DIAGNOSING FORD MISFIRES

Transcription

1 A2003 Ford F-150 V8 rolls into your shop, with the customer complaint that the vehicle runs rough at times. This occurs mostly under light-to-moderate acceleration and the Check Engine light is not illuminated. Perhaps the thought of a no-code driveability problem signals a difficult day ahead. But today you re up for a good challenge and you can t get to the gravy work until this job is done. You road-test the vehicle under a light-to-moderate load and you re able to duplicate the symptom, which is good news. At least you re not chasing a phantom problem. Based on the symptom, it appears the vehicle has a misfiring cylinder. You roll back into the shop and continue following a good diagnostic procedure by performing a thorough visual inspection of items related to engine performance. Unfortunately, you note no obvious issues. So what s next? If the Check Engine light were illuminated, the next logical step would be to connect a scan tool and check for diagnostic trouble codes. But the light is not on. Where should you go from here? If this were a GM vehicle, you d connect a scan tool and review the engine misfire data parameters to pinpoint the cylinder that s causing the intermittent misfire. But this is a Ford, and misfire data parameters are not available in the enhanced data stream. You decide to connect a scan tool anyway and to check for possible pending trouble codes. Unfortunately, no codes are stored. For many techs, this is where a good day usually turns bad really fast. But not for you today. Perhaps you recently attended a scan tool seminar where they discussed using Mode $06 data on Ford vehicles to locate a cylinder that s causing a misfire, and now is a good time to use what you learned. In case you have not heard of Mode $06 data, let s review. Mode $06 data is one of the ten diagnostic modes or categories associated with the OBD II Generic system and is primarily found in the OBD II Generic section of your scan tool. I say primarily because a few vehicle manufacturers now also include Mode $06 data in their factory scan tools. Most OBD II Generic scan tools provide a description of the category but not the actual mode. For example, if the Check Engine light is illuminated and you need to retrieve the fault code, you select the description Read DTCs, which really means you ve selected Mode $03. Fig. 1 on page 38 is of a scan tool screen that displays the different mode numbers and a description of each mode. Mode $06 data is the raw test values the OBD II system uses to evaluate the operating status of various components and systems in the engine management system. Engineers program specific limit values for systems and components in order 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 for Test ID (TID) $01 and Component ID (CID) $11, which relate to Bank 1 Oxygen Sensor functionality. The powertrain control module (PCM) uses a special fuel control switching routine to test the O 2 sensor. The Min and Max columns display the programmed threshold values for pass or fail. The Actual column displays the value for the last test Photoillustration: Harold A. Perry; images: Thinkstock & Wieck Media DIAGNOSING FORD MISFIRES BY BOB PATTENGALE Mode $06 is a PCM and scan tool diagnostic mode that may or may not be familiar to you. Here, we demonstrate how data displayed in this mode can be used to pinpoint the source of an intermittent misfire on Ford vehicles. 36 July 2011

2 performed. In this example, the Actual value is 729, which is greater than the Min value of 512, which means the O 2 sensor passed the last test. If the Actual value was less than 512, the PCM would likely set a P0133 DTC (Bank 1 Slow Response) and illuminate the Check Engine light. Further down the chart are Test ID numbers without a description. Why? In the early stages of OBD II, vehicle manufacturers were not required to use a standardized Mode $06 labeling format, so for the scan tool to decode the data correctly, you need a detailed year, make, model and engine size database. If your scan tool displays only the raw data without definitions, there s no reason to panic; most shop information systems provide Mode $06 definitions. The good news is that a standard has been established, and you ll see this new format on most Controller Area Network (CAN) OBD II vehicles. Now let s get back to our misfiring Ford F-150. The next step is determining which Test IDs are related to the misfire monitor. Let s take another look at Fig. 2, because it might be possible to figure this out without referring to the service information. Looking at the chart, you ll notice there are a series of $51 Test IDs, with the associated Component IDs numbered $01 through $08. A quick guess would have been right: The $51 Test IDs are related to the misfire monitor and Component IDs $01 through $08 represent cylinders 1 through 8. For future quick reference, early-model Ford systems use $51 to monitor misfire, later non-can systems use $53 and CAN systems use $0B for the history/last 10 drive cycles and $0C for the current/last test performed. Test ID $50 is for misfires that cannot be attributed to a specific cylinder and will normally result in a P0300 DTC being set. In Fig. 2, all the $51 TIDs except CID $03 show 0, which means the misfiring cylinder in question is cylinder 3. The Max limit in this case is 1638 and the Actual value is 1177, which means that during the last trip, misfires occurred, but they had not exceeded the limit. This is why we have no DTC for cylinder 3. Once the Actual value exceeds the Max limit, a DTC will be stored; in this case, a P0303 would be stored once the limit is exceeded and then the Check Engine light will illuminate. Keep in mind the Check Engine light will react differently depending on the severity of the misfire. For example, it will illuminate steady for a Type B misfire or flash if there s a Type A catalyst-damaging misfire. Once again, the car manufacturer will establish the limits for Type A and Type B misfires. Ford offers a free resource at that details how its OBD II systems operate. Click on the OBD Theory & Operation tab along the left side of the page. You ll find a list of OBD summary documents from 1996 to the current model year (2011), including diesel engines. The documents offer interesting insight into how Ford OBD II monitors operate, including monitor sequence, which sensors are involved, how long the monitor runs, typical monitor conditions and failure criteria. Following are a few key points about Ford misfire monitoring found in these documents: July

