Your bay welcomes a 2003 Ford F-150 V8, the owner complaining of intermittent rough running, particularly under light to moderate acceleration. The check engine light remains stubbornly off, and the words “no-code driveability problem” might send shivers down the spine of even seasoned technicians. However, armed with the right diagnostic approach, this challenge can become a rewarding victory.
You begin by road-testing the vehicle, simulating the conditions described by the customer. True enough, the symptom manifests – an undeniable misfire. This confirmation is a crucial first step; you’re not chasing a ghost. Back in the shop, a thorough visual inspection follows, scrutinizing components related to engine performance. Yet, no obvious culprits present themselves.
The absence of a Check Engine light might initially feel like a setback. Traditionally, a lit MIL would lead you directly to DTC retrieval. But in this situation, where do you turn next? On a GM vehicle, accessing misfire data parameters via a scan tool would be the immediate next step, pinpointing the problematic cylinder. However, Fords don’t typically offer enhanced misfire data in the standard data stream. Nevertheless, connecting a scan tool to check for pending codes is a worthwhile move, but in this case, none are stored.
This scenario often marks the beginning of a diagnostic headache for many technicians. But if you’re familiar with leveraging Mode $06 data on Ford vehicles, you possess a powerful tool to identify the misfiring cylinder, even without active DTCs. Let’s delve into how Mode $06 can be your secret weapon in diagnosing this Ford F-150 misfire.
Unlocking the Power of OBD2 Mode $06 Data
For those unfamiliar, Mode $06 data is a cornerstone of the OBD II generic system, residing within the ten diagnostic modes. It’s accessible through the OBD II Generic section of most scan tools, and increasingly, vehicle manufacturers are incorporating it into their factory tools.
Scan tools usually present these modes with descriptions rather than mode numbers. For instance, “Read DTCs” equates to Mode $03. Figure 1 visually represents the various OBD II modes and their descriptions as displayed on a typical scan tool.
Mode $06 data provides the raw test values that the OBD II system utilizes to assess the operational status of various engine management components and systems. Engineers program specific pass/fail thresholds for these tests. When a component’s measured value falls outside these limits, a diagnostic trouble code is triggered.
Consider Figure 2, showcasing Mode $06 data from our Ford F-150. The first line item, TID $01 and CID $11, relates to Bank 1 Oxygen Sensor functionality. The PCM employs a fuel control switching routine to evaluate the O2 sensor. The “Min” and “Max” columns define the acceptable range for a passing test. The “Actual” column reflects the value from the last test. In this example, the actual value (729) exceeds the minimum (512), indicating a passing O2 sensor test. A value below 512 would likely result in a P0133 DTC (Bank 1 Slow Response) and illuminate the Check Engine light.
You might notice some Test ID numbers lacking descriptions. This is because early OBD II implementations didn’t mandate a standardized Mode $06 labeling format. Accurate data interpretation necessitates a detailed database specific to the year, make, model, and engine size. If your scan tool only displays raw data, don’t be alarmed; most shop information systems provide Mode $06 definitions. The good news is that newer CAN-based OBD II vehicles adhere to a standardized format, simplifying interpretation.
Pinpointing Misfires with Mode $06: Focus on Component S21 and P0171/P0174 Clues
Returning to our Ford F-150 misfire mystery, the crucial step is identifying the Test IDs associated with the misfire monitor. Examining Figure 2 again, we can deduce this without service information. Observe the series of $51 Test IDs, with Component IDs ranging from $01 to $08. Intuitively, $51 TIDs relate to misfire monitoring, and CIDs $01 through $08 represent cylinders 1 through 8. This is a crucial piece of information for Ford OBD2 diagnostics.
For quick reference, early Ford systems utilize $51 for misfire monitoring, while later non-CAN systems use $53, and CAN systems employ $0B (history/last 10 drive cycles) and $0C (current/last test). Test ID $50 signifies misfires not attributable to a specific cylinder, typically triggering a P0300 DTC. While “component s21 fail” isn’t directly visible in standard Mode $06 data, understanding component IDs is key to interpreting the results. In the context of Ford OBD2 systems and misfires, component IDs usually refer to cylinders.
