An engine misfire is a common cause of check engine lights and major customer concerns. Shaking, no power, and a flashing check engine light are things that make even the most stubborn customers come to see mechanics in a hurry.
In this lesson, I’ll explain engine misfires in detail, breaking down the common, and not-so-common causes of this problem and how to pinpoint them.
After reading, you’ll know more about misfiring engines than those who built them. It’s time to stop throwing coils at misfires and guessing, let’s understand this 100% so you can diagnose and fix engine misfires properly.
Let’s get started…
What is an engine misfire?
The term engine misfire is exactly as it sounds, a cylinder has not gone through the combustion process during an engine revolution.
A particular cylinder (or cylinders) has an issue, preventing it from igniting and producing power, which is a direct cause of a misfire. There can be different types of engine misfires as well.
A dead miss is when the cylinder is completely dead, and a sporadic miss is intermittent, maybe happening every so often but still enough to set a code.
The shaking and roughness you feel while an engine is misfiring is from the dead cylinder failing to contribute to the full revolution of the crankshaft. The cylinder is not pulling its weight which throws the entire engine off balance.
TL,DR: Misfires occur when one or more cylinders fail to achieve combustion properly, resulting in a severe loss of power, rough idling, and poor fuel efficiency.
Understanding the symptoms and causes of engine misfires is crucial for mechanics to know inside and out.
Let’s start with the basics…
What an engine needs to run correctly
Every engine needs fuel, air, compression, and ignition to run… On top of that it all needs to happen at the precise time for complete combustion to occur.
Let’s break down how this all comes together to help us better understand engine misfires.
How does an engine run?
To understand misfires you need to understand how an engine runs. When you know how it works you can reverse engineer why a cylinder is misfiring.
Let’s run through the process…
The Intake Stroke
During the intake stroke, the piston travels downward in the cylinder and the intake valve opens. This creates a vacuum within the cylinder and draws air and fuel through the open intake valve.
Carbureted engines draw air and fuel through the opening intake valve and bring the air/fuel mixture into the cylinder.
Fuel-injected engines rely on the intake stroke for drawing the air needed into the cylinder. The fuel injector provides the fuel, either from the back side of the open intake valve or directly into the combustion chamber.
Engines that inject fuel directly into the combustion chamber are referred to as “direct-injected” engines.
Direct injection is for another article, however the precision of this system is why most modern vehicles are using this technology.
At the end of the intake stroke, the piston has traveled completely down the cylinder and has reached BDC (bottom dead center) the cylinder is now “charged” with the correct air/fuel mixture for efficient combustion…Take that Elon.
The Compression Stroke
There is only one way to go from rock bottom, and that’s up.
Air and fuel will surely burn, but to make the power an engine needs to run the mixture needs to be compressed before being ignited.
As the piston travels back up the cylinder both the intake and exhaust valves close, creating a sealed chamber.
That beautiful air and fuel from the compression stroke begins to feel the squeeze.
When the piston approaches TDC “or top dead center” the ignition coil sends the electrical voltage needed to fire the spark plug.
A quick note: The firing of a cylinder doesn’t usually happen exactly at TDC. Varying firing positions affect power and efficiency greatly, but that’s for another article.
We have a compressed air and fuel mixture… The injector has fired.
This brings us to the next stroke…
The Power Stroke
The ignition system fires the cylinder spark plug and WHAM! That compressed energy ball of fuel and air ignites… Here comes the BOOM!
The energy released drives the piston back down the cylinder at a rapid rate, with a ton of energy behind it.
It’s important to note that there isn’t a massive explosion within the cylinder like people think, it is a very rapid and controlled burn that drives the piston down efficiently.
This stroke is what makes cars move, without this there is no power or energy produced. Every cylinder must complete a successful power stroke to contribute to the 360-degree cycle of the crankshaft or the engine will shake and have a severe lack of power.
A misfiring cylinder doesn’t contribute to the power production of the running engine this is what causes the shaking and drastic power loss.
The takeaway is this… This is the stroke that makes power and keeps the engine running. The piston is forced down by the energy release of igniting the compressed air/fuel mixture.
The exhaust stroke
The intake, compression, and power strokes have been completed, what’s next?
We have burned fuel and there are byproducts in the cylinder, the exhaust stroke has one purpose, clearing the cylinder before the next intake stroke brings in the next batch of fresh air and fuel.
The power stroke has sent the piston back down to BDC and the power produced has been transferred to rotational energy.
As the piston travels back up the cylinder the exhaust valve opens and the intake valve stays closed. The piston in the car’s engine pushes the burned fuel exhaust out through the open exhaust valve, and the entire process starts over again, illustrating the cyclical nature of engine operation.
Common Causes of Engine Misfire: A Comprehensive Overview
So what will create a misfire?
Now that you know how an engine runs let’s take a look at what can cause a misfire.
Loss of ignition
The biggest cause of engine misfires on average is the ignition system. Without anything to ignite the compressed air and fuel, nothing happens.
Let’s break down the components responsible for giving that critical spark every gas engine needs.
