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Old 02-19-2012, 05:57 PM   #1
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Long Term Fuel Trim

After adding long term fuel trim banks 1 and 2 to the parameters I normally display using my AutoTap scanner, I have seen both of these consistently in the range of +8 to + 13. This has been observed over the last several thousand miles. Unfortunately, I have not looked at them in the previous 65000 miles. I wonder if the Ultra power add on had any bearing on this. Performance seems pretty much ok with MPGs consistent around 7.0.

I had believed this parameter was supposed to be close to zero. The O2 sensors seem to be operating within their expected range and fuel pressure is at 55 to 62 psi

So what does the positive readings mean? Any one?

Marty
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Old 02-19-2012, 07:05 PM   #2
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It's not necessarily abnormal for fuel trims to be something other than zero (that's why the trim functionality is there, after all) but excessively rich fuel trims usually point to the O2 sensor seeing an abnormally lean A/F ratio. This can be due to vacuum leaks, or fuel pressure on the low side, etc. It can also be due to a bad O2 sensor of course but more often than not the O2 sensor is simply reporting the problem, not the cause of it.
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Old 02-22-2012, 11:04 AM   #3
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Like any control system, there are two basic fuel modes in the ECM: open loop and closed loop

In a nut shell, while running open loop the ECM just assumes it knows how much fuel is really going in and it assumes that accurate airflow data is being presented to it. It does not check these assumptions against anything. It just calculates how much fuel is needed to achieve a certain A/F ratio based on airflow data. It looks this target A/F ratio up in an open loop fuel table indexed by RPM and engine load.

In closed loop mode, the ECM checks itself by “watching” the front O2 sensor feedback. The factory front O2 sensor switches abruptly around 14.7:1 A/F ratio. The ECM tries to adjust fuel so that the O2 sensor swings back and forth around this target. To do that, the ECM has to have some way to vary its basic fuel calculation. It uses fuel trims for this purpose. Think of these variables as dials if you want. Turn the dial one way and the ECU adds fuel, turn the dial the other way and the ECM reduces fuel.

While running closed loop mode, the ECM uses what's called a “short term fuel trim” (STFT) variable for quick changes. This is the variable it uses to force the O2 sensor to cycle. In a perfect setup, this variable would swing from positive to negative values of about equal amplitude. As it's swinging up and down, the front O2 sensor signal should swing up and down. This is the basic closed loop mode of operation.

The ECM will also watch what's going on with the short term fuel trim variable. If conditions are right and the short term variable has stayed positive or negative long enough, the ECM will move the long term STFT pattern into the long term variable (LTFT). This is called a learning mode. Let's say instead of swinging around 0%, STFT is swinging up and down around -5%. The ECM will eventually take note of this and move that -5% adjustment into the appropriate LTFT variable and then zero out the STFT variable. Since both are factored in, the end result will be the same but STFT should now swing around 0%.

Basically, the closer your LTFT are to 0% learned, the less calculations the ECM has to make to adjust fueling to meet 14.7:1 A/F ratio. GM specifications call for a range of -10% to +10% on your LTFT, but we like to get them within +/-5% or closer to maximize fuel consumption.
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Old 02-22-2012, 12:40 PM   #4
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Thanks smiller and Jon.

Jon what would be the variables you can adjust to move LTFT?

Marty
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Old 02-22-2012, 12:51 PM   #5
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Marty,

There are a lot of variables that can adjust (or throw fuel trims out of whack).

Here are a few:

-Contained in the ECM programming there is a data table for the Mass Air Flow sensor that can be adjusted which will change fuel trim %. This is how we fine tune fuel trims.

-Fuel pressure can be adjusted at the regulator (03 & older 8.1L) to adjust fuel trims. A lower than spec fuel pressure will increase the positive fuel trims and a higher than spec pressure will decrease the fuel trims in a negative fashion. This would include a restriction in the fuel system causing low fuel pressure/low volume.

-A dirty/bad mass air flow sensor will greatly affect fuel trims.

-A weak/bad oxygen sensor will greatly affect fuel trims.

-An intake leak (vacuum leak) will increase the fuel trims in a positive direction at idle.

-An exhaust leak will affect fuel trims depending on how bad.

There are other things that can affect the fuel trims but those are the most common.
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