Misfired Codes - Much has been written on how to find and repair misfire codes, but diagnosis can be even easier if you understand what they are and how they are set. - Motor Age - Automotive

Service Repair

Misfired CodesMuch has been written on how to find and repair misfire codes, but diagnosis can be even easier if you understand what they are and how they are set.
Publish date: Sep 1, 2007
By:  Peter F. Meier
Source: Motor Age

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The cause of a P0316 and P0301 on a Ford Mustang. Notice the stain on the cylinder in front, showing where the bent rod didn't allow the piston all the way to Top Dead Center (TDC), and reducing compression in that cylinder.
Once you have the cylinders identified, take a look at the Freeze Frame information. Keep in mind that the freeze frame data is stored at the time the code is stored and may actually be the vehicle condition some time after the misfire first started. Misfire detection strategies measure the number of misfires over a given number of engine revolutions.

For most manufacturers, misfires are monitored over a 200-revolution counter and a 1,000-revolution counter, in addition to being monitored during initial start-up. The number of misfires detected that would trigger a fault depends on the operating conditions and the emissions regulations in effect when the vehicle was made.




Typically, misfires that would result in emissions levels exceeding that limit would log a Type B misfire, while a misfire level that would lead to overheating of the catalytic converter (with resulting damage) would log a Type A misfire. A Type B misfire is a two-trip code, requiring the same fault to be seen during the next monitor run before turning on the Malfunction Indicator Lamp (MIL), while a Type A is a one-trip code that will cause the MIL to "flash" its warning to the driver.

In any case, a specified number of misfires must be detected before the ECM will consider it a problem, and they will show up in the misfire history for each cylinder.

Fuel trim factor


Isuzu P0301 Freeze Frame
Take a look at the fuel trims stored. Generally, if the total fuel trims are above normal but less than +18 percent, start your diagnosis with the ignition system. If it's greater than +18, you may want to take a look at the engine's mechanical condition and fuel system first.

If you see a code P0316 (misfire detected on the first 1,000 revolutions after start-up), then something is "dead." Suspect an engine mechanical fault first, then move on to a "no fuel" or "no spark" inspection.


Enhanced Misfire Counters
A code P0300 (random cylinder misfire detected) tells you that the allowable misfire count has been exceeded, but not by any one particular cylinder. Look for intermittent faults, weak ignition systems or intake leaks or leaking Exhaust Gas Recirculation (EGR) systems as possible causes.


Ford Misfire PIDS
For the "techies" out there, there are other methods you can use, too. There is a PC-based program that will help you identify the missing cylinders, as well as offer ideas on possible causes based on information it gathers from the oxygen sensor signal. Vacuum-pressure transducers can be used to analyze exhaust gas pulses at the tailpipe. Another program is incorporated into a PC-based lab scope, and allows cylinder power balance testing as well.




In all cases, avoid "tunnel vision" and remember that misfire codes can be caused by a wide variety of culprits. Your first challenge is to identify the affected cylinder(s), then think about what the offending cylinders have in common. Is the problem on only one bank? Is it limited to cylinders that are next to each other? Do the cylinders share an ignition coil? Is the misfire intermittent or repeatable? Does it occur only at idle, at highway speed, or across the board? How was the vehicle being operated when the code was stored? Can you duplicate the problem? Are there any modifications? You get the idea.

Misfire codes may be common, but the causes of misfires are varied and multiple. Verify the integrity (or weakness) of the major sub systems at the beginning of your diagnosis, and you will nail misfire faults easier and faster.

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About the Author

Peter F. Meier


Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. Articles by Peter F. Meier

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Source: Motor Age, 9/1/2007
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