subtract fuel in response to changingengine operating conditions.

The PCM contains a set of preprogrammedmaps which tell it how much fuel the engineshould need under specific conditions of load,manifold absolute pressure (MAP), enginerevolutions per minute (RPM), enginetemperature (ECT), throttle position (TP),etc. If the PCM determines that the engineneeds more or less fuel (based on the exhaustoxygen content information received fromthe oxygen sensor), it “trims” the mixture byadding or subtracting fuel. Fuel trim isexpressed in terms of short term or long termcorrections. GM has also used the termsintegrator and block learn to describe themeasurements that express fuel trim whenviewed on a scan tool.

The GM system assigns a number between 0and 255 to define fuel trim, with 128 beingthe middle point. A short term fuel trim(integrator) reading of 118-138 is considerednormal. Fuel trim can also be expressed aseither a positive (+) or negative (-)percentage. The percentage readingrepresents the difference either above orbelow the anticipated amount. So a shortterm fuel trim reading of +20 percentindicates 20 percent more fuel had to beadded to achieve the proper air-fuel mixture.A -20 percent short term fuel trim indicatesthat fuel had to be removed by shorteningthe injector pulse width to achieve the properair-fuel mixture.

When the engine is operating in closed loop,the oxygen sensor signal is the maininfluence on the PCM’s short term fuel trim(SHRTFT) decisions. For example, theoxygen sensor will produce a lower voltagesignal if the engine has a small vacuum leak.The PCM interprets this as a too lean air-fuelmixture and the injector pulse width isincreased slightly to compensate. Theamount of additional fuel that is added isseen on a scan tool as a positive SHRTFT.

uel trimrefers to thePCM’s ability

to add or subtractfuel in response tochanging engineoperating conditions.

Perhaps you are familiar with what aconventional oxygen sensor waveform lookslike. The oscilloscope trace repeatedly travelsupward to a peak then downward to a valley,as the oxygen sensor reports the amount ofoxygen present in the engine’s exhaust. Thepeaks indicate a rich air-fuel ratio (highoxygen sensor voltage and low oxygencontent), while the valleys indicate a lean air-fuel ratio (low oxygen sensor voltage andhigh oxygen content). When the oxygensensor reports a rich mixture, the fuel systempowertrain control module (PCM) respondsby commanding a lean mixture. And whenthe oxygen sensor reports a lean mixture, thePCM responds by commanding a richmixture. This is classic “closed loop” fuelcontrol, with the PCM always a step behindin its response to the rich or lean voltagesignal from the oxygen sensor.

This would seem to be an inefficient meansof controlling the air-fuel ratio. Why notfind an air-fuel mixture that is close to ideal

(stoichiometry) and keep the mixtureas near to that point as possible? Whymust the fuel system constantlyovershoot its goal as it moves from a

too-rich to a too-lean air-fuel mixture and

CounterCounter PointT H E E L E C T R O N I C, D I A G N O S T I C A N D D R I V E A B I L I T Y R E S O U R C E.

Volume 6 Issue 4, October 2002

continued on page 3

Fuel System Fuel Trim

back again? The answer can be found in thedesign of the catalytic converter. The three-way catalytic converter needs a rich exhaustmixture (low oxygen content) to reduceoxides of nitrogen (NOX) emissions.However, it also needs to receive a leanexhaust mixture (high oxygen content) tooxidize hydrocarbons (HC) and carbonmonoxide (CO) into carbon dioxide (CO2)and water (H2O) vapor. If the exhaust staysrich, the catalytic converter can’t reduce COand HC emissions. And if the exhaust stayslean, the catalytic converter can’t reduce NOX

emissions. Alternating between rich and leanallows all three goals to be reached.

Why Fuel Trim Is Needed

The closed loop setup described abovewould do a pretty good job of controllingemissions if the engine operated in anarrow RPM range at all times. However,due to changes in vehicle speed, load andmany other variables, engine speedfrequently changes through a very widerange. The PCM needs ways to makeadjustments for these changes. One of theseadjustment capabilities is called fuel trim,which refers to the PCM’s ability to add or

WHAT’S INSIDE: FUEL SYSTEM FUEL TRIM…Page 1&3 / FINE TUNING…Page 2 / HOT OFF THE WIRE: 2002 Wire, Cable & Battery Accessories Catalog...Page 2QU QUALITY POINTS: EGR Position Sensor Calibration System…Page 4 / Publisher’s Information…Page 4

F

the amount of voltage the sensor puts on the signalwire to the PCM with the key on engine off(KOEO). Installing the wrong sensor will causesymptoms similar to those produced by a part thatis not within OE specifications.

