Too much or too little air can mess with today's systems

Jan. 1, 2020
All three types of monitors 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.

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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Secondary to protecting the converter from damage is the monitoring of all systems that could potentially cause emissions to increase should they not function as designed. Any problem the ECM discovers that could lead to more (than expected) trash escaping the exhaust pipe will trigger the Malfunction Indicator Lamp (MIL).

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).

Every tech learns early on about proper air/fuel ratio, that magic number where the engine works at its best. This ratio is expressed in terms of air mass (weight) to fuel mass, and for a gasoline engine, that magic number is 14.7 pounds of air for every pound of fuel. This is called the stoichiometric ratio and is different for different fuels (10 percent ethanol fuel, for example, is 13.85:1). It is the ratio that provides just the right amount of fuel to utilize all of the air drawn into the combustion chamber.The actual air supplied versus the theoretical supply needed for stoichiometric is called lambda (also known as the excess air factor), and is represented as 1.0 (meaning 1:1). A lambda less than one indicates that too
little air was supplied, and the mixture is rich. A lambda greater than one indicates the opposite — too much air was supplied, and the mixture is lean. A gasoline engine produces its best power when lambda is 0.85 to 0.95 but its best fuel economy at 1.10 to 1.20 (on manifold injection systems).

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.

What this means is that the ECM is adding 5 percent (+5) or subtracting 5 percent (-5) from that modified base calculation. It also means that the ECM is able to control fuel delivery to the engine, and protect the converter. If you see the STFT trending positive or negative, the ECM is correcting the fuel mixture. It will continue the direction of correction until it sees the oxygen sensor voltage shift in the opposite direction.

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.

If the engine is being fed a mixture that is too lean, for example, the first thing you’ll notice on your scan tool’s data screen is a positive trend in STFT. At the same time, you’ll see a positive trend in LTFT. This is the ECM learning how much of a permanent correction it’s going to take to regain control. Keep watching, and you’ll see STFT numbers getting smaller and once again beginning to alternate between -5 and +5 while LTFT begins to stabilize.

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 start
seeing LTFT numbers exceeding 20 to 25 (+ or -) and STFT is pegged as far as it can go in the same direction, it’s a good bet the ECM is having a hard time controlling those feed gasses. If the trend is positive, expect to find a System Lean code stored, and if it’s negative, look for a System Rich DTC.

System 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.
Keep in mind that the ECM doesn’t set codes lightly. It has to see the same fault occur under the same conditions twice in succession before it turns on the light and bothers the driver. The ECM also becomes your customer. It’s going to test your repair the first chance it gets, and if you didn’t fix the problem the ECM will darn sure let your customer know you missed by turning that annoying MIL right back on.

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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