The engine management system, led by the Engine Control Module (ECM), has one primary task: protect the catalytic converter. If the feed gasses entering the converter from the engine contain too much air or unburned fuel, cat temperature will begin to increase with the potential of permanently damaging the substrate.
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The ECM keeps an eye on its various subordinates by testing them and looking for a passing grade. Groupings of these tests, usually related to a system or component, are called monitors. Monitors are further divided into two types: continuous and non-continuous. Continuous monitors are the misfire monitor, the fuel system monitor and the comprehensive component monitor.
As the name implies, the continuous monitors run all the time, repeating over and over as long as the engine is running. All three keep an eye on engine systems and functions that could result in severe catalytic converter damage if a problem arises. For now, let’s focus on the fuel system monitor.
What We’re Monitoring
The fuel system monitor doesn’t monitor components like the fuel pump or fuel injectors as the name might lead you to believe. It monitors the feed gasses going to the converter by monitoring the amount of fuel fed to the engine. If the ECM detects that too much or too little fuel has been added, and correcting the situation is beyond its abilities, it will flag the appropriate system lean or system rich Diagnostic Trouble Code (DTC).
That seems like a fairly wide range, doesn’t it? We still have to consider emissions. To keep most three-way catalytic converters happy we need feed gasses in a much narrower lambda range, more like 1.000 ± 0.005. Outside of this range, NOx emissions begin to increase and further variance can lead to increased HC and CO emissions as well as potential converter damage.
How We’re Monitoring On most vehicles, fuel control is based on feedback from a conventional oxygen sensor. As Tony Martin writes in a separate feature in this issue, conventional oxygen sensors are really nothing of the kind. An oxygen switch is a more accurate description of how these sensors work. When feed gasses passing by the sensor are at lambda = 1, output voltage is about 0.450 volt. Vary just a bit on either side, and the sensor’s voltage shifts dramatically.The ECM makes its base fuel delivery calculations depending on the type of system in use (speed-density or mass airflow), then commands the injectors open. The feed gasses pass into the exhaust, past the sensor, which then shifts high or low depending on the amount of excess air in the exhaust. The resulting feedback voltage from the sensor lets the ECM know whether its initial calculation was too rich or too lean for the current rpm/load conditions.
The ECM then applies that information as a modifier to the base fuel calculation and attempts to correct enough to make the oxygen sensor voltage shift to the other extreme. If successful, the ECM will switch direction and if not, the ECM will continue its original correction until the voltage shift does occur.
These ECM corrections are what you see on your scan tool as STFT Parameter Identifiers (PIDs), STFT standing for Short Term Fuel Trim. Because the oxygen sensor cannot actually measure the excess oxygen content (or lack thereof), the ECM has to overcorrect in order to know where it is in relation to a lambda = 1 fuel delivery. Typically, STFT will alternate on your scan tool’s screen from +5 to -5.
Learning From STFT
Things change over time, and the ECM is capable of learning to adapt to normal engine wear and tear. If normal corrections don’t cause the expected shift in sensor voltage, the ECM will continue to correct in the same direction (adding or subtracting fuel) until it does. The excess amount of correction is learned by the ECM, and applied in the base fuel calculation as a more permanent means of controlling the feed gasses. This is the Long Term Fuel Trim (LTFT) PID you see on your scan tool.
As with STFT, LTFT numbers represent the percentage correction made. Unlike STFT, these numbers are stored and applied under similar rpm/load conditions. They are a learned correction, and are only modified when they are no longer sufficient enough to allow the ECM to remain in control. So don’t be surprised when you see LTFT numbers in the 10-15 (+ or -) range on your scan tool. As long as STFT is moving back and forth from -5 to +5, the ECM is in command and the cat is getting what it needs to stay healthy.
Out Of Control There are limits, however, on how much of a correction the ECM can make. It varies from manufacturer to manufacturer but if you startSystem Lean codes are caused when the air/fuel mixture has too much air or not enough fuel. System Rich codes are, of course, the opposite; too much fuel or too little air. Diagnosing the cause of either condition starts by checking the conditions under which the code(s) occurred. This information is found in the Freeze Frame records mandated by OBDII. OEM tools or aftermarket tools in “enhanced” mode often provide additional history records you might want to review. Whatever you do, do not clear the existing DTCs until you’ve completed your repair. You’ll wipe out these records as well.
Here are a few tips on diagnosing fuel system codes:
- Incoming air goes unmeasured any time it enters the engine downstream of a Mass Air Flow (MAF) sensor. Tears in the intake boot, cracked vacuum lines, and leaking intake manifold gaskets are all examples of “unmetered” air. Because the fuel charge is based on the measured air, the end result is a lean condition. This easily can be identified as the cause by running the engine at idle and again at 2,500 rpm while monitoring fuel trims (especially LTFT). The greater overall airflow at 2,500 rpm lessens the impact of the leak, and fuel trims will be more normal at the higher engine speed as a result. If Freeze Frame indicates the problem occurred at low speed, try this quick test to see if you’ll be hunting for an exterior leak.
- Contamination of the hot wire or even hot film MAF sensors can skew the accuracy of the sensor signal. Typically, contaminated MAF sensors will under report at idle (less air than is actually getting in) and over report at higher speeds (more air than is really getting in). A volumetric efficiency test is an excellent way to identify a lying MAF sensor.
- MAP sensors are used in speed-density systems to provide input to the ECM, which in turn, calculates the weight of the air entering the engine. Vacuum leaks will not cause fuel trim to go positive to correct. The ECM sees it as just a larger throttle opening, based on the change in the MAP signal.
- The smoke function of your EVAP system is an excellent way to locate vacuum leaks. Use it to pump the intake full of smoke and look for the trail. It is also a good way to check for air leaks in the exhaust. Air leaks upstream of the oxygen sensor can cause the sensor to react to a lean condition that doesn’t exist. And the heat shields used on many manifolds often make these cracks impossible to spot visually.
- Fuel system problems, like leaking injectors or failed fuel pressure regulators often cause System Rich codes. Don’t discount sensors that are lying to the ECM either, especially in a speed-density system.
That’s why the last step to any repair is verification the car is fixed. Use your scan tool, or a battery disconnect, to reset the LTFT corrections stored in the ECM’s Keep Alive Memory and test drive the car under the same conditions you found stored in the original Freeze Frame records. Record STFT and LTFT (for both banks if applicable) and review them in the shop to make sure you’ve returned control of the air/fuel mixture to the ECM.
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