Editor's Note: This article was orginally published Sept. 15, 2015. Some of the information may no longer be relevant, so please use it at your discretion.
In this tool briefing, we are going to discuss the tools and strategies necessary to diagnose the cause of misfires. Since misfires can have many different causes, we will split up the article into three segments and deliver them over the next few PTEN issues. In this month’s article, we will focus on how to easily determine what type of misfire is occurring; mechanical, fuel, or ignition. We will also look at the three primary tools necessary to determine the cause" vehicle information, scan tool information and obtaining information from the customer.
Step 1 – Obtaining Vehicle Specific System Information
Since auto manufacturers utilize differing methods to determine when a misfire is occurring, it is necessary to understand the conditions that cause the DTC to set for that particular vehicle. Knowing what conditions are used to set a DTC will allow the tech to have a better understanding of the strategy the manufacturer uses to determine if a misfire is occurring. Using the information provided from a vehicle information source, such as Mitchell ProDemand, can provide you with not only the conditions necessary to set a DTC, but what conditions can cause a misfire DTC to not set, even if you feel one while you are diagnosing the vehicle.
Let’s start with the reason misfire monitoring is part of OBD-II. The Federal Government mandated in 1996 that vehicle manufacturers notify the driver if a catalyst damaging event is occurring or if emissions were exceeding the FTP (Federal Test Procedure) by 1-1/2 times. Monitoring for misfires can result in both notification of pending catalyst damage or higher emissions.
It is up to the vehicle manufacturer how to determine if a misfire is occurring that will cause the FTP threshold to be exceeded. Because of this, there is not a one-size-fits-all type of strategy to approach misfire diagnosis. For example, most vehicle manufacturers utilize crankshaft speed signals to determine if a misfire is occurring. If the crankshaft speed slows between cylinder firings, it is assumed a cylinder did not have the same combustion rate and therefore is misfiring. Many manufacturers disable the monitor if the ABS or Traction Control System is active because a rough road can cause a tire to bounce on the road, changing crankshaft speed similar to a misfire.
Not all vehicles will do this. We once had a 2009 Toyota Sienna with misfire codes in the shop that, according to the freeze frame data, would only misfire on the first trip of the day. After many diagnostic tests and trips attempting to duplicate the issue, we were unable find a cause. With some research we found a TSB that suggested that a tire severely out of balance could set a misfire DTC. In this case when the tires were cold, they vibrated enough to cause engine speed changes that simulated what the PCM strategy determined to be a misfire. If this vehicle used ABS sensor inputs to disable the misfire monitor, a code would not have set. Understanding the system as well as determining if any TSBs are available can save a lot of time.
Step 2 – Obtain Information from the Customer
I teach a lot of technical classes, and one of my favorite questions to ask technicians is “What should be the first step in your diagnostic procedure?" I get many answers, but not many techs give the correct one, which is “Duplicate the Customer’s Concern." Using a diagnostic worksheet that utilizes information obtained from the driver of the vehicle will give you details that may not be available even from a seasoned technician with the best scan tool. Not understanding what is occurring when the customer experiences the issue can not only add time to the diagnostic process, but may even cause an incorrect or incomplete repair.
A good diagnostic worksheet will also help lead the technician through a complete diagnostic strategy that reduces the likelihood of overlooking a simple solution to the cause of the misfire. The Diagnostic Worksheet, as seen in the sample, can be utilized in the shop to remind technicians to test the basics like battery voltage, fuel pressure, and engine vacuum. All of these items that are easily overlooked because the tech assumes they are okay. It can also be used as a piece of information that can be filed with the repair order, as well as shared with the customer to “justify” the cost of performing diagnostic testing by showing them all of the items that were tested.
Step 3 – Choosing the Correct Tool
Once you understand what the vehicle uses to determine if a misfire is present and obtained enough information from the driver, it is time to get more information. I typically use a code reader to start my diagnostic process. Using a code reader, I can quickly get a lot of information, allowing me to choose the correct tools to continue the diagnostic process. In some cases, it may be quicker to use a manufacturer specific scan tool rather than an aftermarket scan tool. Many of the “factory” scan tools provide great information, but they can be much slower than an aftermarket scan tool. Mode $06 can provide very useful information to diagnose misfires as well as confirm the repair, but many factory tools do not provide Mode $06 data.
Step 4 – Using the Scan Tool
Once you determine which scan tool you will use, it is important to gather information that will assist in the diagnosis as efficiently as possible. The “old days” of just looking at codes are pretty much gone with the advent of CAN bus systems. In addition, most of the newer aftermarket scan tools now provide a lot of valuable information and also are much better at enabling bi-directional controls of components. We will focus the rest of this article on how to maximize the value of your scan tool to assist in diagnosing the type of misfire that is occurring. One of the first things you should look at now is a complete Network Check, or as some scan tools call it, a “Health Check.”
