term fuel trim corrections can be used both while in open loop and closed loop modes. [/ul]
Long term fuel trim is represented as a percentage, just like short term fuel trim, however it is not a single parameter. There is a separate long term fuel trim value that is used for each rpm/load point of engine operation. Long term fuel trim corrections may change depending on the operating conditions of the engine (rpm and load), ambient air temperature and fuel quality (% alcohol, oxygenates, etc.).
When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different rpm and load points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at that rpm/load point.
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