The 5 volt signal to the PCM on the DPFE sensorsignal wire is incorrect. A DPFE sensor functionsvery similar to a manifold absolute pressure sensor.It is nothing more than a voltage divider. Afterlooking at the wiring diagram for the PCM pinouts, I believe you have been testing at the wrongpin at the PCM. The pins for reference voltage andDPFE sensor signal are very close together in theconnector. The wire colors are as follows: theDPFE sensor signal wire is brown with a lightgreen trace, and the 5 volt reference wire is brownwith a white trace. This is a very difficultconnector to back probe, and I’ll admit I’ve mademy share of wire color identification mistakes.

Results: After talking with another parts house, Aldetermined that the sensor he got from the dealerwas indeed the wrong part. He installed the correctsensor and the MIL has stayed off since.

The "Fine Tuning" question from the last CounterPoint concerned a 1992 Chevrolet K-1500. Theproblem began with a no-start, which lead to anignition module and pick-up assemblyreplacement. A couple of weeks later the enginestarted running rough and dying out. The basetiming was off, but the ignition, enginemanagement computer and engine mechanicalsystems checked out okay.

On this vehicle, check the base timing by first

Q: I have a problem with a 1997 Ford Explorer Truck4.0L VIN E that I’ve been working on for the past fewdays. When the truck first came into the shop, it hada diagnostic trouble code (DTC) P0402 stored in thepowertrain control module (PCM). I followed the Forddiagnostic tree for this DTC which recommendedreplacement of the Delta Pressure Feedback ExhaustGas Recirculation (DPFE) sensor because the sensorwas not within specifications.

The local dealership delivered a replacement sensor,but it didn’t look the same as the original that was onthe truck. The original sensor was aluminum and thereplacement was plastic. I called the dealership andthey assured me the part I received was the correctreplacement, so I installed it. I took the truck for aroad test and the malfunction indicator lamp (MIL)came on before I could get out of the driveway.

After returning to the shop, I checked voltage levelsat the PCM. I found 5 volts on the DPFE sensor signalpin at the PCM, so I replaced the PCM. I still have thesame problem: the MIL illuminates and the PCM storesa code P0402. Is the 5 volt DPFE sensor signalnormal? Where do I go from here?

Al OliverezFirestoneDallas, TX

If a replacement part does not look like theoriginal equipment (OE) part, verify whether anydesign changes have been made, before you installit. Refer to the diagnostic tree for DTC P0402.Note under step HE23 that one of two DPFEsensors may be installed on this vehicle. Onesensor has an offset voltage of 1.0 volts and theother has an offset voltage of .55 volts. Offset is

2

Fine Tuning questions are answered byMark Hicks, Technical Services Manager. Please send your questions to: Mark Hicksc⁄ o Wells Manufacturing Corp.,P.O. Box 70, Fond du Lac, WI 54936-0070 or e-mail him attechnical@wellsmfgcorp.com. We’ll send you a Wells shirt if yourquestion is published. So please includeyour shirt size with your question. FineFine Tuning

disconnecting the connector on the tan/black wirelocated under the battery junction block cover.Disconnecting this wire takes any influences fromthe powertrain control module out of thepicture. This is where the "base" in basetiming comes from. Knowing the PCMhas no influence on the timing can be avery useful diagnostic tool. In this case, thebase timing had changed without rotatingthe distributor or having the PCM involved.

On this vehicle, the position of the distributorhousing/pickup assembly in relationship to triggermagnets that are attached to the distributor shaftdetermine the base timing. It is a fairly commonfailure for the magnetic trigger to crack and move onthe distributor shaft. When it moves, the base timingwill also change and disturb the timing curve.

To check for this failure, remove the distributorcap and try to rotate the trigger on the shaft. Ifyou are able to move it in any way, the splines arerounding off and the shaft or distributor must bereplaced.

This is not what happened on this particularvehicle. The distributor shaft bearings at the top ofthe housing began to seize due to a lack oflubrication. The distributor shaft became twistedwhen the camshaft attempted to turn it. The firstsign of either of these problems is an unexpectedchange in base timing. Either answer is correct.

Results: The distributor was replaced and the basetiming returned to normal.

The first correct answer received by e-mail or fax was from:Dan BaumhardtAuto Diagnostics, Inc.West Allis, WI

The first correct answer received by post wasfrom:Eric WhiteEWOO AutomotiveSt. Louis, MO

Q: “I have been working on a 1999 Ford Escortwith a 2.0L engine that has a number two cylindermisfire. The fuel system is operating withinspecifications, but a quick check with a sparktester revealed there was no spark on the numbertwo cylinder.

“I had the same problem with this vehicle abouttwo weeks ago. At that time, the number twocylinder started firing and the engine ran greatafter I replaced the ignition coil. Now it is back inthe shop with the same dead cylinder problem. ThePCM controls the spark on this vehicle. Could thisbe the source of the problem, or is theresomething else I should check?”

