Photo 1: The metal film resistor pictured at the center of this photo is the heart of the hot-wire MAF sensor.
Troubleshooting mass air flow (MAF) sensor problems can
become a major headache for diagnostic technicians because the failure is
usually one that involves a calibration error rather than an outright
electrical or mechanical failure. Because calibration errors tend to be
“gray-area” types of problems, let’s begin by looking at the basics of how a
“hot-wire” MAF sensor measures airflow through an engine running at various
speeds and loads.
In general, a “hot wire” MAF sensor produces an input to the
PCM when a low-amperage electrical current is passed through a metal-film
resistor suspended in the MAF’s air stream. The amperage flow through the
resistor changes because the electrical resistance of the resistor is
responding to the cooling effects of rapidly moving air. This variation in
current flow, which is a base data input, is then translated into a voltage or
frequency signal that is sent as a secondary data input to the PCM. In some
applications, the PCM translates the base data input into a grams-per-second
(GPS) data line indicating the metric weight of the air flowing into the
engine. See Photo 1.
Some MAF sensors also include an intake air temperature
(IAT) sensor that helps the PCM calculate air density. In other MAF designs,
the IAT is located downstream from the MAF sensor. Although there are different
configurations of MAF sensors, most current hot-wire MAFs share the same basic
FACTORS AFFECTING CALIBRATION
“False air” leaking through cracks in the ducting that
connects the MAF to the throttle body is a common problem affecting MAF sensor
calibration. In most cases, false air leaks will cause low-speed stalling or
Contamination of a MAF’s resistors is, by far, the leading
cause of MAF calibration errors. Oil, dirt or even paper filaments detaching
themselves from poor-quality air filters can accumulate on the metal-film
resistors suspended in the intake air stream.
2: When not serviced correctly, some types of oiled-media aftermarket
air filters tend to contaminate hot-wire MAF sensors.
In most cases, contamination tends to insulate the
metal-film resistor from the air stream, which makes it run hotter than normal.
This generally forces the MAF to underestimate the engine’s intake airflow. On
the other hand, a large particle stuck on the resistor, such as an insect or
particle of vegetable chaff, can cause the metal-film resistor to radiate more
heat than it should, causing the MAF to overestimate intake airflow. See Photo
Turbulence in the intake air stream can also affect MAF
sensor calibration. For example, a reverse pulse wave in the intake duct caused
by a poorly seating intake valve or cylinder misfire can momentarily reverse
the airflow into the MAF. While diagnosing such problems are beyond the scope
of this text, remember that intake manifold tuning valve failures, valve-timing
problems, and restricted intake or exhaust systems usually reduce airflow
through the engine. See Photo 3.
Photo 3: The primary function of this MAF inlet screen is to reduce turbulence occurring in the intake air stream.
At the other end of the turbulence issue, remember that the
original equipment air filter and intake air box are specifically designed to
reduce turbulence into the MAF sensor assembly. Replacing the original
equipment air intake system with various customized intake systems can increase
air turbulence and, thereby, cause a calibration error resulting in an engine
It’s obvious that MAF calibration errors can be difficult to
diagnose because the PCM’s programmed diagnostic strategy often doesn’t have
enough data inputs from other sensors to rationalize or analyze MAF sensor
performance. Consequently, many master diagnostic technicians have devised a
number of diagnostic strategies that, in one form or another, can be used to
diagnose MAF calibration errors. With “indicative” saying “maybe” and
“definitive” saying “pass or fail,” I’ll give you my opinion of how effective
each method of analyzing MAF performance might be.
GRAMS PER SECOND
Off the top, I’ll say that a few entry-level Asian
nameplates use the GPS method to analyze MAF performance. For example, an OE
procedure might include GPS readings at idle, at 1,500 rpm, and perhaps at
2,500 rpm, to analyze MAF performance. Some aftermarket trainers have also
suggested that the GPS number on a good MAF will equal the engine’s
displacement in liters at idle speed. In other words, 3 GPS at idle would be
correct on a 3L engine. According to my own experience, grams-per-second is an
indicative, rather than definitive, analysis of MAF performance.
