Posted by Matthew Yu on Thu, Jan 07, 2010 @ 07:03 PM
There are so many electrical and mechanical components in your vehicle these days that work together in order to maximize engine performance and to minimize emissions. The fuel mixtures are controlled by an electrical computer called the Fuel Trim (FT) that receives information about the amount of oxygen present in the engine's exhaust from the oxygen sensor and sends signals to your Powertrain Control Module (PCM). The PCM then interprets and processes this information and informs the fuel delivery system to output the amount of fuel accordingly into the combustion chamber. The target is to produce a stoichiometric air-fuel ratio such that no extra fuel (rich) or extra carbon monoxide (lean) is produced. Fuel Trim was originated from OBD II to control the amount of pollutants released from your exhaust pipe according to the standards set by the FDA. OBD II computer basically uses the Fuel Trim system to compensate for the engine problems by adjusting the amount of time the fuel is injected into the combustion chamber (Engine Piston).
Why would something as simple as your air-fuel mixture need to be monitored? The original "closed loop" system that previously sent information to the PCM from the oxygen sensor did not allow the PCM a quick enough response to the rich and lean voltage signal. If engines operated at the same speed, conditions, load, RPM, etc. then the "closed loop" setup would work fine. Since there are many other variables to consider in the operation of your vehicle the PCM must be able to make adjustments to meet these standards. Fuel Trim controls the amount of oxygen and fuel mixture needed in response to changing engine operating condition.
In OBD II there are two types of Fuel Trim:
- Short Term Fuel Trim (SFT)
- Long Term Fuel Trim (LFT)
Short Term Fuel Trim adjust the air-fuel ratio in response to brief errors that occur during normal operation. For example, the oxygen sensor will produce a lower voltage signal if the engine has a small vacuum leak. The PCM interprets this as a too lean air-fuel mixture and the injector pulse width is increased slightly to compensate. Long Term Fuel Trim is designed to add or subtract fuel over a longer period of time. If the vacuum leak in the previous example were to continue or grow larger, fuel delivery would have to be further increased and continued for a longer period of time. The typical FT values may range from -10 to 10. If the FT value is negative the air-fuel mixture is too rich, it needs to go lean by adding more oxygen; to convert hydrocarbons and carbon monoxide into carbon dioxide and water vapors. If the FT value is positive the air-fuel mixture is too lean, it needs to go rich by adding less oxygen; to reduce the amount of nitrogen oxide emissions into the environment. The target of the computer is to make set FT values to zero (perfect mixture).
Different makes/models terminology for Fuel Trim can vary, some examples are:
- In Honda, the function should be called "PCM Reset" under Engine
- In Toyota, it is called "ECM Reset" under "Engine"
- In BMW, it is called "Reset Adaptation" under "Service Functions" menu
- In Land Rover, it is called "Clear Adaptation" under "Engine"
- In GM, it is called "Fuel Trim Reset" under "Special functions" of the Engine
- In Ford, it is called "Reset Live Memory" under "Engine"
- In Chrysler, it is called "Clear Adaptation"
You can use a scan tool and read your vehicles current FT values under OBD II software then go into the manufactured specified software to reset the fuel trim in order to the computer to "relearn" its original values. See below for screenshots from the Autoboss V30 / PC Max for a BMW:
1) Select Diagnose
2) Choose "Service function"
3) Then select "Clear Adaptations"
Once you have cleared the adaptation, go back to confirm there are no longer any DTC's and you've completed the reset of your Fuel Trim!