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Short Term Fuel Trim And Long Term Fuel Trim

 The Purpose of Fuel Trim

The internal combustion engine is not 100% efficient. There will always be some byproducts of combustion such as HC, CO, O2, CO2, and NO. The least amount of leftover emissions occurs when the mixture is at 14.7 to 1 air fuel ratio. Even at this ratio, there are too many harmful pollutants.

Short Term Fuel Trim

Auto makers created a device that would clean up the byproducts of internal combustion. This device is called the catalytic converter. The catalytic converter works on the theory of oxidation and reduction. The term oxidation means to add oxygen. The term reduction means to take oxygen away. More specifically, oxidation means oxygen (O2) is added to the HC and CO to make it H20 and CO2. Reduction is when the oxygen is removed from the nitrogen (NOX) which reduces NOX to N2 and 02. The catalytic converter must perform both of these functions. It is necessary to utilize a fuel metering system that can continuously switch from rich to lean.

This fuel system is called the closed loop system. By switching the mixture rich and lean, the catalytic converter can efficiently perform oxidation and reduction at the appropriate times. When the mixture is slightly rich, reduction occurs and NOX is converted to N2 and 02. When slightly lean, oxidation occurs. CO is converted to CO2 and HC is converted to H20. The auto makers had this in mind when they created the O2 sensor. The job of the O2 sensor is to tell the PCM when the mixture is rich so the PCM can deliver a lean command (shorter pulse width.) Conversely when the mixture is lean, the PCM will deliver a richer mixture (longer pulse width). The PCM constantly changes the mixture from rich to lean based on the input from the O2 sensor. This process is what allows the catalytic converter to work. Note: when rich= reduction; when lean= oxidation.

This is where fuel trim comes in. Fuel trim is the internal mechanism within the PCM that switches the fuel mixture from rich to lean. It has to do this a minimum of 2 to 5 times a second. Fuel trim is broken down to two types, short term and long term. Since the standardization of OBDII, fuel trim is found on most cars.

Fuel Trim How It Works - Short Term

Short term fuel trim monitors the O2 sensor and makes live corrections based on O2 sensor voltage. It has the ability to quickly change the mixture when it sees low or hi  voltage from the O2 sensor. It can only correct over a short period of time. The starting point is 0. 0 represents the cleanest air fuel mixture 14.7 to 1. The objective of the PCM is to keep short term fuel trim averaging 0. Not right on 0 but an average of 0. An average of 0 would represent a slightly rich and slightly lean mixture. Therefore the average air fuel mixture would equal 14.7 to 1. While averaging 14.7 to 1 the combustion process is clean and the catalytic converter does not have to work hard.

Here is an example of how it works. You remove the vacuum line to the fuel pressure regulator. The fuel mixture will go rich. The O2 voltage will move high towards 900MV. Short term fuel trim will see the high voltage from the O2 sensor and command a lean correction. You will see short term fuel trim go to a number lower than 0. Negative numbers represent the PCM removing fuel from the system. If you create a vacuum leak, O2 voltage will go low. PCM will command a rich command. Short term fuel trim will move higher than 0 adding fuel. It is this correction that makes the O2 sensor go up and down under normal running conditions. Any voltage higher than 450MV from the O2 sensor is considered rich and below 450MV is considered lean.

This switching voltage from the O2 sensor is what gives us the cross count reading on our scan tools. Cross count represents the amount of times the voltage switches above and below the 450MV threshold in 1 second. It is the job of short term fuel trim to make this happen. When this happens a minimum of 2 to 5 times per second the catalytic converter will be most efficient.

Short term fuel trim has limited capabilities. It can only correct for short term problems. If the problem with the fuel system is a long term problem, PCM will have to make a correction for a longer period of time. Thus this is where long term fuel trim helps out.

Fuel Trim  How It Works - Long Term

Auto makers knew that over time cars would wear out. Injectors would leak, engines would suffer from compression loss, normal wear and tear would occur, and everybody drives differently. Trying to control the mixture with STFT would be insufficient. That’s where LTFT comes in. LTFT has the ability to correct the fuel over the long term. Its job is to monitor the short term fuel trim. It will move in either direction of 0 to lean or richen the mixture.

Here is an example of how it works. The fuel system return line has become restricted. The fuel pressure is 15 psi higher than specs allow. This has created a long term rich problem. STFT would have come way down to correct the problem, below 0. Since this is a long term fuel problem the STFT fails to control the mixture. O2 sensor voltage stays high. Since the O2 sensor voltage stays high STFT will remain low below 0. It will stay low because it will be continually trying to correct the rich mixture problem. Note the STFT only looks at the O2 sensor voltage and tries to correct anything above and below 450MV. With this example, the PCM will determine that a long term fuel correction is needed. The long term fuel trim’s job is to monitor short term fuel trim and keep it averaging 0. It has the ability to add or subtract large amounts of fuel over a long period of time. With this example, the LTFT will come down a notch at a time until it sees the STFT move back to an average of 0. With the STFT back to 0, the PCM has successfully corrected the fuel mixture. Once the LTFT has successfully corrected the mixture, PCM will lock this parameter in memory.

This memory is known as block cell. Also known as adaptive strategy. One thing to remember, if the PCM is successful in keeping the STFT averaging 0, the car would most likely pass the smog test. The only way you would know there is a problem is by the low number on LTFT. Another point to remember, if LTFT can’t make the correction than PCM will give up and set a rich code. Once it has set the code for rich, STFT will go back to base number of 0 and stay there in open loop condition.




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