In a recent conversation I was reminded of the challenges some technicians face when dealing with MAF sensor diagnosis. If you’re someone who deals with MAF DTC’s, or system too lean/rich codes on a regular basis, you’re likely well versed in the subject. However, if you only occasionally come across these DTC’s, are new to the automotive repair industry, or are someone who just likes to tinker, then you will want to read on. MAF sensor diagnosis is simple and easy.
Let’s begin by reviewing what the MAF sensor does. In the last article (click here for a refresher: http://modenook.com/auto-repair-in-boston/the-basics-of-no-start-diagnosis) we discussed the fact that the engine needs air, fuel, spark, and compression in order to run. Since the vehicle’s engine control computer (or powertrain control computer – PCM) needs to deliver the appropriate amount of fuel to go with the air the engine is taking in, it needs to know exactly how much air is moving through the intake system. The job of the MAF sensor is to tell the ECM/PCM how much air is moving through the intake. How does it do this?
The engine control system supplies the MAF sensor a specific level of current flow through a wire located in the intake airstream. Located next to this wire is a temperature sensor whose job it is to measure the temperature of the wire. As the throttle is open, and more air flows through the intake it carries heat away from the wire. The engine control system, through the temperature sensor, sees this drop in temperature of the wire and sends more current through the wire trying to keep the wire at a constant temperature. The ECM/PCM monitors this current to determine how much air is flowing into the engine. The more current required to keep the wire at a constant temperature, the more air that must be flowing. So what goes wrong?
Anything that can affect the temperature of the wire will affect how much air the ECM/PCM “thinks” it is seeing. If a piece of fuzz from an air filter were to attach itself to the sensing wire, this would serve to insulate the wire. The result might be that the ECM/PCM wouldn’t see a representative drop in wire temperature due to the insulation effect of the contamination as air flows past it. In this case, the engine would be receiving more air than the ECM/PCM thinks it is.
The engine control system will deliver an amount of fuel equal to what it thinks is sees for air (in our example too little fuel). The engine feedback system (O2 sensor) then sees lots of extra oxygen left over after combustion (remember: too little fuel combining with more oxygen than the computer can see will result in extra oxygen left over to be sensed by the O2 sensor) and decides it should deliver more fuel during the next injector pulse. The result is that fuel trims (the computer’s decision as to add or subtract fuel) will be skewed. In the case of too much oxygen in the exhaust being sensed, fuel trims will be climbing as the ECM/PCM adds fuel. So how is MAF sensor diagnosis simple?
The MAF sensor input is what the ECM/PCM uses to calculate how much load the engine is under. The more air, the more load is assumed. Volumetric efficiency is a term used to describe the comparison of how much air can actually fit in the engine compared to how much air can theoretically fit in the cylinders (based on cylinder dimensions). There are a number of things that can affect how much air is actually drawn into the engine: intake design, throttle plate opening, air filter, engine speed, valve opening speed, timing, and length, etc. The key to diagnosing MAF sensor problems is in determining the volumetric efficiency of the engine and determining if any variances are due to an engine mechanical condition, or bad reporting of how much air is flowing into the engine.
Some instructors will suggest that you to go online and find a VE calculator, plug in the values and get a number (expressed as a %). This is a valid way of determining if the volumetric efficiency is wrong. I contend that there’s an easier way. Your scan tool provides a data parameter (PID) called “calc load”, “engine load”, or “load” also expressed as a percentage. This is an engine load value that the computer calculates using MAF sensor input and pre-programmed values. It is essentially a VE number without having to do the extra work.
If volumetric efficiency (VE) is too high, or too low, then we have a problem with the mechanical portion of the engine, or a problem reporting airflow (i.e. MAF sensor). Normally aspirated engines generally produce an engine load value of 80% to 90%. The only time that the “load” PID is accurate in terms of measuring VE is when the throttle is fully open. During a test drive, open the throttle all the way and record the “load” value on the scan tool. If the value is higher, or lower, than the 80% to 90% range you’ve got a problem.
Now look at the Long Term Fuel Trim (LTFT) values. If the problem causing improper load calculation is mechanical then the value of air flowing into the engine will be measured correctly (the MAF sensor is just reporting what it sees). This means that the fuel delivered by the ECM/PCM will be correct. Fuel trims values will then be normal.
If the problem causing improper load calculation is from improper measuring of airflow into the engine, fuel trims will be “wrong” (i.e. too far positive or too far negative). Therefore, when diagnosing a bad MAF sensor, compare the “engine load” value to fuel trims. If fuel trims AND load values are wrong, you have a bad MAF sensor. If only the load value is wrong, you have a physical problem with the system.
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