Spark plugs
Spark plugs have one job… Igniting the air-fuel mixture in the combustion chamber. Spark plugs are the final step in the ignition process.
Spark plugs wear and eventually cause misfires if not replaced. As they wear, the electrode gap becomes bigger, which can cause an engine to misfire due to inadequate spark. This means higher voltage is needed to jump the gap.
The engine will most likely have an intermittent miss, especially under hard acceleration.
Another issue that causes spark plug-related misfires is carbon tracking.
Carbon tracking occurs when ignition wires or coil boots “leak” voltage out around the outside of the spark plug. The result is a hard miss, either consistent or sporadic, depending on how bad the tracking is.
Carbon tracking shows up on the porcelain portion of a spark plug, it resembles a pencil-like black line that you usually can catch your fingernail in. If you find this, the plug and coil boot (or wires) must be replaced before any further diagnosis.
Ignition coil and wires (if applicable)
Ignition coils, now commonly referred to as coil packs in modern vehicles, are responsible for generating the high voltage needed to create a spark at the spark plugs.
If an ignition coil fails, it can’t produce the voltage needed. This can cause both hard and intermittent misfires.
You will often see coils that work but do not provide enough voltage. Or a coil that will only start to misfire when the engine is scorching, such as after running for 45 minutes or so.
Wiring and ECU signals
A misfire caused by a lack of ignition isn’t always caused by a bad plug or coil. There is much more that the ignition system needs to get those two components working.
Firstly, today’s engines have crank and camshaft sensors. These sensors send the ECU signals it uses to determine the exact position of the crankshaft and camshafts.
The computer uses this data to fire each injector and ignition coil at the correct time. This data is also used to control the stop-start features on newer cars. The ECU pre-fires the cylinder it knows is in position to assist the starter.
This is why these modern cars can immediately start when you need to drive off from a stop light.
If the crankshaft position sensor fails, or there is a break in the harness, the injectors and ignition coils will not fire in most cases. All is well here because the engine won’t run at all, let alone miss.
The problem occurs when there is a missing signal to one coil or a break or high resistance in the wiring harness. Confirming spark output and injector pulse width is essential on all cylinders when tackling a misfire.
In the case of a sporadic or random misfire that is very difficult to pinpoint a lab scope may be used to monitor the coil and injector, this is very advanced and will be covered in a different article soon.
Fuel Mixture too rich or lean
The air and fuel mixture needs to be within a specific range for combustion to happen efficiently.
The correct air-fuel mixture is referred to as stoichiometric or 14.7 to 1.
This is the perfect mixture for efficiency.
If the air/fuel mixture within the cylinder is too rich or lean, a misfire can result. Let’s take a look at both situations quickly.
Mixture too lean
The first and most common for creating an engine misfire is a lean mixture.
A lean mixture means there is a lack of fuel to match the air volume, there is not enough fuel to the amount of air within the cylinder.
Telltale signs of a misfire caused by a lean condition are random multiple misses throughout all of the cylinders and lean fuel trim codes, such as…
P0171 System too lean Bank 1
You may also hear “lean backfire” in the intake as well depending on how lean the mixture is.
I will be covering fuel trim in much more detail in coming articles, this is a great time to sign up for my email newsletter so you don’t miss out!
Mixture too rich
The opposite of lean is rich, rich is excessive fuel within the cylinder. When there is too much fuel from a stuck injector or excessive fuel pressure the result can be a misfire not only in one cylinder, but all if the rich mixture is caused by something else besides a single fuel injector issue.
Now that we know how an improper air/fuel mixture can cause misfiring, let’s break down some common causes of an out-of-whack mixture.
Vacuum leaks and air intake problems
Vacuum leaks allow unmetered air to enter the engine, creating a lean condition. Similarly, air intake problems such as a clogged air filter or a malfunctioning mass airflow sensor can result in a rich mixture and misfire.
Even excessive fuel pressure or a leaking fuel pressure regulator can cause a rich condition.
Air that enters the engine after the mass air flow sensor via a vacuum leak is not accounted for by the engine ECU, it has no way of knowing the proper fuel trim adjustment.
As a result, the computer drives the trims (either rich or lean) until it passes a limit, once the ECU drives the injector rate past the limit without achieving a stable fuel mixture a code is set.
Low compression
Fuel – air + ignition… Still a dead misfire on one or more cylinders?
This is when you start to question the compression within the cylinder. Remember, without compressing the air-fuel mixture before firing it will not achieve the proper ignition needed to create that all-important power stroke.
With confirmed ignition and fuel, it’s time to perform a compression test on the cylinder to determine the internal health of the engine.
Here are the common reasons for low cylinder compression…
- Burned valves
- Worn piston rings
- Major piston damage
- Engine severely out of time
- Blown head gasket, or cracked cylinder head
- Other major damage within the engine
Perform a compression test to determine the overall health of the engine. Engines vary in compression ratios, for example, a turbo-charged engine has a lower compression ratio to offset the boost pressure that is introduced into the cylinders.
The lowest acceptable compression reading on a single cylinder is 130 psi, below this issues will start to develop on most engines.