In Figure 2, all $51 TIDs except CID $03 show 0, indicating cylinder 3 as the misfiring culprit. The Max limit is 1638, and the Actual value is 1177, signifying misfires occurred during the last drive cycle but hadn’t exceeded the limit. This explains the absence of a DTC for cylinder 3. Once the Actual value surpasses the Max limit, a P0303 DTC would be stored, and the Check Engine light would illuminate.
The Check Engine light’s behavior varies depending on misfire severity. It will illuminate steadily for a Type B misfire or flash for a catalyst-damaging Type A misfire. Vehicle manufacturers set the thresholds for Type A and Type B misfires. Furthermore, lean codes like P0171 (System Too Lean, Bank 1) and P0174 (System Too Lean, Bank 2), while not directly misfire codes, can be related. A lean condition can sometimes cause or exacerbate misfires, and investigating these codes alongside misfire symptoms is crucial for a comprehensive diagnosis, especially when dealing with OBD2 Ford systems.
Ford provides valuable free resources at www.Motorcraftservice.com detailing their OBD II system operation. The “OBD Theory & Operation” tab on the left offers OBD summary documents from 1996 to the current model year, including diesel engines. These documents provide insights into Ford OBD II monitors, including monitor sequences, involved sensors, run times, conditions, and failure criteria.
Key takeaways from Ford misfire monitoring documents include:
- Fuel Tank Level: Misfire monitoring requires a fuel tank level above 15%. Low fuel levels suspend misfire monitoring.
- Profile Correction Learn: Clearing PCM memory necessitates a Profile Correction Learn to compensate for engine mechanical variations, potentially requiring extended road tests on newer vehicles. The OBD II document specifies that the PCM needs at least three 60- to 40-mph no-braking decelerations for Profile Correction Relearn, which a typical short road test won’t achieve.
- Catalyst Protection: The Check Engine light might not immediately illuminate for misfires if the count is below the limit. Ford’s catalyst protection strategy involves disabling the fuel injector on a misfiring cylinder to prevent catalyst damage. The cylinder then acts as an air pump, mitigating catalyst failure risk.
A thorough understanding of system operation is paramount for efficient driveability diagnostics. Knowing the “when, where, and how” of tests is critical. These Ford OBD documents offer insights beyond basic trouble tree charts. In no-code driveability situations, Mode $06 data is key to identifying misfiring cylinders, and understanding component IDs like S21 (though not standard, it could refer to a specific test or component within a Ford diagnostic context, warranting further investigation if encountered in specific Ford documentation) can be valuable. Consult your scan tool’s user manual to locate Mode $06 data if you’re unsure.
Beyond Mode $06: Factory Scan Tools and Power Balance Tests
Dealership technicians often utilize factory scan tools for cylinder identification. Figure 3 shows a Ford factory scan tool screen capture, demonstrating a Power Balance test. This test displays all cylinders in a chart, comparing per-cylinder RPM to a baseline. In this example, cylinder 3 shows reduced RPM, indicating its diminished contribution. While the misfire cause remains undetermined, cylinder 3 warrants further investigation.
Investigating the Root Cause: Fuel, Ignition, and Mechanical Factors
With cylinder 3 identified as the misfiring cylinder, the next step is pinpointing the cause. Misfires stem from issues within the fuel, ignition, or engine mechanical systems. Deductive reasoning is crucial for efficient diagnosis, guiding you to prioritize checks and tests. The intermittent nature of the misfire in this case points towards factors that can fluctuate.
Let’s consider the fuel system first. A shorted fuel injector would likely trigger an injector fault DTC alongside a potential P0303 misfire code. Service information would prioritize resolving the injector fault. Fuel injectors delivering excessive or insufficient fuel could lead to P0171 or P0174 fuel mixture DTCs (lean codes), which, as mentioned, can be related to misfires. A completely non-functional, non-shorted injector might cause a P0303 and potentially a P0171 (lean mixture) code.
Based on this logic, the fuel system is less likely the primary culprit for an intermittent misfire, although fuel delivery issues cannot be entirely dismissed, especially when considering potential lean codes like P0171 and P0174. Wildcards like water in the fuel or intermittent fuel injector wiring problems remain possibilities, but are less probable.
