Service Repair | |||
Short-Term Fuel Trim and Long-Term Fuel Trim Because traditional O2 or lambda sensors cannot actually measure the residual oxygen content of the exhaust gases, but rather "switch" across 0.450v (or lambda equals 1.0) with the greatest voltage change occurring at this point, the ECM will try to get the sensor to do just that — switch. This is accomplished by making constant adjustments to the injectors' on time, or pulse width. This pulse width modification is done after all the base rpm and load corrections are made, and in reaction to the last commanded pulse width given. This is the Short-Term Fuel Trim PID (STFT) you see on your scan tool. When the O2 sensor is reporting lean, the ECM adjusts STFT "positive," or adds fuel. This addition causes the mixture to become richer and the oxygen content to be lower, which in turn causes the O2 sensor to "switch" to a higher voltage. This "switching" across 0.450v is what keeps the catalyst fed with its own lambda mixture, allowing the cat to do its job of keeping emissions under control.STFT is listed on OBD2 vehicles as a percentage modifier to injector pulse width. For example, a STFT of "5" indicates that the ECM is increasing pulse width by 5 percent. A STFT of "-5" indicates that the ECM is reducing pulse width by 5 percent. When monitored, you should see the STFT PID constantly moving from positive to negative numbers, with "0" being average. (Note: some ECM strategies may bias this center point just lean or rich of "0" for additional NOx control. Other set points are used to compensate for skewed readings caused by remaining combustible gases reacting with the O2 molecules at the sensor itself). If this is true, the ECM has established "fuel control," and the cat is happy. The amount that manufacturers will allow STFT to vary from "0" varies, but a general rule is no more than +/- 5 percent. As the engine components age and normal wear and tear set in, the original base map programmed into the ECM has to be modified. This occurs when the ECM sees that STFT has to stay positive or negative to maintain the O2 sensor switch across 0.450v. If STFT has to shift from +6 to +15, for example, it is taking additional fuel to keep lambda where it should be. The ECM sees this, and logs a more permanent correction to its injector pulse width calculations. That is noted on your scan tool as Long-Term Fuel Trim (LTFT). |