Mike MetzH.J. Biebold GarageBuffalo, NY

The first reader to respond with the mostaccurate answer via e-mail or fax, and the firstreader to respond with the most accurate answer viasnail-mail, will receive a Wells golf shirt. The answerwill appear in the next issue.

Wells Manufacturing Corp. has released its 2002 Wire,Cable & Battery Accessories Catalog. Over 100 new partnumbers have been added since the publication of the2001 catalog. New catalog product categories includeElectrical Tape and Nylon Loom Clamps.

Tape is offered in several varieties, including VinylElectrical Tape, Professional Grade Electrical Tape,Amalgamating (Cold Shrink) Electrical Tape and Self-Fusing Silicone Tape. Nylon clamps are tough, resilient,lightweight and resistant to aging and most chemicals.

For more information about the Wells ManufacturingCorp. line of products, contact your Wells parts supplier.

Diagnose The Problem Win A ShirtDiagnose The Problem Win A Shirt

Hot off theHot off the WireWire, Cable & BatteryAccessories Catalog

Vehicle 2SHRTFT = -10%LONGFT = -30%Explanation: The air-fuel mixture is richbecause LONGFT is removing 30 percent ofthe anticipated amount of fuel to achieve theproper air-fuel mixture. Even after removing30 percent, SHRTFT still has to make afurther adjustment of 10 percent to attemptto lean out the mixture. This is not a normalcondition. Look for a defective fuel injector,a defective fuel pressure regulator or arestriction in the intake air passage.

Vehicle 3SHRTFT = +10%LONGFT = ±2%Explanation: These readings are normal. It isnormal for SHRTFT to add or subtract up to20 percent of the anticipated amount of fuelto achieve and maintain the proper air-fuelmixture. LONGFT is ±2 percent, indicatingno large long term adjustments are necessary.

Computer Fuel Trim Strategies

Under normal conditions, SHRTFT is whatkeeps the engine near the best overall air-fuelratio of 14.7:1 (stoichiometric). SHRTFTvalues are retained in the PCM’s volatilememory and are erased as soon as the ignitionkey is switched OFF. These values “start fromscratch” each time the engine is restarted.

LONGFT is a learned value that is determinedby trends in short term fuel correction.LONGFT values are stored in the PCM’sKeep Alive Memory (KAM) and stay thereunless they are reset with a scan tool or erasedwhen the PCM’s power source is disconnected.After an engine management problem hasbeen corrected, always clear the codes. Thisalso erases the PCM’s long term fuel correctioninformation, allowing it to start fresh.

The PCM constantly monitors bothSHRTFT and LONGFT to determine if thefuel system is operating normally. IfSHRTFT goes beyond its limit in an attemptto control the mixture, LONGFT will alsobe affected. If SHRTFT correctioncontinues, LONGFT will experience a majorchange. Both SHRTFT and LONGFT cancompensate for a rich or lean condition, butthey can only take it so far. When thecombined SHRTFT and LONGFTcorrections exceed their adaptive limit, thePCM recognizes they are “maxed out” and adiagnostic trouble code (DTC) is stored. Themaximum allowable combined fuel trimadjustment varies from one vehiclemanufacturer to the next, but a DTC willusually store when the combined SHRTFTand LONGFT are greater than 25-35percent. On OBD II vehicles, a freeze frameof SHRTFT and LONGFT data is stored atthe moment of a fault that results in a DTCbeing stored.

Fuel correction problems will often be causedby a component failure that sets a separateDTC. For example, a failed oxygen sensorwill probably set its own DTC on a latemodel fuel system. When it does, the fuelsystem may be out of range but will not setan additional fuel system rich or lean code,because the fuel system monitor won’t rununtil the oxygen sensor fault has beencorrected. There are many other possiblecauses for excessively rich or lean fuelmixtures. Problems with the mass air flowsensor (MAF), MAP or ECT may also triggera component DTC when they fail. However,there are quite a few components that are notdirectly monitored by the PCM that can alsocause a fault. Some of these include:• Leaking or plugged fuel injectors,• Clogged fuel filters,• Weak fuel pumps and faulty pressure

regulators,• Fuel line obstructions,• False air leaks between the MAF sensor and

throttle body,• A contaminated MAF sensor,• Fuel dilution in the crankcase,• Faulty PCV valves or plumbing,• Plugged air filters or other air intake

obstructions,• Low compression,• Improperly adjusted valve or ignition

timing,• A restricted exhaust,• Exhaust or vacuum leaks that trick the

oxygen sensor.

