A2003 Ford F-150 V8 rolls into your shop, withthe customer complaint that the vehicle runsrough at times. This occurs mostly underlight-to-moderate acceleration and the CheckEngine light is not illuminated. Perhaps thethought of a no-code driveability problem sig-nals a difficult day ahead. But today youre up for a good chal-lenge and you cant get to the gravy work until this job is done.

You road-test the vehicle under a light-to-moderate loadand youre able to duplicate the symptom, which is goodnews. At least youre not chasing a phantom problem. Basedon the symptom, it appears the vehicle has a misfiring cylin-der. You roll back into the shop and continue following agood diagnostic procedure by performing a thorough visualinspection of items related to engine performance. Unfortu-nately, you note no obvious issues.

So whats next? If the Check Engine light were illuminat-ed, the next logical step would be to connect a scan tool andcheck for diagnostic trouble codes. But the light is not on.Where should you go from here?

If this were a GM vehicle, youd connect a scan tool andreview the engine misfire data parameters to pinpoint thecylinder thats causing the intermittent misfire. But this is aFord, and misfire data parameters are not available in theenhanced data stream. You decide to connect a scan toolanyway and to check for possible pending trouble codes.Unfortunately, no codes are stored.

For many techs, this is where a good day usually turns badreally fast. But not for you today. Perhaps you recently at-tended a scan tool seminar where they discussed using Mode$06 data on Ford vehicles to locate a cylinder thats causing amisfire, and now is a good time to use what you learned.

In case you have not heard of Mode $06 data, lets review.Mode $06 data is one of the ten diagnostic modes or cate-gories associated with the OBD II Generic system and isprimarily found in the OBD II Generic section of your scantool. I say primarily because a few vehicle manufacturersnow also include Mode $06 data in their factory scan tools.

Most OBD II Generic scan tools provide a description ofthe category but not the actual mode. For example, if theCheck Engine light is illuminated and you need to retrievethe fault code, you select the description Read DTCs,which really means youve selected Mode $03. Fig. 1 onpage 38 is of a scan tool screen that displays the differentmode numbers and a description of each mode.

Mode $06 data is the raw test values the OBD II systemuses to evaluate the operating status of various componentsand systems in the engine management system. Engineersprogram specific limit values for systems and components inorder to determine when a fault code should be set.

For example, the screen capture in Fig. 2 shows Mode$06 data from a Ford F-150. The first line shows data forTest ID (TID) $01 and Component ID (CID) $11, whichrelate to Bank 1 Oxygen Sensor functionality. The power-train control module (PCM) uses a special fuel controlswitching routine to test the O2 sensor. The Min and Maxcolumns display the programmed threshold values for passor fail. The Actual column displays the value for the last test

36 July 2011

Mode $06 is a PCM andscan tool diagnosticmode that may or maynot be familiar to you.Here, we demonstratehow data displayed inthis mode can be usedto pinpoint the sourceof an intermittentmisfire on Ford vehicles.

Mode $06 is a PCM andscan tool diagnosticmode that may or maynot be familiar to you.Here, we demonstratehow data displayed inthis mode can be usedto pinpoint the sourceof an intermittentmisfire on Ford vehicles.

DIAGNOSINGFORDMISFIRESBY BOB PATTENGALE

DIAGNOSING FORDMISFIRESBY BOB PATTENGALE

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performed. In this example, the Actual value is 729, which isgreater than the Min value of 512, which means the O2 sen-sor passed the last test. If the Actual value was less than 512,the PCM would likely set a P0133 DTC (Bank 1 Slow Re-sponse) and illuminate the Check Engine light.

Further down the chart are Test ID numbers without adescription. Why? In the early stages of OBD II, vehiclemanufacturers were not required to use a standardizedMode $06 labeling format, so for the scan tool to decode thedata correctly, you need a detailed year, make, model andengine size database. If your scan tool displays only the rawdata without definitions, theres no reason to panic; mostshop information systems provide Mode $06 definitions.The good news is that a standard has been established, andyoull see this new format on most Controller Area Network(CAN) OBD II vehicles.