3 Fig. 1 Mode $06 data is available on most OBD II Generic scan tools, but the location and description of the information will vary from tool to tool. In this example, Mode $06 is clearly labeled for easy access. Consult your scan tool manual for additional details about the location of Mode $06 data. The Fuel Tank level must be above 15% for misfire monitor operation. It s not uncommon to be working on a vehicle that s low on fuel, but if the fuel level is less than 15%, misfire monitoring is suspended. Some technicians are in the habit of swapping coils, clearing PCM memory and then road-testing the vehicle for component or fault verification. After clearing the PCM memory, a Profile Correction must be learned to correct Circle #25 Fig. 2 Mode $06 data will be displayed in many formats. This scan tool capture shows all the available items in a simple, easy-to-read chart. If your scan tool doesn t provide all the definitions pertaining to Mode $06, consult your service information for details. 38 July 2011

4 Peak kv Ignition Start of Burn Secondary Ignition Burn Time Fig. 3 Ford deal ership technicians use a factory scan tool to locate the misfiring cylinder. The dropout in the pattern shown in this screen capture indicates that cylinder 3 is not contributing as much as the others. The good news: You don t have to purchase a factory scan tool to get the job done. for mechanical variations in the engine. This might require an extended road test on later-model vehicles. The misfire section of the OBD II document provides a key piece of information that will help reduce the amount of time it takes for Profile Correction Learn to complete. The PCM needs to see at least three 60- to 40-mph no-braking decelerations for Profile Correction Relearn, which means a standard road test around the block will not get this done. Many techs have wondered why on some Fords the Check Engine light does not come on right away when a misfire is present. This is primarily due to the misfire count not exceeding the limit. But there s another way Ford protects the catalytic converter during misfire conditions. The misfire section of the OBD II document explains that if the PCM determines a cylinder is misfiring, the fuel injector can be turned off to protect the catalyst from damage. Once the fuel injector is turned off, the cylinder is now basically an air pump, which reduces the potential for catalyst failure. The key to efficient driveability diagnostics is thorough system understanding. Knowing when, where and how a test is performed is critical for success. Consulting these documents will provide insight that goes beyond trouble tree charts. In no-code driveability situations, using Mode $06 data is the key to determining which cylinder or cylinders are misfiring. If you re not sure where to find Mode $06 data on your scan tool, review the user s manual. You might be wondering how dealership technicians deal with cylinder identification issues. Fig. 3 above is a screen capture from the Ford factory scan tool. Ford offers a Power Balance test procedure, which shows all cylinders in a chart and compares per-cylinder rpm to a baseline. In this example, cylinder 3 Fig. 4 This is a secondary ignition pattern from a Ford with an intermittent misfire at idle. The pattern is easy to get when you use a C-O-P adapter, as shown in the photo. shows reduced rpm, which indicates that cylinder 3 is not contributing as much as the other cylinders. We don t know the cause of the misfire at this point, but we know cylinder 3 needs to be checked. The next step is to determine the cause of the misfire. There are many individual issues that can create cylinder misfires, and they fall into three basic categories fuel, ignition and engine mechanical. This is where deductive reasoning will save you time by helping you determine which checks or tests should be performed first. We know from our road test that the misfire is not present at all times, so we need to consider what can cause intermittent misfires. Or maybe in this case we should consider what would cause a constant misfire condition. By having this information, we can determine where to look for intermittent misfires. Let s look at fuel first. If the fuel injector were shorted, we would likely have an injector fault DTC and a possible P0303. Service information would point to resolving the fuel injector fault first, which would resolve the P0303. If the fuel injector were providing too much or not enough fuel, a P0171 or P0174 fuel mixture DTC would be stored. If the fuel injector in question 40 July 2011

