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A flashing Check Engine light and a P0301 to P0312 diagnostic trouble code is a sure-fire indication that one or more cylinders are misfiring. Occasional misfires may pass unnoticed, but a steady misfire is hard to miss. The engine usually feels rough, lacks power and uses more gas than usual. Sometimes a misfire may feel like a vibration. A misfire also will cause a huge increase in hydrocarbon (HC) emissions out the tailpipe, which can cause a vehicle to fail an emissions test. Misfires that turn on the Check Engine light and log a cylinder-specific fault code are the easiest to diagnose. The OBD II system will identify the cylinder(s) that are not contributing their normal dose of power and set a code that corresponds to the cylinder’s firing order. A code P0303, for example, would tell you that the #3 cylinder is misfiring. To figure out which cylinder is #3, refer to a diagram of the engine’s firing order or look for markings on the intake manifold, spark plug wires or ignition coils (if the engine has a coil-on-plug ignition system).

When you have a misfire code for a particular cylinder, you can focus on one of three things: an ignition problem, a fuel injector problem or a compression problem.

Ignition problems that can cause misfires include worn or fouled spark plugs, bad plug wires, moisture on the plugs or plug wires, or a weak or dead ignition coil (in multi-coil DIS or COP systems). If there’s no spark or a weak spark, the air/fuel mixture may not ignite.

Engine Storytelling
Spark plugs can be easily checked by removing and inspecting them. “Reading the plugs” will tell you what’s going on inside the cylinder. Heavy black oily carbon deposits indicate an oil consumption problem (probably worn valve guides or piston rings). Fluffy black deposits indicate a rich fuel mixture (check for a leaky fuel injector, too much fuel pressure or a bad oxygen sensor). Thick brown deposits may be a clue that the engine spends a lot of time idling or is only driven on short trips. Switching to a hotter heat range spark plug may solve the idle fouling problem. A melted electrode would indicate pre-ignition and too much heat in the combustion chamber (check the cooling system). Physical damage to the electrodes or insulator would tell you the plugs are the wrong ones for the application. A spark plug with heavy whitish to brown deposits may indicate a coolant leak either past the head gasket or through a crack in the combustion chamber. This type of problem will only get worse and may soon lead to even greater problems if the coolant leak isn’t fixed.

Bad spark plug wires are another common cause of misfires. After 50,000 or so miles, OEM carbon-core ignition wires may have too much internal resistance that weakens the spark and increases the risk of misfire. High-mileage plug wires also can develop cracks that leak current to ground or to other wires, shorting the spark before it can reach the plug. Inspect the wires closely, and measure their resistance. Replace any wires that are worn, damaged, do not fit properly or have resistance that exceeds specifications. If more than one plug wire is bad, replace the entire set.

A condition known as “lean misfire” can occur if there is not enough fuel in the combustion chamber. Causes of lean misfire that only affect one cylinder include a dirty fuel injector, an open or shorted fuel injector, or a problem in the fuel injector driver circuit (wiring or PCM). Compression problems that may cause a misfire include a burned exhaust valve, bent intake or exhaust valve, or leaky head gasket.

Intermittent Misfires
The hardest type of misfires to diagnose are the ones that come and go and set no code. The misfire may only occur under certain operating conditions, or it may be totally random — which can drive you nuts. The underlying causes are still the same: ignition, fuel or compression. But the trick is trying to duplicate the conditions that cause the misfire so you can nail it down.

Intermittent misfires are almost always caused by a weak spark or a lean fuel mixture. That piece of knowledge may not tell you what exactly is causing the misfire, but it should lead you toward a coil problem or an injector problem.