Now lets get back to our misfiring Ford F-150. The nextstep is determining which Test IDs are related to the mis-fire monitor. Lets take another look at Fig. 2, because itmight be possible to figure this out without referring to theservice information. Looking at the chart, youll notice thereare a series of $51 Test IDs, with the associated ComponentIDs numbered $01 through $08. A quick guess would havebeen right: The $51 Test IDs are related to the misfiremonitor and Component IDs $01 through $08 representcylinders 1 through 8.

For future quick reference, early-model Ford systems use$51 to monitor misfire, later non-CAN systems use $53 andCAN systems use $0B for the history/last 10 drive cycles and$0C for the current/last test performed. Test ID $50 is formisfires that cannot be attributed to a specific cylinder andwill normally result in a P0300 DTC being set.

In Fig. 2, all the $51 TIDs except CID $03 show 0, whichmeans the misfiring cylinder in question is cylinder 3. TheMax limit in this case is 1638 and the Actual value is 1177,which means that during the last trip, misfires occurred, butthey had not exceeded the limit. This is why we have noDTC for cylinder 3. Once the Actual value exceeds the Maxlimit, a DTC will be stored; in this case, a P0303 would bestored once the limit is exceeded and then the Check En-gine light will illuminate.

Keep in mind the Check Engine light will react differentlydepending on the severity of the misfire. For example, it willilluminate steady for a Type B misfire or flash if theres aType A catalyst-damaging misfire. Once again, the car manu-facturer will establish the limits for Type A and Type B mis-fires.

Ford offers a free resource at www.Motorcraftservice.comthat details how its OBD II systems operate. Click on theOBD Theory & Operation tab along the left side of the page.Youll find a list of OBD summary documents from 1996 tothe current model year (2011), including diesel engines.

The documents offer interesting insight into how FordOBD II monitors operate, including monitor sequence,which sensors are involved, how long the monitor runs, typi-cal monitor conditions and failure criteria.

Following are a few key points about Ford misfire moni-toring found in these documents:

37July 2011

The Fuel Tank level must be above15% for misfire monitor operation. Itsnot uncommon to be working on a ve-hicle thats low on fuel, but if the fuellevel is less than 15%, misfire monitor-ing is suspended.

Some technicians are in the habit ofswapping coils, clearing PCM memoryand then road-testing the vehicle forcomponent or fault verification. Afterclearing the PCM memory, a ProfileCorrection must be learned to correct

DIAGNOSING FORD MISFIRES

Fig. 1 Mode $06 data is available on most OBD II Generic scan tools, but thelocation and description of the information will vary from tool to tool. In this ex-ample, Mode $06 is clearly labeled for easy access. Consult your scan tool man-ual for additional details about the location of Mode $06 data.

Fig. 2 Mode $06 data will be displayed in many formats. This scan tool capture showsall the available items in a simple, easy-to-read chart. If your scan tool doesnt provideall the definitions pertaining to Mode $06, consult your service information for details.

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Circle #25

for mechanical variations in the engine.This might require an extended roadtest on later-model vehicles. The misfiresection of the OBD II document pro-vides a key piece of information thatwill help reduce the amount of time ittakes for Profile Correction Learn tocomplete. The PCM needs to see atleast three 60- to 40-mph no-brakingdecelerations for Profile Correction Re-learn, which means a standard road testaround the block will not get this done.

Many techs have wondered why onsome Fords the Check Engine light doesnot come on right away when a misfireis present. This is primarily due to themisfire count not exceeding the limit.But theres another way Ford protectsthe catalytic converter during misfireconditions. The misfire section of theOBD II document explains that if thePCM determines a cylinder is misfiring,the fuel injector can be turned off toprotect the catalyst from damage. Oncethe fuel injector is turned off, the cylin-der is now basically an air pump, whichreduces the potential for catalyst failure.

The key to efficient driveability diag-nostics is thorough system understand-ing. Knowing when, where and how atest is performed is critical for success.Consulting these documents will pro-vide insight that goes beyond troubletree charts.