5 was not providing any fuel at all and is not shorted, then a P0303 would be present, and perhaps a P0171 (lean mixture fault) as well. Based on this reasoning, the fuel system is not likely to be the source of the intermittent misfire. There are always exceptions to the rule, and possible wild-card ideas might be water in the fuel or fuel injector wiring circuit issues, but neither of those is a likely scenario. Skipping over ignition problems for a moment, let s move on to mechanical engine issues. Mechanical issues like low compression, jumped timing and valve train problems are more likely to create a permanent misfire condition. Again, exceptions to the rule and all that, and other causes of a misfire could be sticking valves or faulty lifters, but I don t believe these are the most likely causes. You probably already have a good idea that the ignition system is the most likely cause of the intermittent misfire, but which part of the ignition system is at fault PCM, ignition coil, coil boot/spring or spark plug? Some techs are coil swappers. If you are, too, don t worry; you re not alone. But how often have you have swapped coils only to find that the misfire is gone on the original cylinder and all you did was trade one misfiring cylinder for another. The misfire may be gone for only a short time, then shows up in the swapped position or returns to the original cylinder. Swapping coils is not a very accurate Fig. 5 These screen captures and photo show another method of testing Ford ignition coils. A lab scope is connected to the control side of the coil. A low-current probe can be used to check for a shorted coil, but is not much use for secondary ignition tests. or efficient way to locate the source of a misfire and might end up wasting valuable time. What s the best way to test for intermittent ignition misfires? Connect an engine analyzer/lab scope to the misfiring cylinder before disturbing the connections between the ignition coil, spring, boot and spark plug. Most scope manufacturers offer a variety of ways to access ignition signals and you need to pick your best display options. Fig. 4 on page 40 shows a secondary ignition pattern taken from a Ford F- 150 at idle with an intermittent misfire using a coil-on-plug (C-O-P) wand. The C-O-P secondary ignition pattern looks very similar to a single-cylinder distributor ignition pattern and they are, with one exception: Ford uses a multispark strategy at idle speed. This will go into a single-spark program off idle, which is useful to know if you re conducting loaded diagnostics. I ve labeled the common sections of the ignition pattern for general review, but here are a few tips when dealing with Ford misfires: If you have a DTC for a specific cylinder and the code comes right back after you clear it, the most likely cause is a shorted coil or a secondary ignition issue that s so bad you might even hear it arcing if you listen closely. Shorted coils are easy to pick out using the primary ignition pattern. C-O-P ignition systems are more sensitive to issues in the secondary ignition area. The point labeled Start of Burn in Fig. 4 is a good spot to focus on when looking for resistance in the boot, spring and spark plug. If the Start of Burn voltage is higher than what s shown here, there s more resistance; if the Start of Burn voltage is lower, there s less resistance. Intermittent issues are generally going to show up in the secondary ignition section, but most of the time you ll need to road-test the vehicle. Don t spend too much time looking for a misfire in Fig. 4 because none is present. Remember, the misfire happens only when the vehicle is driven, so we need to attach a C-O-P device suitable for road tests and retest while driving. If you don t have a suitable road test attachment available, Fig. 5 above shows an alternative method, which entails connecting your lab scope to the control side of the ignition coil. The image you re 42 July 2011

6 looking for is similar to the blue trace, which is basically the secondary pattern from Fig. 4 displayed in a lower voltage range. A low-current probe is also connected, but is not necessary for intermittent secondary misfire issues. The left image in Fig. 5 shows the cylinder without a misfire. The right image is captured as we begin to accelerate the vehicle, duplicating the conditions that caused the customer s complaint. The first secondary ignition firing Fig. 6 A C-O-P adapter can be used to quickly pinpoint shorted coils. The image at right is a known-good ignition pattern; the others are examples of shorted coils. The diagnostic focus is on the primary ignition section of the pattern. Normal Shorted Shorted event is turned into a sea of solid blue, and it looks like we have times where the secondary voltage goes high and then very low. The second and third firing events also show unstable secondary burn. Fig. 6 above shows a few more examples of shorted coils Fig. 7 below shows the misfiring cylinder and its cause: a damaged ignition oil boot. The white area in the photo indicates a burn-through in the rubber boot where the secondary voltage escaped. Fig. 7 The intermittent misfire issue on this Ford vehicle was a hole in the side of the boot. The damage in this example is very obvious, but similar carbon traces inside the boot also may occur. If you re not careful, swapping coils will make you think the problem magically went away. It didn t; it may appear a few days or weeks later. Inside the rubber spark boot is a spring connecting the coil to the spark plug. When the boot becomes soft, the voltage is able to find a path of less resistance. Most of the time the damage is not that obvious. In some cases, a carbon trace will develop inside the boot and the secondary voltage will travel down the side of the spark plug to ground. Testing the secondary ignition before disturbing the coil will allow you to see what s truly at fault. In this example, we determined the source of the problem was in the secondary ignition area and a new boot and spring should fix the problem. In review, the key to quickly finding intermittent misfire issues is having a consistent strategy and being comfortable with your equipment. Knowing how to use Mode $06 to find the cylinder that s misfiring is a critical step in the process and takes only a few min- This article can be found online at 44 July 2011