SUCCESSFUL MAF SENSOR DIAGNOSIS

Motor,  Jul 2006  by Bell, Sam

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A broad range of seemingly unrelated or contradictory driveability complaints may arise from MAF sensor performance faults. Use this guide to navigate out of a diagnostic thicket or, better still, to avoid one entirely.

Once in a while we may encounter a total failure of a MAF sensor, one that is, perhaps, short circuited or internally open. Much more common, however, are failure modes in which the MAF sensor has become unreliable, underreporting or overreporting the true airflow into the engine. Indeed, as we shall see, many MAF sensor failures actually result in both underreporting and overreporting!

Before we get down to brass tacks, a brief review of the basics of MAF systems is in order. Fuel control systems for most modern gasoline engines are centered either on MAF or MAP (manifold absolute pressure). MAF systems, which, as their name suggests, measure the weight of incoming air and then meter the appropriate amount of fuel to ensure efficient combustion, are potentially more- precise, although MAP systems, which calculate have requirements based on engine load, have historically demonstrated greater reliability.

As you already know, combustion is most efficient when the ratio of air to fuel is approximately 14.7:1 by weight. Mass and weight are essentially synonymous in the presence of a sufficiently strong gravitational field such as the Earth's. Thus, knowing the weight of the air entering the engine allows the engine controller to meter the exact amount of fuel required to achieve efficient combustion. The controller commands the fuel injectors to open for an amount of time calculated to be sufficient to allow the correct weight of fuel to enter the engine, providing that the fuel's pressure is known. Fuel delivery is fine-tuned by applying fuel trim corrections derived from the closed-loop feedback of the oxygen sensor(s).

If the entire system is working as designed, fuel trim corrections, expressed as a percentage deviation from the base fuel delivery programming, will be within 10% (either positive or negative) of the programmed quantity. In the absence of a MAF-specific diagnostic trouble code (DTC), what would first lead us to even suspect that a faulty MAF sensor might underlie a particular drive-ability problem?

To function correctly, all of the air entering an engines combustion chambers must be "seen" by the MAF sensor. This means that any vacuum or air leak downstream of the sensor will result in insufficient fuel metering, causing a lean condition in open-loop operation and higher-than-normal fuel trim values in closed-loop. When we encounter a MAF sensor-equipped vehicle exhibiting these symptoms, we need to check for unmetered airflow first. Remember too, that unmetered airflow may not require an external air leak. An incorrectly applied or faulty PCV valve can result in incorrect MAF data where the PCV intake through the breather hose is upstream of the MAF.

So, the first two rules of MAF sensor diagnosis are:

1. Find and eliminate all external air or vaacuum leaks downstream of the MAF sensor. When in doubt, use a smoke machine, or lightly pressurize the intake manifold and spray with a soap & water solution.

2. Verify that the manufacturer-specilied PCV valve is correctlly installed and functioning as designed. (This is one instance where precautionary replacement may he cost-justified.)

Only after these two steps have been completed can you safely proceed with other diagnostics. The foremost due that the limit lies with the MAF sensor itself will be excessive fuel trim corrections, usually negative at idle, more or less normal in midrange operation and positive under high load conditions (see "How Contamination Affects Hot-Wire & Hot-Film MAF Sensors" on page 32).

While there are several distinct MAF sensor technologies ranging from hot-wire or hot-film to Karman vortex and Corialis sensors, and while MAF sensor outputs may take the form of variable frequency, variable current or a simple analog voltage, the diagnostic principles remain largely the same.

Let's start with Ford vehicles, for a couple of reasons. First, they are so widespread that most of us are familiar with them. Second, most MAF sensorequipped Ford products make use of a PID (Parameter IDentification) called BARO (barometric pressure). Up to 2001 models, this was an inferred, or calculated, value generated by the PCM (powertrain control module) in response to the maximum MAF flow rates observed on hard wide-open throttle (WOT) acceleration. Where this calculated BARO PID is available, it is of great diagnostic value, since it can confirm MAF sensor accuracy, if only under high flow rate conditions.

To use the BARO PID, yon must first know your approximate local barometric pressure. You might consult the BARO PID on a known-good MAP sensor-equipped vehicle. Alternatively, your local airport can provide this data. Do not rely on local weather stations, however, since these usually report a "corrected" barometric pressure. If weather information is the only available source, a rule of thumb is to subtract about 1 in. of mercury (1 in.Hg) for every 1000 ft. of elevation above sea level. This will yield a rough estimate of your actual local barometric pressure. For greater accuracy, yon can purchase a functional barometer for something less than $40. Compare this data with the BARO PID. A large discrepancy here-say, more than 2 in.Hg-should direct your suspicions toward the MAF.