Shop Class: How to Diagnose an Engine Misfire

Misfire!

Alex Steele
May 18, 2018
Contributors: ASE Master Technician
Photographers: Truck Trend Staff
We’re obviously not talking about a jammed semi-automatic or dud ammo, but an engine misfire is fundamentally the same thing. A cylinder supposedly filled with compressed air and fuel—akin to gunpowder packed into a bullet cartridge—fails to combust as expected.
Lots of engines will experience a misfire during their lifecycle, and there are all kinds of causes. Typical symptoms include rough running, loss of power and fuel economy, and the unwelcome check-engine light flashing on the instrument panel.
Many system failures can be the source, but let’s break misfires themselves down into three basic types. What we call a “dead-miss” describes a complete misfire with no combustion taking place whatsoever. A partial misfire means there’s some kind of a burn, but significantly incomplete combustion. An intermittent misfire (dead or partial) occurs only sometimes, be it under certain conditions or indiscriminately. All of the above can affect one or multiple cylinders, dependent on the root cause.
Photo 2/7   |   Shop Class Diagnose Engine Misfire

Ignition

A common instigator of a misfire in a gasoline engine is no spark jumping the gap of a spark plug to ignite the air/fuel mixture, i.e., nothing out of the primer at the base of a bullet cartridge. A variety of faults can make this happen.
Repeat after us: electromagnetic induction. This is the principle that enables an ignition coil (transformer) to boost a small amount of voltage to something closer to 40,000 volts, enough to efficiently jump a spark-plug gap and ignite air/fuel.
Every ignition coil uses a primary and secondary circuit. The primary side is made up of a thicker coil (windings) of wire wrapped around a secondary coil with a much higher number of thin windings, both surrounding an iron core. Battery voltage (and a ground to complete the circuit) is applied to the primary windings and a magnetic field is produced. When the circuit is opened, the field collapses and induces a higher voltage into the secondary coil. This voltage is directed to the spark plug, firing the cylinder.
What breaks? We’re going to bypass discussing distributors, points, and coil-packs, and focus on the most common coil-on-plug systems. Each coil is mounted on top of each spark plug within a cylindrical metal tube in the cylinder head. Keeping each coil independent enhances control and monitoring of ignition.
Photo 3/7   |   Shop Class Diagnose Engine Misfire Coil On Plug Systems
The weak point is typically the coil itself due to the high-temperature environment directly above the combustion chamber, along with the high-voltage being produced.
A short in the primary or secondary windings of the coil will cause a no-spark misfire, constant or intermittent.
The primary circuit uses a 12-volt power source and individual transistors within the powertrain control module. These transistors trigger the coil to complete the primary circuit’s path to ground, determining strength and timing of the spark. PCM failure, loss of the 12-volt source, or a short in any of the wiring can cause a misfire.
The secondary side is heavy-duty, handling high voltage to and through the spark plug. This is where we have to keep in mind that electricity follows the path of least resistance—always. So strong insulation is critical to channel the voltage correctly and successfully jump the plug gap.
Picture a coil-on-plug sitting inside its tube on top of the spark plug. The conductor between the coil and plug is surrounded by heavy insulation. Just a minute crack in the insulation can allow the electrical current to jump to the surrounding metallic tube, and never make it to the spark plug. The same theory applies if the tube was contaminated with oil, water, or anything that may allow an easier path to ground.
Photo 4/7   |   Shop Class Diagnose Engine Misfire Spark Plug Wires
Ditto at the spark plug: Once the electrical current reaches the terminal on top of the plug, a ceramic casing insulates it all the way until the spark jumps the gap between the plug’s center and side electrodes. A crack in the casing can provide a lower-resistance route to ground and kill that cylinder.
Fouled spark plugs convey the same situation. Current can travel through carbon buildup or excessive fuel (conductors) around the insulator and never reach the big gap between the two electrodes.

Fuel

Sticking with present-day stuff, we’re excluding carburetors and throttle-body injection from the fuel topic.
Fuel injectors, whether used with standard sequential multiport or direct-injection systems, actually share some commonalities with ignition coils. They are both individually controlled by the PCM and help out with accurate strength and timing in correlation with piston and camshaft position, but this time we’re dealing with fuel instead of spark.
Injectors are electric solenoids that open spring-loaded valves with fast pulses, hence spraying fuel into the combustion chamber via the back of the intake valve or straight into the chamber (direct injection). Just like ignition coils, an electrical failure inside the injector, the PCM, or wiring in between can cause a misfire.
Photo 5/7   |   Shop Class Diagnose Engine Misfire Fuel Injectors
The mechanical end of an injector is subject to failure too. The valve may become a tad blocked, disrupting the fine umbrella-shaped spray pattern needed for ideal combustion, or it could be totally plugged with debris and cause a dead miss. An injector can also not seal properly when closed and leak fuel into the combustion chamber when it shouldn’t.
Straight water, diesel, and sometimes even alcohol in a gas engine won’t burn. But a mix of contaminants with gasoline can cause a variety of symptoms, including misfires, often accompanied by various degrees of power loss and hesitation. It all depends on what and how much of the bad stuff was pumped into the tank.
User error is a cause of fuel contamination. More often than you might expect, folks fill up with diesel or E85 (in a non-flex fuel engine). Meanwhile, water and other contaminants in a gas station’s underground tank are a far-from-rare occurrence.
Some of us who still work with carbureted engines fully understand the corrosive properties of ethanol in gas. Let a carb sit for a few months with 10 percent ethanol gasoline in the bowl, and you’re ripe for another rebuild due to plugged jets. Possibly not as notable because of updated fuel-system materials, and the fact that regular run-time and fuel flow deters corrosion, ethanol does have adverse effects on injected engines. We’ve seen several instances where engines sat for about a year and failed to restart, with all injectors corroded shut.
A lack of fuel pressure due to a bad pump(s) or blocked passages can cause multi-cylinder misfires, but you’ll typically be focusing on lack of power and bogging from the lean condition.