Random misfires are another type of misfire that can be very difficult to diagnose. Random misfires may or may not set a P0300 code (it depends on their severity). In any event, a random misfire means the engine has multiple cylinders that are misfiring at random intervals. The problem may appear to jump from cylinder to cylinder, and the cause in most instances is the same as an intermittent misfire: a weak spark or a lean fuel mixture. If the ignition system checks out okay and is delivering a hot spark, something is upsetting the fuel mixture such as a vacuum leak, a leaky EGR valve, a faulty fuel pressure regulator, dirty fuel injectors, a plugged fuel filter or a weak fuel pump.

Tracking Misfires

No engine fires every cylinder 100% of the time. Misfires can occur at idle, when the engine is pulling hard under load, at high rpm and during throttle transitions as the air/fuel mixture changes. A few misfires are to be expected under these conditions, and should cause no major performance problems or significant increase in emissions. But if the misfires get out of hand and occur too often, they can make the engine idle or run rough, stumble when accelerating, waste gas and fail an emissions test.

On 1996 and newer vehicles that are OBD II compliant and have misfire detection, OBD II tracks and counts misfires. The misfire monitor runs continuously when the engine is started. On most applications, OBD II uses the crankshaft position sensor (CKP) to detect subtle variations in the speed of the crankshaft between cylinder firings. If the crank suddenly slows a bit, it indicates a misfire. But some of the older OBD II systems can be fooled by the vibrations produced by driving on a rough road and set a false misfire code. Newer systems are smarter and less apt to be fooled by road vibrations.

On some vehicles, the amperage of the spark current is analyzed when each spark plug fires to determine if the mixture burned or not.

When OBD II detects a misfire, it stores operating data such as engine speed, load and warm-up status. While the misfire is happening, the MIL lamp may flash to alert the driver. Because this might distract the driver, the OBD II system will set a temporary misfire code after the second such occurrence. From that point on, the MIL lamp should flash every time the misfire returns. If the same thing happens on the next trip, the MIL lamp should blink as before and remain on even when the misfire ceases.

If the misfire problem has gone away and does not reoccur on the second or following trips, the OBD II system may erase the temporary misfire code and forget the entire episode. The code also may be erased if no misfires are encountered under similar driving conditions during the next 40 drive cycles.

Knowing this, you should always look at the history freeze-frame data when diagnosing a misfire code. If the code set when the engine was cold, chances are the OBD II system is being overly sensitive and there may be no misfire problem. Check for any technical service bulletins (TSBs) that may be out on the vehicle for false misfire codes. On vehicles that have a history of being overly sensitive, the fix may be reflashing the PCM with new instructions.

On some cars, (Volkswagens, for example), it’s possible to set false random or individual misfire codes when doing a cranking compression test. If this happens, just clear the codes after the test so the MIL lamp doesn’t come on later.

Multiple Codes
If you find an injector code as well as a misfire code, the misfire is probably due to a bad fuel injector. Likewise, if you find a misfire code for a cylinder and also an ignition coil code for a multi-coil distributorless ignition system or coil-on-plug ignition, you can probably assume a bad coil is causing the misfire.

In cases where there’s a cylinder misfire code but no other codes, the ignition or fuel delivery system may be borderline and not yet bad enough to set a code of its own. A shorted or open fuel injector solenoid, or a shorted or open coil will usually set a code, but a dirty or weak fuel injector or a weak coil probably won’t set a code.

Scoping It Out
An oscilloscope is a good tool for identifying and analyzing misfires, but using a scope takes some know-how and experience. Suffice it to say that anybody can “see” a misfire on a scope, but figuring out whether the cause is ignition or fuel requires an understanding of ignition waveforms. Once you’ve identified the problem cylinder, you can look at the primary and secondary patterns for that cylinder to get more diagnostic information. The dwell section of the firing line just ahead of the spark will tell you what’s going on in the primary side of the system. The spark line in the firing section after the spark occurs will tell you what’s happening in the cylinder and coil.