A thorough understanding of how the systemreceives and processes information may helpyou to determine which of these failures isbehind the system’s inability to properlycontrol the air-fuel ratio.

Long Term Fuel Trim

SHRTFT adjusts the air-fuel ratio inresponse to brief changes that normallyoccur during engine operation. Long

term fuel trim (LONGFT) is designed to addor subtract fuel over a longer period of time. Ifthe vacuum leak in the previous example wereto continue or grow larger, fuel delivery wouldhave to be further increased and continue for alonger period of time. The PCM incorporatesLONGFT for this purpose.

SHRTFT continues to add fuel tocompensate for the vacuum leak. But if theleak continues for longer than a fewseconds, the PCM also adjusts LONGFTto compensate for the lean mixture. Whenthe PCM makes an adjustment toLONGFT, SHRTFT continues to be usedto make quick changes in the air-fuelmixture as needed to provide the catalyticconverter with an alternating rich and leanexhaust.

Fuel Trim Diagnostics

A scan tool can be used to read bothSHRTFT and LONGFT data.LONGFT data has more value fordiagnostic purposes because itrecords what has occurred over a

longer period of time and represents agreater amount of mixture correction. Forexample, if a reading of +20 percentLONGFT is indicated, we know the PCMhas commanded the delivery of 20 percentmore fuel than the calibrated amount as itattempts to achieve the proper air-fuelmixture. The LONGFT number willalways indicate what the PCM had to do toachieve the proper mixture.

Three examples of scan tool fuel trim readingsand possible diagnostic explanations follow.Examples of real vehicle readings are shown inFigures 1 and 2.

Vehicle 1 SHRTFT = +5%LONGFT = +20%Explanation: The PCM is responding to alean condition. LONGFT indicates theprogrammed amount of fuel had to beincreased by 20 percent to achieve the

proper air-fuel mixture. At thispoint, SHRTFT could “toggle” themixture rich and lean to achievethe best catalytic converter

efficiency. Possible causes for this conditionare a manifold vacuum leak, misfire or lowfuel pressure.

3

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Fuel System Fuel Trim

Figure 1: With moderate engine load, SHRTFTresponsds by adding fuel. LONGFT is also adding fuel.

Figure 2: At low engine load, SHRTFT is subtractingfuel. LONGFT is now adding less fuel than in Figure 1.

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WELLS MANUFACTURING CORP.P.O. Box 70Fond du Lac, WI 54936-0070

Publisher’s Information

Wells’ President ..............William AllenVice President of Sales ....Gavin SpenceTechnical Services Manager .. Mark HicksNewsletter Editor ...............Karl SeyfertCounter Point is a quarterly publication of WellsManufacturing Corp., P.O. Box 70, Fond du Lac, WI54936-0070. Letters and comments should bedirected to: Counter Point Editor, c/o Wells Manufacturing Corp., P.O. Box 70,Fond du Lac, WI 54936-0070.© COPYRIGHT 2002 WELLS MANUFACTURING CORP.All rights reserved. No reproduction in whole or part is permittedwithout the written consent of Wells Manufacturing Corp.

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QualityQuality PointsEGR Position SensorCalibration System Due to the many sophisticated onboard systems ontoday’s vehicles, components matching the latestoriginal equipment manufacturers’ (OEM)specifications must always be installed whenreplacement becomes necessary. If a poorlycalibrated component is installed, it may triggerthe malfunction indicator lamp (MIL) or cause thevehicle to fail an emissions test. One of the waysWells Manufacturing Corp. ensures that all of itscomponents match OEM specs is during the finalcalibration phase of the manufacturing process.

For example, after a Wells exhaust gas recirculationposition sensor is manufactured, it is placed in acustom-designed calibration and test machinecradle. Next, it is electrically connected to thecomputer using the OE connector. Using the sametype of connector that will be used when thecomponent is installed in the vehicle ensures a goodconnection. The component’s position shaft is thendepressed until it reaches a predetermined calibration voltage. The computer processes thisinformation to determine the amount of shaft

milling that is required. The sensor is then slid intoposition and the mill does its job. Returning to theoriginal position, the shaft is again operated throughits entire range. The computer subsequently graphsand records the voltage versus sensor shaft distancetraveled, and the data is compared to OE specs. Thecritical shaft dimension, output voltage andconnector pin integrity are all tested and calibrated.An exact match turns on the green light and thecomponent passes.

This calibration and testing is performed on 100percent of the exhaust gas recirculation positionsensors Wells manufactures. We are proud to saythis calibration system was entirely envisioned,designed and built by Wells personnel.

Mike Syms monitors the operation of the Wellsexhaust gas recirculation position sensorcalibration and test machine. Shaft mill in inset.

INSIDE:Fuel Trim