In no-code driveability situations, us-ing Mode $06 data is the key to deter-

mining which cylinder or cylinders aremisfiring. If youre not sure where tofind Mode $06 data on your scan tool,review the users manual.

You might be wondering how dealer-ship technicians deal with cylinder iden-tification issues. Fig. 3 above is a screencapture from the Ford factory scan tool.Ford offers a Power Balance test proce-dure, which shows all cylinders in a chartand compares per-cylinder rpm to abaseline. In this example, cylinder 3

shows reduced rpm, which indicates thatcylinder 3 is not contributing as much asthe other cylinders. We dont know thecause of the misfire at this point, but weknow cylinder 3 needs to be checked.

The next step is to determine thecause of the misfire. There are many in-dividual issues that can create cylindermisfires, and they fall into three basiccategoriesfuel, ignition and enginemechanical.

This is where deductive reasoningwill save you time by helping you deter-mine which checks or tests should beperformed first. We know from ourroad test that the misfire is not presentat all times, so we need to consider whatcan cause intermittent misfires. Ormaybe in this case we should considerwhat would cause a constant misfirecondition. By having this information,we can determine where to look for in-termittent misfires.

Lets look at fuel first. If the fuel in-jector were shorted, we would likelyhave an injector fault DTC and a possi-ble P0303. Service information wouldpoint to resolving the fuel injector faultfirst, which would resolve the P0303. Ifthe fuel injector were providing toomuch or not enough fuel, a P0171 orP0174 fuel mixture DTC would bestored. If the fuel injector in question

DIAGNOSING FORD MISFIRES

Fig. 3 Ford deal ership technicians use a factory scan tool to locate the mis-firing cylinder. The dropout in the pattern shown in this screen capture indi-cates that cylinder 3 is not contributing as much as the others. The good news:You dont have to purchase a factory scan tool to get the job done.

Fig. 4 This is a secondary ignition pattern from a Ford with an intermittent misfire atidle. The pattern is easy to get when you use a C-O-P adapter, as shown in the photo.

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Peak kVStart of Burn

Burn TimePrimaryIgnition

Secondary Ignition

was not providing any fuel at all and isnot shorted, then a P0303 would bepresent, and perhaps a P0171 (lean mix-ture fault) as well.

Based on this reasoning, the fuel sys-tem is not likely to be the source of theintermittent misfire. There are alwaysexceptions to the rule, and possiblewild-card ideas might be water in thefuel or fuel injector wiring circuit issues,but neither of those is a likely scenario.

Skipping over ignition problems for amoment, lets move on to mechanicalengine issues. Mechanical issues like lowcompression, jumped timing and valvetrain problems are more likely to createa permanent misfire condition. Again,exceptions to the rule and all that, andother causes of a misfire could be stick-ing valves or faulty lifters, but I dont be-lieve these are the most likely causes.

You probably already have a goodidea that the ignition system is the mostlikely cause of the intermittent misfire,but which part of the ignition system isat faultPCM, ignition coil, coilboot/spring or spark plug?

Some techs are coil swappers. If youare, too, dont worry; youre not alone.But how often have you have swappedcoils only to find that the misfire is goneon the original cylinder and all you didwas trade one misfiring cylinder for an-other. The misfire may be gone for onlya short time, then shows up in theswapped position or returns to the origi-nal cylinder.

Swapping coils is not a very accurate

or efficient way to locate the source of amisfire and might end up wasting valu-able time. Whats the best way to testfor intermittent ignition misfires? Con-nect an engine analyzer/lab scope to themisfiring cylinder before disturbing theconnections between the ignition coil,spring, boot and spark plug.

Most scope manufacturers offer avariety of ways to access ignition signalsand you need to pick your best displayoptions.

