An air leak into the intake manifold will not cause any change in the air/fuel ratio since the air leak is seen by the ECM the same as opening the throttle blade a little. The speed density system plays by some different rules. The lambda sensor that is mounted in the exhaust stream will see this extra oxygen and cause a positive trend to the fuel trim. This is because the air leak is allowing air to enter the engine without being first measured by the MAF sensor (also referred to as “unmetered air”). However, air leaks that happen any place between the engine side of the MAF and the intake valves will cause the system to run lean. The EGR acts as cylinder filler, and the air that is entering the engine has the proper amount of fuel injected to keep the air/fuel ratio correct. Things like restricted air intakes, restricted exhaust, and EGR flow will not affect the air/fuel ratio. It uses a manifold absolute pressure sensor, engine volume, and throttle position to calculate the weight of the air flow into the engine. The speed density method is a little more complicated. The Mass Air Flow (MAF) sensor measures the weight of the air entering the engine, and the ECM controls the fuel injectors to achieve the correct air/fuel ratio. There are two different methods used today: Since the gasoline engine is an air pump, it’s easiest to start with the calculation of the air mass. To be able to calculate the air/fuel ratio, the ECM needs a starting place. All of these sensors input the data into the Engine Control Module (ECM), which processes the data and in turn operates the fuel injectors.
To accomplish this small feat, the fuel control system will use sensors like engine coolant temperature, intake air temperature, throttle position, manifold absolute pressure, mass air flow and lambda sensors in the exhaust. The main object of fuel control is to control the air/fuel ratio so the engine will run smooth, efficient and produce good power all while producing a clean flow of exhaust gasses.
Another benefit of graphing data is you have a record of the broken vehicle, and you are also able to use the same test data to verify the problem was fixed properly.
It has been said that a picture is worth a thousand words, and in this case, that saying has never been truer. While I am talking about scan tools, I do recommend you use a scan tool that will graph the information since the trend of the data can be as important as the numbers as we troubleshoot drivability problems. About 10 years ago, I was introduced to fuel trim data, and the power of the scan tool.īefore I start this discussion, I want to go over a little of the theory of fuel trim, where it comes from, what is it doing, and what it will tell us as we graph out the data on the scan tool. My impression of this data was, “It is meaningless.” I think this opinion was formed from lack of information and lack of training. Thinking back over the years, my first encounter with fuel trim information was the General Motors block learn and integrator. Editor's Note: This article has been updated from its original 2012 version.