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ATCAT
ATCAT, Master Technician
Category: Car
Satisfied Customers: 16
Experience:  ASE Master & Undercar Specialist GM Master Tech/ GM Diesel Tech Honda Silver Tech Foreign&Domestic
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short term fuel trim and long term fuel trim means what

Customer Question

short term fuel trim and long term fuel trim means what?
Submitted: 8 years ago.
Category: Car
Expert:  ATCAT replied 8 years ago.
Customer

Good question! There are a few types of fuel trim control. All operate bascially the same although descriptions vary from from product line to product line. This info is newer model GM specific and basic operation, but should help you to understand what fuel trim is and how it works.

 

Long Term Fuel Trim Description

 

The long term fuel trim consist of groups of fuel control calibrations or cells. Each cell contains the short and long term fuel injector control values required to maintain the correct air and fuel mixture for a particular long term engine operating condition. The control module monitors the short term fuel trim values in order to determine if the short term fuel trim values are indicating a trend of rich or lean corrections to the fuel control. If the control module determines that the short term fuel trim is able to maintain control and is near 0 percent (128 counts), the long term fuel trim within that particular cell will indicate a value of 0 percent (128 counts). If the control module determines that the fuel system is operating rich, long term fuel trim will compensate for the rich condition by decreasing the injector pulse width, and a long term fuel trim value of less than 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating lean, long term fuel trim will compensate for the lean condition by increasing the injector pulse width and a long term fuel trim value of more than 0 percent (128 counts) is indicated. If the control module determines that the long term fuel trim values indicate a trend toward a rich or a lean correction within that cell, the control module will switch to the appropriate cell, either rich or lean, and the short term value will return to 0 percent (128 counts).

 

 

Short Term Fuel Trim Description

 

The short term fuel trim is used by the control module in order to control the short term injector pulse width, or the injector ON time. When the control module is in the Closed Loop mode, the control module monitors the fuel trim oxygen sensors signals in order to determine if the fuel system is operating rich or lean. If the control module determines that no change in fuel delivery is needed in order to maintain correct short term fuel control for a particular engine operating condition, a short term fuel trim scan tool data value of 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating rich, short term fuel trim will compensate for the rich condition by decreasing the injector pulse width and a short term fuel trim value of less than 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating lean, short term fuel trim will compensate for the lean condition by increasing the injector pulse width and a short term fuel trim value of more than 0 percent (128 counts) is indicated.

When the control module is in the power enrichment (PE) mode, the short term fuel trim value will indicate 0 percent (128 counts) in order to prevent the fuel trim from compensating for the commanded rich condition of the PE mode.