Once in a while we mayencounter a total fail-ure of a MAF sensor,one that is, perhaps,short circuited or inter-nally open. Much morecommon, however, are failure modes inwhich the MAF sensor has become un-reliable, underreporting or overreport-ing the true airflow into the engine. In-deed, as we shall see, many MAF sen-sor failures actually result in both un-derreporting and overreporting!

Before we get down to brass tacks, abrief review of the basics of MAF sys-tems is in order. Fuel control systemsfor most modern gasoline engines arecentered either on MAF or MAP (man-ifold absolute pressure). MAF systems,which, as their name suggests, measurethe weight of incoming air and thenmeter the appropriate amount of fuel toensure efficient combustion, are poten-tially more precise, although MAP sys-tems, which calculate fuel requirementsbased on engine load, have historicallydemonstrated greater reliability.

As you already know, combustion ismost efficient when the ratio of air tofuel is approximately 14.7:1 by weight.Mass and weight are essentially synony-mous in the presence of a sufficientlystrong gravitational field such as theEarths. Thus, knowing the weight ofthe air entering the engine allows theengine controller to meter the exactamount of fuel required to achieve effi-cient combustion. The controller com-mands the fuel injectors to open for anamount of time calculated to be suffi-cient to allow the correct weight of fuelto enter the engine, providing that thefuels pressure is known. Fuel delivery isfine-tuned by applying fuel trim correc-tions derived from the closed-loop feed-back of the oxygen sensor(s).

If the entire system is working as de-signed, fuel trim corrections, expressedas a percentage deviation from the basefuel delivery programming, will be with-in 10% (either positive or negative) ofthe programmed quantity. In the ab-sence of a MAF-specific diagnostic trou-ble code (DTC), what would first leadus to even suspect that a faulty MAFsensor might underlie a particular drive-ability problem?

To function correctly, all of the air

entering an engines combustion cham-bers must be seen by the MAF sen-sor. This means that any vacuum or airleak downstream of the sensor will re-sult in insufficient fuel metering, caus-ing a lean condition in open-loop opera-tion and higher-than-normal fuel trimvalues in closed-loop. When we en-counter a MAF sensor-equipped vehi-cle exhibiting these symptoms, we need

to check for unmetered airflow first.Remember, too, that unmetered airflowmay not require an external air leak. Anincorrectly applied or faulty PCV valvecan result in incorrect MAF data wherethe PCV intake through the breatherhose is upstream of the MAF.

So, the first two rules of MAF sensordiagnosis are:

1. Find and eliminate all external air

28 July 2006

MAF DIAGNOSIS

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SUCCESSFUL

MAF SENSORDIAGNOSIS

BY SAM BELL

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.

or vacuum leaks downstream of theMAF sensor. When in doubt, use asmoke machine, or lightly pressurizethe intake manifold and spray with asoap & water solution.

2. Verify that the manufacturer-speci-fied PCV valve is correctly installed andfunctioning as designed. (This is one in-stance where precautionary replace-ment may be cost-justified.)

Only after these two steps have beencompleted can you safely proceed withother diagnostics. The foremost cluethat the fault lies with the MAF sensoritself will be excessive fuel trim correc-tions, usually negative at idle, more orless normal in midrange operation andpositive under high load conditions (seeHow Contamination Affects Hot-Wire& Hot-Film MAF Sensors on page 32).

While there are several distinct MAFsensor technologies ranging from hot-wire or hot-film to Karman vortex andCorialis sensors, and while MAF sensoroutputs may take the form of variablefrequency, variable current or a simpleanalog voltage, the diagnostic principlesremain largely the same.

Lets start with Ford vehicles, for acouple of reasons. First, they are sowidespread that most of us are familiarwith them. Second, most MAF sensor-equipped Ford products make use of aPID (Parameter IDentification) calledBARO (barometric pressure). Up to2001 models, this was an inferred, orcalculated, value generated by the PCM(powertrain control module) in re-sponse to the maximum MAF flowrates observed on hard wide-openthrottle (WOT) acceleration. Wherethis calculated BARO PID is available,it is of great diagnostic value, since itcan confirm MAF sensor accuracy, ifonly under high flow rate conditions.

To use the BARO PID, you mustfirst know your approximate local baro-metric pressure. You might consult theBARO PID on a known-good MAPsensor-equipped vehicle. Alternatively,your local airport can provide this data.Do not rely on local weather stations,however, since these usually report acorrected barometric pressure. Ifweather information is the only avail-able source, a rule of thumb is to sub-tract about 1 in. of mercury (1 in./Hg)for every 1000 ft. of elevation above sealevel. This will yield a rough estimate ofyour actual local barometric pressure.For greater accuracy, you can purchasea functional barometer for somethingless than $40. Compare this data withthe BARO PID. A large discrepancyheresay, more than 2 in./Hgshoulddirect your suspicions toward the MAF.

