AfterMarketNews AfterMarketNews Auto Care Pro AutoCareCareerHub Brake&Frontend BodyShopBusiness Counterman EngineBuilder Fleet Equipment ImportCar Motorcycle & Powersports News Servicio Automotriz Shop Owner Tire Review Tech Shop Tomorrow's Tech Underhood Service Speedville

Remember the Filters During an AutomaticTransmission Fluid Evacuation Service

Fluid evacuation systems in the automatic transmission service industry are commonly being used in lieu of traditional transmission service methods. The use of this method to replace the transmission fluid is a clean, efficient and safe way to remove...

Read more...

Oldsmobile: Engine Overheats, Replaced Engine Coolant Thermostat

Model: 2003 Oldsmobile Alero Complaint The customer states the engine is overheating. Sometimes DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control will be present. Cause Observed the engine coolant temperature...

Read more...

Avoid Alternator Returns, Make Peace With Your Parts Suppliers

If the charging voltage is low, or the alternator isn’t putting out enough current to keep up with the electrical loads that are placed upon it, don’t automatically assume the starter or alternator is bad and needs to be replaced (unless you’re...

Read more...

Brake Job: 2010 Ford Fiesta

What’s so significant about the 2010-current Ford Fiesta brakes? First, the Ford Fiesta is one of the first sub-compacts on the market with the “Full Monty” of ABS/ESC/EBC/TC to come as standard. 2014-current models even have torque vectoring. Second,...

Read more...

10 Alignment Tips to Help You See Past The Angles

1. Talk to the driver. Always ask questions at the time the vehicle is written up. Find out why customers think they need an alignment. 2. Take notes. Nothing is worse than a repair order that just says “perform alignment” or “needs alignment.”...

Read more...

What to Know About Modern Air Ride Systems

GM, Ford, BMW and just about every other manufacturer has a 10-year-old vehicle on the road with an air ride suspension at the rear or at all four corners. These systems could be as exotic as a Land Rover Defender or as common as a Ford Explorer....

Read more...

Hyundai: P0715 Code Leads to Replaced Transmission Speed Sensor

Complaint The customer states the check engine light is on. Cause Connected a scan tool and found codes P0715 — Input/Turbine Speed Sensor “A” Circuit. At idle, allowed the vehicle to reach operating temperature, checked the transmission fluid...

Read more...

Vehicle Unit Increase; Import-Rich VIO Are A Boon for Import Specialists

Need a dose of good news for your business? Then hear this: The influx of new vehicles in the market will lay the foundation for bountiful aftermarket service and repair opportunities. And, with import vehicle sales continuing an ­upward climb, there...

Read more...

Honda: Engine Shuts Off, But Power Mode Stays In ‘On’ or ‘Accessory’

Applies To: Honda Insight models with one-push start Customers might complain that when they shift into park and shut off the engine, that the power mode stays in On or Accessory. Honda says the culprit could be a misadjusted shift cable. If the...

Read more...

Modern Fuel Control Diagnostics: Sorting Through the Datastream

When confronted with a fuel control problem on a modern vehicle, contributing writer Gary Goms normally uses short- and long-term fuel trims to determine if the problem is caused by one of many failures such as a vacuum leak or a leaking fuel pressure...

Read more...

Hunter Releases New Technical and Training-Specific Channel on YouTube

Hunter Engineering Company has launched a new technical and training channel on YouTube, called the Hunter Learning Channel. The Hunter Learning Channel features extensive training video playlists for every product category manufactured by Hunter....

Read more...

Ingersoll Rand Becomes ‘Official Power Tools of NASCAR’

NASCAR and Ingersoll Rand have announced a multi-year partnership designating Ingersoll Rand as the “Official Power Tools of NASCAR.” The multi-faceted agreement also designates Ingersoll Rand as an Official Partner of the International Motorsports...

Read more...
Home x-old ACPN Troubleshooting Mass Air Flow (MAF) Sensors

Print Print Email Email

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.

BASIC PRINCIPLES
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 operating principles.

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 rough-idling complaints.

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.

Photo 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 2.

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 performance complaint.

DIAGNOSTIC STRATEGIES
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.

VE TESTING
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 results.

CALCULATED VALUES
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 number.

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.

Photo 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. 

The following two tabs change content below.

Gary Goms

Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry. Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He also belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).
Latest articles from our other sites:

Remember the Filters During an AutomaticTransmission Fluid Evacuation Service

Fluid evacuation systems in the automatic transmission service industry are commonly being used in lieu of traditional transmission service methods. The use of this method to replace the transmission fluid...More

Oldsmobile: Engine Overheats, Replaced Engine Coolant Thermostat

Model: 2003 Oldsmobile Alero Complaint The customer states the engine is overheating. Sometimes DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control will be present. Cause Observed...More

Brake Job: 2010 Ford Fiesta

What’s so significant about the 2010-current Ford Fiesta brakes? First, the Ford Fiesta is one of the first sub-compacts on the market with the “Full Monty” of ABS/ESC/EBC/TC to come as standard....More

10 Alignment Tips to Help You See Past The Angles

1. Talk to the driver. Always ask questions at the time the vehicle is written up. Find out why customers think they need an alignment. 2. Take notes. Nothing is worse than a repair order that just...More

Hyundai: P0715 Code Leads to Replaced Transmission Speed Sensor

Complaint The customer states the check engine light is on. Cause Connected a scan tool and found codes P0715 — Input/Turbine Speed Sensor “A” Circuit. At idle, allowed the vehicle to reach...More

Vehicle Unit Increase; Import-Rich VIO Are A Boon for Import Specialists

Need a dose of good news for your business? Then hear this: The influx of new vehicles in the market will lay the foundation for bountiful aftermarket service and repair opportunities. And, with import...More

MAHLE Service Solutions Will Introduce New Touch Screen Displays 
And A/C Machines At MACS 2015 In Orlando

MAHLE Service Solutions showcases its new interactive touch screen displays and new A/C units for processing R1234yf and R134a refrigerants at the 2015 MACS (Mobile Air Conditioning Society) Training Event...More

The Equipment And Tool Institute Reports Record High Membership Level

The Equipment and Tool Institute is starting off 2015 strongly with a record high membership statistic of 100 Members comprised of 88 Full Members and 12 Associate Members. ETI’s reputation in the...More