VOLTAGE AND FREQUENCY
Voltage tests are similarly more indicative rather than
definitive. In many applications, idle speed voltages should hover around 0.7
volts. The “air gulp” test uses a labscope to display the MAF output voltage
increase during a snap-throttle test, and should show the voltage rising from
about 0.7 to a higher voltage that’s dependent upon the condition of the MAF
and of the engine and exhaust system. In my opinion, voltage and frequency
testing yields indicative rather than definitive results when attempting to
diagnose MAF calibration problems.
Volumetric efficiency (VE) testing assumes that the
indicated GPS should agree with the calculated air volume or GPS flowing
through the engine. A metric VE calculator using grams-per-second airflow can
be located by using an Internet search engine. Keep in mind that VE calculators
applied to naturally aspirated, stock engines generally produce definitive
Some on-board diagnostic systems display a calculated
barometric pressure value and/or a calculated load value. Calculated load is a
value produced by the PCM by rationalizing inputs from (among others) the MAF,
engine speed and throttle position sensors.
Photo 4: A calculated load of 55% at 43 mph cruising speed could be considered normal for this 2002 Toyota 4Runner.
While calculated load varies among
most vehicles, the calculated load at wide-open throttle at higher engine
speeds should be at least 80%. If recorded calculated load can be compared with
an identical engine configuration, so much the better. Since it’s less likely
that the throttle position and engine speed inputs are faulty, calculated load
values of less than 80% are indicative of a calibration problem with the MAF
sensor or of a restriction in the engine’s fuel
, air intake or exhaust system.
See Photo 4
Some applications also use the MAF, TP and engine speed
inputs to estimate barometric pressure (BARO). The BARO value might be
expressed as a frequency (Hz) or as inches of mercury (“Hg). In any case, if
the recorded barometric pressure isn’t equal to local barometric pressure, it’s
indicative of a calibration problem with the MAF sensor or a restriction in the
engine’s intake or exhaust system.
BASIC FUEL TRIM ANALYSIS
Engineers program a “fuel map” into the PCM that indicates
the exact amount of fuel required to meet hundreds of different operating
conditions. Since this fuel map is monitored by the oxygen or air/fuel ratio
(AFR) sensors, fuel might need to be added or subtracted to bring the oxygen or
AFR sensors back to “center” or to a chemically correct stoichiometric value.
More fuel being added to the programmed fuel map value results in a positive
“fuel trim” number, while subtracted fuel results in a negative fuel trim
Most MAF calibration problems are indicated by the classic
P0171 and P0174 DTCs. Fuel trim analysis can be tricky because intake manifold
vacuum leaks and insufficient fuel supply can also set these DTCs. Negative
fuel trim numbers caused by minor intake vacuum leaks generally disappear under
heavy engine loads. Positive fuel trims caused by leaking fuel pressure
regulators and injectors also tend to disappear under increased engine loads.
5: Short-term fuel trims of 16-17% indicate that the PCM on this 2001
Mazda is adding fuel to maintain a normal 14.7:1 air/fuel ratio.
Short-term fuel trims reflect the immediate demands of the
engine, while long-term fuel trims are an average of short-term fuel trims.
Fuel trims of plus or minus 10% are considered normal, while plus or minus 25%
will generally set an appropriate DTC. See Photo 5.
But negative fuel trims at higher engine loads can also be
caused by dirty MAF sensors or by low fuel pressures. Consequently, it’s
important to eliminate fuel delivery issues by testing fuel pump pressures and
volumes before assuming that the MAF sensor is truly defective. In most cases,
a defective MAF sensor will reveal itself through a combination of the
evaluation techniques mentioned above.
Last, it’s important to understand that the metal film
resistors generally lose their calibration due to normal wear and tear. While a
careful cleaning might restore a MAF sensor’s basic calibration, that level of
calibration might not pass one of the many different exhaust emissions tests
enforced throughout the U.S. So, if you’re in doubt, it’s best to always
replace a MAF sensor suffering from a suspected calibration defect.
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