Confirm your hypothesis as follows:First, make sure you have followed thesteps outlined in the two rules above.Next, record all freeze frame data andall DTCs, including pending DTCs. Ifthe OBD monitor readiness status foroxygen sensors shows READY, proceedto the next step. If it doesnt, refer tothe procedures in the following para-graph now. Next, perform a KAM(Keep Alive Memory) reset and drivethe vehicle. Make sure your test drive

29July 2006

includes at least three sustained WOTaccelerations. (Its not necessary tospeed to accomplish a sustained WOTacceleration. Rather than a WOT snapfrom idle, an uphill downshift at 20 to30 mph is usually sufficient. The WOTprescription can be met at throttleopenings as low as 50% to 70%.) TheBARO PID should update from its de-fault reading by the end of the thirdWOT acceleration. If its now close toyour local barometric pressure, theMAF sensor is not likely to be faulty. IfBARO is not close, try one of the clean-ing techniques explained in the sidebarKeeping It Clean on page 34, thenagain reset KAM and take a test drive. Ifthe BARO is still out of range, a replace-ment MAF sensor is in your customersfuture. Unfortunately, in many 2002 andlater Fords, the calculated BARO PID issupplanted by a direct BARO reading

taken from a sensor incorporated intothe ESM (EGR System Management)valve, greatly lessening its diagnostic val-ue for our current purposes.

If the oxygen sensor monitor statusshowed INCOMPLETE above, youllhave to verify O2 sensor accuracy andperformance before performing theKAM reset procedure. Use a 4- or 5-gasanalyzer to determine whether theair/fuel ratio is correct in closed-loopoperation. The notes about lambda ()below should help.

Outside of the Ford family, MAFsensor diagnosis is more difficult. Largefuel trim correctionseither positive ornegativeare often the only initialpointer to MAF sensor problems.Again, any and all air leaks downstreamof the MAF sensor must be repairedfirst. Since accurate fuel trim correc-tions depend on correct O2 sensor out-

puts, you must verify the functionalityof these sensors first. The easiest andfastest way to do this is by checkinglambda, a type of measure of the air/fuelratio. (For a detailed explanation, seemy article in the September 2005 issueof MOTOR.) If the O2 sensors are func-tioning correctly, lambda at idle shouldbe very nearly equal to 1.00 in closed-loop. You may wish to check this also at1500 to 1800 rpm to verify adequatemixture control off idle. Once lambda isfound to be correct, the O2 sensors areproven good. Then any fuel trim adjust-ments must result from unmetered orincorrectly metered airflow or from in-correct fuel delivery.

Distinguishing between fuel deliveryproblems and MAF sensor problemscan be very frustrating. Start by verify-ing fuel pressure and volume. (Thosewho rely on pressure alone may regret

30 July 2006

SUCCESSFUL MAF SENSOR DIAGNOSIS

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it.) Use your scan tool to record criticaldata PIDs and graph them for analysis.Here are a couple of examples:

In Fig. 1 on page 30, taken during aperiod of closed-loop operation, short-term fuel trims (blue and green traces)for each bank were above 13% at 1100rpm (red trace), yet dropped sharplynegative at 3600 rpm, proving that inade-quate fuel delivery was not the problem.The values indicated in the legend box-es correspond to the readings obtained

at the indicated cursor position (verticalblack line). The vertical white line indi-cates the trigger point for the recording.Subsequent diagnostics focused on theMAF sensor and the PCV system.

Take a look at the scan data graphshown in Fig. 2. It shows a car whosefaulty fuel pump was unable to deliversufficient fuel under high load condi-tions. Notice the very low O2 sensorreadings (displayed in blue) corre-sponding to the cursor (black vertical

line just to the right of the zero timestamp). Fuel pressure was within specat idle and at about 2000 rpm, but vol-ume was very low. The sudden drop-off in O2 activity in response to hardacceleration is a characteristic ob-served in many instances of MAF sen-sor faults as well.

Ultimately, known-good snapshots,waveforms and other data sets are in-valuable. Take a look at the scan snap-shot in Fig 3. Does it show good fueltrim and appropriate MAF sensorreadings?

Since total fuel trim stays well withinthe 0 10% range throughout thetrace, its a good bet that the MAF sen-sor is working well, at least under thesampled conditions.

How about the data set shown inFig. 4? In fact, the snapshot was takenduring open-loop, closed-throttle de-celeration when fuel was not being in-jected, so the O2 sensor PID makessense. Its actually a substituted defaultvalue inserted whenever the vehicle isin closed-throttle decel mode. Whatabout the reported MAP value? Areading of 4.00 in./Hg shows very highengine vacuum, which jibes with thereported TPS PID. The fuel trim datais within the usually accepted range of0 10%. Good data can come in a vari-ety of formats.

