Diagnosing Engine Misfires

Counterman · November 17, 2022

Engine misfires are among the most common issues that a technician must diagnose. However, the biggest problem – from a diagnostic standpoint – is that it’s almost an automatic reaction to blame misfires on the ignition system. Understanding why this is a problem, and the reason it happens, can help you differentiate between ignition-related and non-ignition-related misfires.

There are a couple deep-seated reasons for the blame. To begin with, years ago, before fuel injection and distributorless ignition, the majority of all noticeable misfires were caused by an ignition problem – either a bad spark plug or plug wire. Note that I said noticeable misfires. On today’s vehicles, ignition problems indeed are a common cause of engine misfires … but also a common misdiagnosis. This is where it starts to get interesting.

Carburetors, distributors and plug wires got us through the first 80-plus years of the automobile, and did a great job at it. But the bottom line is that they were simply inefficient fuel and ignition systems. Compared to today’s vehicles, they ran terrible! Once you had a few miles on them, they idled rough, maybe had to warm up a little longer to avoid a stumble, and most cars had their own little idiosyncrasies for starting.

This was the type of performance we were used to, so nothing registered to us as a misfire until it was a “dead” miss. This is a term we as technicians use to refer to a misfire that renders a cylinder completely ineffective, or completely “dead.” These types of misfires shake the entire vehicle to the point where even the most absent-minded of vehicle owners knows something is wrong.

It Was Easy Once

Back in the day, spark plugs had relatively short service intervals – around 10,000 miles – and plug wires were wide-open to abuse and wear. So, not only were they the frequent cause of misfires, but it wasn’t uncommon to open the hood and see a spark jumping from a plug wire to ground, or between wires. This made it easy to diagnose, and if there was no visual cue such as this, a common trick was to use insulated spark plug wire pliers and pull off one wire at a time to see if it made a difference. If you got no change, you found the suspect cylinder. If you got the daylights knocked out of you, you needed a new set of insulated pliers!

This was a sloppy yet effective diagnosis, but by no means an insult; I’ve done it a “million” times. Now, let me get back to the diagnostic problems of a misfire. For complete and correct combustion to occur, you must have the proper amount of air, fuel and spark; you must have the correct amount of compression; and the ignition timing must be correct. So, the possible causes of incomplete combustion could be too much or too little air; too much or too little fuel; lack of or not a strong enough spark; spark at the wrong time; or a mechanical problem that causes insufficient compression.

On old cars, incomplete combustion was common (one reason emissions were so bad), but there was no computer, no electronics to tell us, and as I mentioned, we were used to engines that ran a little rough and didn’t have the best drivability. On today’s vehicles, complete combustion is critical for emissions and fuel economy. This is why the engine control module (ECM) continuously monitors for complete combustion, and while there’s a lot to that, we’ll just focus on the misfire monitor.

The operation of the misfire monitor is primarily based on crankshaft pulses. The ECM continuously monitors crankshaft speed via the crankshaft sensor, and if it sees even the slightest deviation, its logic deduces that some form of incomplete combustion (i.e., a misfire) has occurred. Since any type of incomplete combustion will cause an increase in emissions, a diagnostic trouble code (DTC) is stored, and the “Check Engine” light is illuminated.

While the driver of the vehicle might not have noticed anything wrong, the ECM did, and now there’s an annoying light glaring at the driver, enticing them to seek service. As a technician, when we pull the DTCs on a vehicle that seems to run fine, it’s not even a slight surprise to see misfire codes, which are always P0300 for random misfire, or P0301 and up (the last number indicating the cylinder from which the misfire occurred).

When You Assume …

With all of that said, now let’s focus on the diagnostic problem of assuming that a misfire is caused by ignition. Problems such as the slightest pinhole vacuum leak or the slightest loss of fuel delivery from an injector will be detected by the PCM, storing a misfire trouble code. If we allow ourselves to automatically blame it on ignition, a percentage of the time we will be wrong. However, since we know that ignition problems still are common, there are certain signs we look for and certain things we can do to determine if the problem is ignition-related.

To begin with, one telltale sign of an ignition misfire is the flashing “Check Engine” light. As part of emission control, all ECMs will store trouble codes when a misfire is detected. But, when the ECM determines there’s imminent danger of damage to the catalyst, it flashes the “Check Engine” light to get the immediate attention of the driver. This occurs when it picks up a “dead” misfire, with the logic that quite possibly there is excessive unburned fuel coming out of the dead cylinder, which will damage the catalyst.

These “dead” misfires again can be felt by the driver, so a shaking vehicle and a flashing “Check Engine” light go hand-in-hand. While not always, this is usually an ignition problem, and quite often the engine will idle smooth, but under acceleration begin to misfire and flash the light. Most misfire monitors are accurate with the reported cylinder, and a common practice for diagnosis is to swap the coil and plug from the suspect cylinder with those from another. More often than not, the misfire will move, and while it feels the same, the ECM will instantly report the misfire from the other cylinder.

Rolling back the clock again, even in the age of the carburetor, technicians knew that misfires could be caused by multiple factors aside from ignition, and if a car had a misfire that wasn’t blatantly obvious, we relied on an oscilloscope to be able to “look” at the operation of the ignition system on a screen, which allowed us to compare the efficiency of all cylinders and deduce whether the problem was ignition-related, or caused by another factor.

These tools and techniques still are relevant today, but there are more special-tool requirements for more vehicles, as opposed to one set of test leads that you could hook up to any make or model. As technicians, we always take into account all of the factors that can cause a misfire, so as not to misdiagnose it. However, we may not always have the required equipment to view ignition waveforms, so we continue our inspection with more signs that can indicate an ignition-related misfire.

Spark Plugs and Critters

Naturally, worn spark plugs can cause a misfire. If you know the mileage on them, it can help you decide whether it’s time for replacement. Once they’re removed, excessively worn electrodes are a good clue.

It’s always important to look for wiring damage. Coil wiring can be inviting for rodents, so a close inspection always is a good idea. Carbon tracking is another tell-tale sign of an ignition misfire. The high-voltage spark produced in the coil finds its path to ground by jumping across the gap of the spark plug. Electricity always takes the easiest path to ground, and if it finds an easier way than the spark plug gap, it’ll go there.

Spark plug wire or coil boots are designed to insulate the high voltage, but problems can cause the spark to travel under the boot and ground to the base of the spark plug. Carbon is a byproduct of an electrical spark, and a high-voltage spark traveling between the wire boot and plug will leave its “footprint” via a carbon track.

Carbon tracking also can be visible inside a distributor cap, and where you find it, you can be sure of an ignition misfire. A final visual indicator of an ignition misfire can be oil in the spark plug wells. Many of today’s cars are designed with spark plugs in the center of the combustion chamber. It’s common in these designs for the well seals to leak, and oil can work its way up underneath the coil boot and cause a misfire.

During any type of diagnosis, we always use sight, sound and smell as our most important tools. Even if you can’t see it, you get used to the sound of a jumping spark, and this can be an indicator of an ignition misfire. You also might smell excessive fuel at the tailpipe. There are other indicators with scan-tool data that can give you a clue as to whether you have an ignition misfire, but you have to be careful.

If you have an ignition misfire and excessive fuel is being introduced into the exhaust, the engine-management system will recognize this as well and make adjustments to fuel delivery, which can alter engine data and the smell from the exhaust.

As you can see, this is a really deep hole, and the bottom line is engine misfires can be difficult to diagnose at times due to the sensitivity of modern engine-management systems. However, a thorough inspection and careful mindset will allow you to head off an ignition misfire before getting too deep into additional diagnostics.