The value of this is not only getting a clear picture of all the computer modules seeing if any codes are present, but checking for “U” codes. A “U” code indicates a loss of communication between modules and, in many cases, can lead you directly to an issue. For example, most vehicle manufacturers will disable the misfire monitor when the fuel level is below 1/8 of a tank.
It would be possible to feel a misfire when there are no misfire codes present, if the fuel level sensor cannot communicate due to a CAN communication issue, the misfire monitor will be disabled, therefore not allowing a misfire DTC to set. So if there were a “U” code for a Rear Integration Module present, the Fuel Sender could not provide information to the PCM, disabling the misfire monitor. Repairing the “U” code first, would enable the monitor and allow a DTC to set, pointing to which cylinder is having an issue.
There are many other functions the scan tool can perform, but the order you do them can save time down the road. Many technicians will clear the codes after obtaining them, but you need to remember that when you clear codes, you also erase Freeze Frame Data, reset Adaptive Strategies and reset the Monitor Status. Let’s address the value of each. Freeze Frame Data can provide you with information the PCM was receiving at the time a DTC was set. If it is erased, you lose valuable information of what the sensors were reporting, and the information necessary to drive the vehicle under the same conditions that created the misfire.
Resetting the Adaptive Strategy allows both Short (STFT) and Long (LTFT) Term Fuel Trims to return to their baseline. Fuel trim data allows the technician to understand how fuel is being burned in the combustion chamber and how the PCM is compensating for it. Observing Fuel Trim on each bank can give additional clues as to which cylinder, or bank of cylinders, is having the issues. Fuel Trim can also be used to see if there is a vacuum leak causing the misfire. If you noted a LTFT value of +25 percent at idle, and +4 percent it would indicate a vacuum leak, requiring more fuel at idle, but not effecting the required fuel mixture as much at higher RPM’s.
Obtaining the status of each monitor can also assist in determining what could be causing a misfire. The value of looking at the status of the monitors is twofold. First and foremost, seeing which monitors have not completed can assist in understanding some of the data stream information better. For example, if the fuel monitor has not yet run, Fuel Trim corrections in the data stream may not help in the diagnosis. Additionally, providing the monitor status to the customer and advising them that all of the PCM’s “Self- Tests” have not yet completed and if the check engine light comes on shortly after your repair it does not mean the same issue is present.
Next in line would be looking at Mode $06 data. Mode $06 is typically found in the scan tool’s OBD Generic test functions. MODE $06 data consists of raw data that is provided to the PCM from various non-continuously monitored components, such as misfire. Looking at mode $06 data can provide you with an ongoing count of how many misfires are occurring on each cylinder. This information is not only valuable to determine which cylinder is misfiring, but can be used to confirm the vehicle has been repaired properly. Some older scan tools did not easily provide mode $06 data easily, but the newer tools on the market have improved significantly not only the ease of obtaining the data, but in translating the hexadecimal raw data into easily understood information.
Lastly, let’s look at the data stream information useful to determine the cause of a misfire. It is important to keep in mind that the data displayed on tour scan tool is digitized. That means depending on how many PIDs are being displayed, the data you are seeing may not actually be “live” pieces of information. Basically, the more PIDs you are looking at, the slower the information will be updated to the scan tool.
This is also important to understand if you are observing graphs such as Oxygen Sensors. The information updates much slower and is very different from what you would see on a lab scope. Looking to see if the cam and crank signals are synched can also be useful, in some cases if they are not in synch, misfire detection will not be enabled. Observing Oxygen Sensor data and fuel trims are key in understanding what type of misfire is occurring.
If the system is indicating a lean condition, then focusing on fuel delivery issue would be the most direct approach. Observing MAP Sensor fluctuations could indicate mechanical failures causing the misfire. Note the battery voltage to make sure it is sufficient to deliver enough current to operate the ignition and fuel systems.
Many newer vehicles now have PIDs for current on each fuel injector or ignition coil. If this data is available, it can be used to determine if there is a specific issue on an individual cylinder.
Another PID that you should check is to see if any injectors are disabled. Since OBD-II requires limiting emissions, many vehicle manufacturers will disable fuel injectors once a misfire is detected. This is done so as to not put unburned fuel into the catalytic converter, possibly damaging it. Looking to see if an injector has been disabled by the PCM can prevent lost time looking at a non-functioning injector, as the cause of a misfire when it may be due to ignition or mechanical issues, but the injector has been turned off by the PCM and will remain off until the misfire code has been cleared.
In the next two articles, we will combine information gained from the scan tool, diagnostic worksheet, and the vehicle information and use it to guide you through a quick diagnostic process using more advanced tools such as lab scopes, pressure/vacuum transducers, and other methods to determine the direct cause of a misfire.