Of course, waveform captures fromyour scope are often all that are neededto confirm a faulty MAF sensor. In ourshop, weve found that a snap-throttleMAF test for Ford products should al-ways produce a peak voltage of at least3.8 volts DC. The snap-throttle test is

32 July 2006

SUCCESSFUL MAF SENSOR DIAGNOSIS

Fig. 5 Fig. 6

Hot-wire and hot-film MAF sen-sors calculate airflow based onmonitoring the current re-quired to maintain a constant tem-perature in the sensing element.When dirt accumulates, the addi-tional surface area allows greaterheat dissipation at low airflow rates.The dirt, however, also functions asan insulator, with an overall net re-sistance to heat transfer at very highairflow rates.

At idle and under relatively lowflow/load rate conditions where themajority of operation may takeplace, the surface area effect usual-ly predominates, causing a rich con-dition with fuel trim correctionsusually in the range of 10% to5%. At sustained high flow/loadrates, the insulative effect usuallytakes over, causing a lean mixtureneeding fuel trim corrections ashigh as +30%.

Worse still is a complex case of

mass confusion that may arise un-der hard acceleration when long-term negative fuel trim corrections,learned in closed-loop under low-flow-rate conditions, are appliedprecisely when positive fuel trim cor-rections would be more appropriate.So, for example, when the systemgoes to open-loop during hard accel-eration where the MAF is alreadyunderreporting airflow by up to30%, the PCM may subtract an addi-tional 10% to 15% (LTFT) from thenormal fuel delivery calculation,leaving the system as much as 45%leaner than desired!

In midrange operation, the twoeffects (surface area and insulativeproperties) may roughly cancel eachout, with fuel trims being more orless normal. Additionally, the exactchemistry and configuration of dirtbuildups can vary, changing the bal-ance of power between the surfacearea and insulative effects.

How Contamination Affects Hot-Wire &Hot-Film MAF Sensors

performed the same way asfor ignition analysis. The ideais not to race the engine, butsimply to open the throttleabruptly to allow a momen-tary surge of maximum air-flow as the intake manifoldgets suddenly filled with air.Its critical that the throttle beopened (and closed) as quick-ly as possible during this test.

The waveform in Fig. 5 onpage 32 is from a known-goodMAF sensor. Note the peakvoltage of 3.8 volts. The rapidrise and fall after the throttlewas first opened is normaland reflects the initial gulp ofair hitting the intake manifoldwalls and suddenly reaching maximumdensity, greatly reducing subsequentflow. The exact shape of the waveformmay vary from model to model, basedon intake manifold and air duct(snorkel) design.

Whats the relationship betweenMAF and engine speed? As Fig. 6shows, rpm and airflow rate track oneanother closely under the moderate ac-celeration conditions during which thisscreen capture was taken. The similarityof the shapes of the two traces shown

here suggests, but does not prove, thatthe MAF sensor is functioning well un-der these conditions. If the airflow re-port was consistently increased or de-creased by the same factor, say 10% oreven 50%, the shape of its graph wouldremain the same.

Consider the additional plots pre-sented in Fig. 7 above. Does the extradata shed any light on the MAF sensorsaccuracy? Or is this just an example oftoo much information?

Since short-term and long-term fuel

trims remain within singledigits throughout, we can bereasonably sure that the MAFsensor is functioning correctly.Do we really benefit fromlooking at the O2 sensor datahere? We could probably doalmost as well without it, sincewe have both STFT andLTFT, but the O2 trace (blue)serves as an additional cross-check on the validity of thefuel trim calculations. Moreimportantly, the O2 sensortrace proves both that an ap-propriately rich mixture wasobtained on hard accelerationand that applied fuel trimcorrections were effective

throughout the captured data set.I said at the outset that hard failures

were relatively rare, but they do occurfrom time to time, and I owe it to you todiscuss this type of failure as well as in-termittent failures. Open-circuited orshort-circuited MAF sensors usually seta trouble code, most frequently P0102or P0103 (low input and high input, re-spectively). P0100 is a nonspecific MAFsensor circuit fault, while P0104 indi-cates an intermittent circuit failure.Checking scan data is a vital first steptoward successful diagnosis of any ofthese codes. On pre-OBD II vehiclesespecially, unplugging a faulty MAFsensor will often restore a minimum de-gree of driveability as the PCM revertsto TPS, rpm and/or MAP as fuel deter-minants. Certain mid-80s GM vehicleswere notorious for intermittent MAFsensor failures. These usually could beeasily recreated by lightly tapping with asmall screwdriver on the MAF sensorhousing at idle. A noticeable stumbleoccurring with each tap clinches thecondemnation (Fig. 8, page 36).