Fig. 4 on page 40 shows a secondaryignition pattern taken from a Ford F-150 at idle with an intermittent misfireusing a coil-on-plug (C-O-P) wand. TheC-O-P secondary ignition pattern looksvery similar to a single-cylinder distribu-tor ignition pattern and they are, withone exception: Ford uses a multisparkstrategy at idle speed. This will go into asingle-spark program off idle, which is

useful to know if youre conductingloaded diagnostics.

Ive labeled the common sections ofthe ignition pattern for general review,but here are a few tips when dealingwith Ford misfires:

If you have a DTC for a specificcylinder and the code comes right backafter you clear it, the most likely cause isa shorted coil or a secondary ignition is-sue thats so bad you might even hear itarcing if you listen closely. Shorted coilsare easy to pick out using the primaryignition pattern.

C-O-P ignition systems are more sen-sitive to issues in the secondary ignitionarea. The point labeled Start of Burn inFig. 4 is a good spot to focus on whenlooking for resistance in the boot, springand spark plug. If the Start of Burn voltageis higher than whats shown here, theresmore resistance; if the Start of Burn volt-age is lower, theres less resistance.

Intermittent issues are generally go-ing to show up in the secondary ignitionsection, but most of the time youll needto road-test the vehicle.

Dont spend too much time lookingfor a misfire in Fig. 4 because none ispresent. Remember, the misfire happensonly when the vehicle is driven, so weneed to attach a C-O-P device suitablefor road tests and retest while driving.

If you dont have a suitable road testattachment available, Fig. 5 above showsan alternative method, which entails con-necting your lab scope to the control sideof the ignition coil. The image youre

DIAGNOSING FORD MISFIRES

Fig. 5 These screen captures andphoto show another method of testingFord ignition coils. A lab scope is con-nected to the control side of the coil. Alow-current probe can be used tocheck for a shorted coil, but is notmuch use for secondary ignition tests.

42 July 2011

looking for is similar to the blue trace,which is basically the secondary patternfrom Fig. 4 displayed in a lower voltagerange. A low-current probe is also con-nected, but is not necessary for intermit-tent secondary misfire issues.

The left image in Fig. 5 shows thecylinder without a misfire. The right im-age is captured as we begin to acceler-ate the vehicle, duplicating the condi-tions that caused the customers com-plaint. The first secondary ignition firing

event is turned into a sea of solid blue,and it looks like we have times wherethe secondary voltage goes high andthen very low. The second and third fir-ing events also show unstable secondaryburn. Fig. 6 above shows a few more ex-

amples of shorted coilsFig. 7 below shows the misfiring cylin-

der and its cause: a damaged ignition oilboot. The white area in the photo indi-cates a burn-through in the rubber bootwhere the secondary voltage escaped.

Inside the rubber spark boot is a springconnecting the coil to the spark plug.When the boot becomes soft, the voltageis able to find a path of less resistance.Most of the time the damage is not thatobvious. In some cases, a carbon tracewill develop inside the boot and the sec-ondary voltage will travel down the sideof the spark plug to ground.

Testing the secondary ignition beforedisturbing the coil will allow you to seewhats truly at fault. In this example, wedetermined the source of the problemwas in the secondary ignition area and anew boot and spring should fix theproblem.

In review, the key to quickly findingintermittent misfire issues is having aconsistent strategy and being comfort-able with your equipment. Knowinghow to use Mode $06 to find the cylin-der thats misfiring is a critical step inthe process and takes only a few min-

DIAGNOSING FORD MISFIRES

Fig. 7 The intermittent misfire issue on this Ford vehicle was a hole in the side ofthe boot. The damage in this example is very obvious, but similar carbon traces in-side the boot also may occur. If youre not careful, swapping coils will make you thinkthe problem magically went away. It didnt; it may appear a few days or weeks later.

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This article can be found online atwww.motormagazine.com.

Fig. 6 A C-O-Padapter can beused to quickly pin-point shorted coils.The image at rightis a known-good ig-nition pattern; theothers are exam-ples of shortedcoils. The diagnosticfocus is on the pri-mary ignition sec-tion of the pattern.

PrimaryNormal

PrimaryShorted

PrimaryShorted