Moving to mechanical engine issues, problems like low compression, jumped timing, or valve train issues typically induce permanent misfires. While sticking valves or faulty lifters are possible, they are less likely to cause intermittent misfires compared to ignition system faults.
Ignition System: The Prime Suspect for Intermittent Misfires
The ignition system emerges as the most probable source of the intermittent misfire. The question then becomes: which ignition component is at fault – PCM, ignition coil, coil boot/spring, or spark plug?
Coil swapping, while common practice, is not always efficient and can be misleading. It might temporarily shift the misfire to another cylinder or mask the issue. A more accurate approach involves utilizing an engine analyzer/lab scope to examine the ignition waveform on the misfiring cylinder before disturbing any connections.
Lab scopes offer various methods for accessing ignition signals. Figure 4 displays a secondary ignition pattern captured from the Ford F-150 at idle, exhibiting an intermittent misfire, using a coil-on-plug (C-O-P) wand. C-O-P secondary patterns resemble single-cylinder distributor ignition patterns, with a key difference: Ford employs a multispark strategy at idle, transitioning to a single-spark program off-idle, a useful detail for loaded diagnostics.
Key points when analyzing Ford misfire ignition patterns:
- Persistent DTCs: A recurring DTC for a specific cylinder immediately after clearing often points to a shorted coil or a severe secondary ignition issue, potentially audible as arcing. Primary ignition patterns are effective for identifying shorted coils.
- Secondary Ignition Sensitivity: C-O-P systems are more susceptible to secondary ignition problems. The “Start of Burn” point in Figure 4 is crucial for assessing resistance in the boot, spring, and spark plug. Higher “Start of Burn” voltage indicates increased resistance, and lower voltage suggests decreased resistance.
- Intermittent Faults: Intermittent issues typically manifest in the secondary ignition section, often requiring road testing for accurate capture.
Figure 4 doesn’t depict a misfire. Remember, the misfire occurs under driving conditions, necessitating road testing with a suitable C-O-P road test attachment.
If a road test attachment is unavailable, Figure 5 presents an alternative method: connecting the lab scope to the ignition coil’s control side. The desired waveform resembles the blue trace, essentially Figure 4’s secondary pattern displayed at a lower voltage range. A low-current probe is also connected, but not essential for intermittent secondary misfire diagnosis.
The left image in Figure 5 shows a cylinder without a misfire. The right image captures the waveform during vehicle acceleration, replicating the customer’s complaint. The first secondary ignition firing event becomes a solid blue mass, with fluctuating secondary voltage. The second and third firing events also exhibit unstable secondary burn. Figure 6 provides additional examples of shorted coils.
Figure 7 reveals the misfiring cylinder’s cause: a damaged ignition coil boot. The white area indicates a burn-through in the rubber boot, where secondary voltage escaped. Inside the boot, a spring connects the coil to the spark plug. When the boot degrades, voltage finds a path of least resistance. Often, the damage is less obvious, sometimes manifesting as a carbon trace inside the boot, allowing voltage to ground along the spark plug.
Testing the secondary ignition before disturbing the coil allows for accurate fault identification. In this case, the issue resided in the secondary ignition area, and replacing the boot and spring should resolve the problem. The potential relationship with P0171 and P0174 codes in this scenario could be due to unburnt fuel from the misfire affecting oxygen sensor readings over time, although these codes are not directly caused by a faulty boot.
Conclusion: Mastering Mode $06 for Efficient Ford Misfire Diagnosis
In summary, efficiently diagnosing intermittent misfires hinges on a consistent strategy and proficiency with diagnostic equipment. Mastering Mode $06 data to pinpoint misfiring cylinders is a critical, time-saving step achievable with commonly available scan tools. Combining Mode $06 analysis with lab scope techniques and a logical approach to fuel, ignition, and mechanical system checks ensures accurate and effective diagnosis of even elusive Ford misfires, potentially preventing misdiagnosis related to codes like P0171 and P0174 by addressing the root misfire issue.