Mechanical

Here we get down to the bare-bones engine breakdowns that can trip a misfire, most of the time a more expensive repair to boot.
A misfire from a mechanical source almost always boils down to a lack of compression: either a loss of seal at the piston rings in the cylinder bore, valvetrain failures, or a leak in the combustion chamber.
Cylinder Bore
The seal between the piston rings and cylinder wall may be compromised simply by high mileage, and they’re just worn out. But there are causes of premature failures such as poor machining and/or assembly, overheating, wash-down hampering ring lubrication, scoring from debris, and lack of maintenance. The bottom line is too much pressure is lost past the piston to achieve an efficient burn and/or too much oil enters the combustion chamber, dousing the fire or fouling the spark plug.
Photo 6/7   |   Shop Class Diagnose Engine Misfire Piston Ring
Valvetrain
Valve function is probably the most common source of a mechanical miss, and some engine designs require periodic valve-clearance adjustments. When the clearance gets out of specifications, misfires may result due to valve timing being thrown off or insufficient clearance not allowing a valve to fully seat and maintain compression.
A bad valve or valve seat—burnt, broken, or misaligned—will leak compression. A worn-out cam lobe will not open a valve far enough to allow enough incoming air or outgoing exhaust. Incorrect cam/valve timing throws the whole system out of whack. The intake and exhaust valves are no longer opening and closing in precise correlation with piston travel. This can result from a timing belt or chain jumping teeth, damage causing the miss-position of cam and crankshaft gears and sprockets, or even a variable valve timing problem.
Other Compression Leaks
After eliminating piston rings and valvetrain problems, where else could the pressure go?
Typically, it’s the head gasket, which seals compression, coolant, and oil between the cylinder head and block. A failed gasket may leak compression enough to cause a misfire, either alone or between two side-by-side cylinders and killing both at the same time. This can be due to age, block or head warpage from an overheat, poor assembly, or low-quality parts.
A really common scenario is a gasket leak between the cylinder bore and a coolant passage. During the cylinder’s power stroke, combustion gases are forced into the coolant system, but on the intake stroke, coolant is drawn into the combustion chamber. That coolant contamination with air/fuel impedes combustion.
Cracks (typically the result of way too much heat) in the head or block can cause nearly identical symptoms.

Diagnosis

Some of the old methods still hold true, and the first step is determining which cylinder is missing.
A cylinder balance test is an old-faithful misfire detection tool. Disable the combustion process of each cylinder one-by-one while observing engine speed (rpm). This can be accomplished by disconnecting coils or injectors. Example: All cylinders drop 300 rpm when shut down, but No. 3 doesn’t affect engine speed at all. We just narrowed it down to a dead-miss at cylinder 3. The same test, but this time cylinder 3 drops only 100 rpm, confirms a partial miss at the suspect cylinder.
Photo 7/7   |   Shop Class Diagnose Engine Misfire Service Light
The use of an oscilloscope goes way back. It displays voltage signals in wave forms that can be viewed on a monitor. Engine scope diagnostics look at the primary and secondary ignition voltage of each cylinder. This can not only determine which cylinder is missing but also point toward a cause (with a little expertise).
Today’s PCMs have the ability to determine a misfire, which cylinder is missing, and store a designated trouble code to aid in diagnosis. The most used platform views engine speed at the crankshaft position sensor. As the crankshaft rotates, propelled by the power stroke of each cylinder, a uniform engine speed pattern is inputted to the PCM. The PCM views speed increases from each cylinder’s combustion in the firing order. When a cylinder doesn’t fire, the decrease in crankshaft speed is documented in counts. This allows the PCM to determine what cylinder and how severe the misfire might be.
Concluding which, how bad, how often, and how many cylinders misfire directs us in tracking the cause.
Diagnosing a dead-miss can be a quick swapping of ignition coils with an adjacent cylinder. If the misfire is now at the other cylinder, you’ve successfully condemned that coil. The same thing applies with the swapping of one injector with another. Once determining both spark and fuel aren’t the problem, compression testing could be the next step.
Once in a while, diagnosis can get involved and require numerous tests, parts swapping, or even engine teardown to find a cause. The key is taking everything into consideration, but starting with the basics. The PCM’s data list gives a wealth of information that an experienced technician will utilize to crack the tough cases.

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