Misfires can be spotted by observing a secondary parade pattern that shows the firing lines for all the cylinders on one trace. The peak firing voltage for a misfiring cylinder with a shorted plug or plug wire will be lower than the others. A worn or fouled spark plug will show a higher than normal firing voltage on the scope. But don’t forget that low compression and an overly rich fuel condition also can cause the same kind of variations in firing voltage readings.

If a normal or lower than normal firing voltage drops even lower when the throttle is snapped, it may indicate a plug wire that is shorting to ground.

If one of the peak firing voltages is spiking significantly higher than the others, the misfiring cylinder may have a bad plug wire with excessive resistance, a badly worn spark plug or a plug with an excessively wide gap. Lean fuel conditions also can cause firing voltages to jump.

Note: If the spark burn lines for all the cylinders are higher than normal (indicating a lean fuel mixture), the underlying cause would be something that affects all cylinders such as an intake manifold leak, leaky vacuum hose, leaky EGR valve, leaky throttle gasket or low fuel pressure (weak fuel pump or bad pressure regulator).

A rich fuel mixture in an individual cylinder is less common, but can occur if the fuel injector leaks. A more common condition would be a rich condition in all cylinders caused by a dead oxygen sensor or coolant sensor that prevents the computer from going into closed loop, or a faulty fuel pressure regulator or plugged fuel return line that routes too much pressure to the injectors. Something else to look at in the spark burn line is the amount of “hash” it contains. A good cylinder will show a relatively clean line with little hash. A lot of hash, on the other hand, occurs when ignition misfire or lean misfire are present.

The duration or length of the spark burn line can provide more clues about what’s going on inside the cylinder. The duration of the spark line for a “good” cylinder should usually be 1.3 milliseconds or longer at idle with a 0.045” to 0.050” plug gap. A shorter spark line would indicate a weak spark possibly due to a lean fuel condition (dirty injectors or vacuum leaks) or low compression (burned exhaust valve or bad head gasket). If the duration of the spark line is longer than about 2 milliseconds, the air/fuel mixture is running abnormally rich. If the burn line is shorter than about 0.75 milliseconds, the cylinder is running too lean.

The last thing you want to look at in the primary ignition pattern is coil oscillations. If the coil is good, there should be at least two and preferably three or more oscillations after the burn line. Fewer oscillations would indicate a faulty coil.

More Checks
On older vehicles with a distributor, check the distributor cap and rotor for cracks and carbon tracks that can short the spark to ground.

If firing voltage is low (weak spark), measure the coil’s primary and secondary resistance with an ohmmeter. If out of specifications, replace the coil. Also, make sure the coil is receiving normal voltage from the PCM or ignition module. If the supply voltage is low, check the charging system for anything that may be reducing the alternator’s output (battery and alternator cable connections, ground connections, and voltage output).

One thing to keep in mind here is that low coil output also can be caused by a faulty ignition module or coil driver circuit in the PCM. A lot of coils are replaced unnecessarily because problems like these are overlooked or misdiagnosed.

Simple checks that can be made to identify a misfire caused by a fuel delivery problem include checking the injectors and fuel pressure. You should hear a buzzing noise if the injectors are pulsing. A test light or logic probe also should flash when connected to each injector circuit.

You also can observe injector performance on a scope, and check its response to changes in the air/fuel mixture. First of all, a flat line would tell you the injector is dead or is not receiving voltage (depending where the line falls on the screen). If the injector is working, the line should drop when the injector turns on, then peak when the current is switched off.

The injector scope pattern will tell you how long the injector is on. If you make the air/fuel mixture artificially lean by momentarily pulling off a vacuum hose, and/or artificially rich by feeding some propane into the manifold, you should see a corresponding change in the injector on-time as the computer responds to input from the oxygen sensor. No change would tell you either the O2 sensor is dead or there’s a problem in the computer.

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Larry Carley

Larry Carley has more than 30 years of experience in the automotive aftermarket, including experience as an ASE-certified technician, and has won numerous awards for his articles. He has written 12 automotive-related books and developed automotive training software, available at
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