Of course, backprobing the MAFsensor connector for voltage drops atboth the power and ground terminalsKOER is a required step before any fi-nal condemnation. The coincidence ofVBATT and MAF both showing 0.0volts cannot be ignored. Neither shouldthe mouse nest in the MAF, nor thegnawed wires throughout the enginecompartment.

Why is this a hard diagnosis? Conta-

34 July 2006

SUCCESSFUL MAF SENSOR DIAGNOSIS

Fig. 7

Most MAF sensor failures re-sult from contamination.Sometimes the dirt is visible,but more often its not. Technicianshave tried a variety of cleaners, withmixed success. Many use an aerosolbrake/electrical parts cleaner, wait-ing until the MAF sensor is cold. AFord trainer in my area swears bythe most popular consumer glasscleaner. Several top technicians re-port good results from steam clean-ing, while others prefer a spray in-duction cleaner.

The vast majority of technicianswarn that the MAF sensor may bedamaged by any type of cleaningwhere the electrical connector is notheld upright. This is particularly truewhere strong chemicals are used, asthey may pool and work their way

into the delicate electronic circuitry. To avoid future contamination, be

wary of oiled air filters or any thatappear likely to shed lint. Poor seal-ing of air filter housings may con-tribute to contamination. Neverspray an ill-fitting air filter with a sili-cone lubricant or sealer; such spraysare likely to render the MAF sensorinaccurate. If an engine produces ex-cessive blowby gases, these may con-taminate the MAF sensor, as well. Besure any specified filter breather ele-ment is installed. If none is specified,but oil accumulates in the air intakehousing, the MAF sensor or associat-ed intake ducts, be sure to investi-gate and remedy the cause to pre-vent repeat failures. Be sure to checkmanufacturers TSBs, the iATNarchives and other sources as well.

Keeping It Clean

minated MAF sensors oftenoverreport airflow at idle (re-sulting in a rich condition andnegative fuel trim corrections)while underreporting airflowunder load (resulting in a leancondition and positive fueltrim corrections).

This double whammy makesdiagnosis more difficult for anumber of reasons: First, manytechnicians incorrectly elimi-nate the MAF sensor as a po-tential culprit because they ex-pect it to show the same bias(either over- or underreport-ing) throughout its operating range. Sec-ond, a lack of a direct MAF fault DTC(such as P0100) is often mistaken tomean that the MAF sensor must begood. Third, the symptoms mimic(among other possibilities) those of a ve-hicle suffering from low fuel pump out-put coupled with slightly leaking injec-tors or an overly active canister purgesystem. Even sluggish, contaminated or

biased oxygen sensors may cause similarsymptoms. Without appropriate testing,its hard to distinguishjust by driv-ingamong certain ignition or knocksensor faults and MAF sensor malfunc-tions. Additionally, since MAF sensorsare somewhat pricey, many techniciansare afraid to condemn them, fearing ei-ther the customers or the boss wrath iftheir diagnosis is not borne out. Perhaps

the biggest obstacle is lack of acomprehensive database ofknown-good waveforms, volt-ages and scan data againstwhich to compare the suspect.

My own data set featuresknown-good scan data andscope captures made KOEO,at idle and on snap-throttle. Ingeneral, these three data pointsshould be sufficient to identifya faulty MAF sensor even be-fore it sets a fuel trim code.

A bad Bosch hot-wire MAFsensor may be the result of afailed burn-off circuit. Dont

simply replace the sensor; make surethe burn-off is functional. (The purposeof the burn-off is to clean the hot-wireof contaminants after each trip.) Burn-off is usually a key OFF function afterengine operation exceeding 2000 rpm.Burn-off circuit faults may be in thePCM or a relay. The hot-wire shouldglow visibly red during burn-off.

So what can we conclude from allthis? A broad and seemingly unrelatedor even contradictory range of fuel sys-tem-related driveability complaints mayarise from MAF sensor performancefaults. Fuel trim data showing excessivecorrections from base programmingcasts strong suspicion on MAF sensorperformance issues. After recording allDTCs and freeze frame data, many ex-perienced techs recommend unplug-ging a suspect MAF sensor to see if ba-sic driveability is improved. Scopetraces at idle and on snap-throttle accel-eration help verify MAF sensor guilt orinnocence.

As usual, a library of known-goodscan data and waveforms is invaluable.The Min/Max voltage feature on yourDMM may not be fast enough to catchactual peak voltage on a snap-throttletest, but is usually sufficient for verify-ing performance of frequency-generat-ing (digital) MAF sensors. If your scopeis capable of pulse-width triggering, us-ing that function will provide exact cap-tures of digital MAF sensors in snap-throttle testing.

36 July 2006

SUCCESSFUL MAF SENSOR DIAGNOSIS

Fig. 8

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