short term fuel trim question cliff?
07-15-2010, 10:12 PM
#1
4th Gear Member
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Location: oregon
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short term fuel trim question cliff?
OK so ive been thinking for a bit since i rebuilt the motor that the car seems to be running pretty rich, i can smell the fuel. I know i changed stuff i just didnt think it would make a difference. In any case, i used my sct to watch my short term and long term, in both my factory setting and my 91 canned tune. Now im not sure i fully understand what the numbers mean as far as if they are voltage readings or what? But in stock short term was around .8-.9 idle, and would maybe break 1.0 on throttle blip. But in the canned tune, its .9-1.0 and between 1.0 and 1.1ish on blip. That too me seems to be a big difference in richness. So i cant turn it down even a little, because they locked everything. And my car runs nicer with the canned tune. But im trying to figure out, at this point, seeing the difference between the two, should i consider a dyno tune? also can someone explain this so i can understand how big the change they made is? Maybe cliffk and his hyrogliphic massivly in depth answers? hehe thanks
07-15-2010, 10:13 PM
#2
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Join Date: Oct 2006
Location: Texas
Posts: 10,551
do you have a wideband? if you dont, you need to stop screwing around with anything fuel...Its a canned tune, it will inherently be rich. Nobody will sell you an internet tune that is on the low 13.1 side of things.. your probably around 12.4-12.6.. which is about as save as you can get with a n/a setup.
Last edited by H0SS302; 07-15-2010 at 10:15 PM.
07-16-2010, 12:02 AM
#3
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im actually thinking it may be richer than 12.5...but then again, i dont really know. Im tempted to buy a wide band, but the more i get into tuning the more i realize that the sct is crap....whats the point of having the knowledge to improve things if i cant actually change the paramiters. And a wide band with a sniper set up is more money than i want to spend right now. I had no intention of taking fuel out without the right equipment anyway. I was more wondering based on those numbers and the little i mentioned, would it be worth it to tune the car. i mean i guess thats kinda a moot point since tuning a bolt on mustang probebly isnt worth the dollar per HP, but then again ive spent plenty on stuff that didnt do much.
07-16-2010, 06:58 AM
#4
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Location: Ohio
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an accurate tune is always worth it in my opinion...even on a bolt on car. If you do not want to spend the cabbages on a wideband and sniper, try to get in with a local tuner that will be willing to work with you as you continue to mod your car. A dyno tune will definitely improve things for you.
07-16-2010, 02:45 PM
#5
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Join Date: Dec 2006
Location: Saint Augustine, FL
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OK so ive been thinking for a bit since i rebuilt the motor that the car seems to be running pretty rich, i can smell the fuel. I know i changed stuff i just didnt think it would make a difference. In any case, i used my sct to watch my short term and long term, in both my factory setting and my 91 canned tune. Now im not sure i fully understand what the numbers mean as far as if they are voltage readings or what? But in stock short term was around .8-.9 idle, and would maybe break 1.0 on throttle blip. But in the canned tune, its .9-1.0 and between 1.0 and 1.1ish on blip. That too me seems to be a big difference in richness. So i cant turn it down even a little, because they locked everything. And my car runs nicer with the canned tune. But im trying to figure out, at this point, seeing the difference between the two, should i consider a dyno tune? also can someone explain this so i can understand how big the change they made is? Maybe cliffk and his hyrogliphic massivly in depth answers? hehe thanks
Here's the description of the Fuel Trim monitor, from the shop manual:
Fuel Trim
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:
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.
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.
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.
Edit:
It just struck me that internally, in the tune code, fuel trims are referenced as value from 0.0% to 100% with 50% being considered "0"--I.e. no correction needed. Delta Force's Recon application used to show them that way.
If the data logger you are using is doing this as well then 0.8 to 0.9 would be +30% (0.8 - 0.5) to +40% (0.9 - 0.5) trims--this would be REAL rich and lead me to believe that the front O2 sensors are stone cold dead, or something is really messed up in the tune...
Last edited by cliffyk; 07-16-2010 at 02:52 PM. Reason: revisit the issue...
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