Tech tip: Add a small amount of clean engine oil to the cylinder being tested if the initial reading is low, the piston rings may be washed down with fuel, and this will greatly affect the compression readings.
Cylinder compression is often overlooked, especially by unseasoned techs. A compression test will show you the overall mechanical health of the engine and help you determine what is causing the engine to misfire.
What happens next when cylinder compression is too low?
A dead cylinder with low compression indicates a mechanical misfire, meaning something is preventing the pressure build-up within the cylinder to accommodate proper firing on that cylinder.
Once you determine there is low compression the fun doesn’t stop, you need to find out why.
This is where a cylinder leak-down test separates the technician from the wannabe. A cylinder leak-down test pressurizes the cylinder while it is at TDC of the COMPRESSION stroke.
Remember… In this position, the piston is at the top of its stroke and all valves are closed. I will explain what a leak-down test is and how to do it in a later article, for now, let’s focus on principals first.
Tech tip: When you introduce pressurized air into the cylinder the vehicle’s engine might, (or will) turn over as the pressure pushes the piston down. Unless the leak is so large no pressure can build up.
What a cylinder leak-down Test Reveals
A compression test tells us if cylinders in the engine have issues, and a leak-down test tells us WHERE the leak is. This is done by pressurizing the cylinder with compressed air and measuring the amount of “leak down” or air escaping the cylinder.
In essence, a leak-down test shows us internal engine problems.
Anything over 10% leak down indicates issues and may be a sign that you have engine damage.
After the leak-down test is set up you can simply feel for airflow out of the exhaust, intake manifold via an airbox or vacuum hose, or from the dipstick.
What does this indicate?
Let’s break it down… As long as you have the test set up correctly and the engine is in the proper position, ie both the intake and exhaust valves closed fully there should be minimal leakage past the piston rings and that’s it.
Airflow out of the exhaust? One or more of the exhaust valves are burnt, or not seated properly.
Feel or hear air escaping through the intake manifold? Same deal, just with an intake valve.
Smell oil and feel flow from the oil cap or dipstick? It’s getting past the piston rings, this is known as “blow-by.”
Keep in mind there are situations where things are tougher to confirm, such as cracked cylinder heads. Either way, the principle is the same, be sure you are set up correctly and find out where the air is escaping.
Restricted exhaust
What happens when ignition is there, you have fuel, compression seems OK and only one cylinder bank seems to be missing while monitoring live data?
This is a strong indication of a restricted exhaust on that cylinder bank. A clogged or broken catalytic converter core is blocking off the exhaust.
This causes very high back pressure that chokes the engine, when the engine can’t get rid of exhaust gases efficiently, it can’t breathe in fresh air to burn… Here comes the engine misfire.
An exhaust back pressure test is done by removing the front oxygen sensor BEFORE the converter suspected of being restricted.
A high temp rubber plug is held against the hole as a helper revs the engine quickly and immediately lets off the gas as the engine approaches redline.
If the converter is clogged the gauge will peg, indicating the exhaust pressure can’t flow through the exhaust as it fights to escape through your gauge.
This test can save a lot of headaches, remember misfires on all cylinders of one bank indicate a strong possibility of a clogged exhaust.
Engine timing and mechanical wear or failure
Now that we have covered what engines need to run and fire properly on all cylinders…
- Air and fuel
- Compression
- Ignition
Camshaft timing issues
The order and timing of when all of this happens is critical. Another major problem that can cause misfires is crankshaft to camshaft correlation … ie engine timing.
The crankshaft and the camshafts are connected via a timing chain or belt, gearing on some engines. The point here is… These components need to be perfectly synchronized for combustion to happen.
As the crankshaft rotates it’s driving each piston up and down within the cylinders. The camshaft (or camshafts) turn to open and close the valves.
For perspective, when the piston of a cylinder reaches TDC (top dead center) of the compression stroke, the camshaft lobes are sitting at their lowest point to the valve rocker or stem. This means that there oblong camshaft lobe is not contacting the valve, opening it, allowing compression to occur.
When there is an issue with camshaft timing the position of the camshafts changes and causes valve lift to happen early or late. This can cause misfires simply because the compression stroke is compromised due to leakage past valves at the wrong position when the engine needs to fire.
Major internal engine damage
Then there is serious engine damage this can be a hole burned in a piston or some other issue that requires a complete rebuild or replacement.
Preliminary Steps to Diagnose Engine Misfire
Diagnosing engine misfires requires a systematic approach to identify the root cause of the missing cylinder. Here is a step-by-step guide to diagnosing engine misfires:
1. Retrieve trouble codes: Use an OBD-II scanner to retrieve any trouble codes stored in the engine ECU. These codes provide information about the specific cylinder(s) experiencing misfire.
Using codes you can tell what cylinder is missing by the code number…
P0300 means there is a random misfire across many cylinders
P0301 would be cylinder #1
P0302 cylinder #2 … and so on, up to the number of cylinders your engine has.
Use any other codes present to help narrow down the issue. For example, if you have a P0300 (Multiple cylinder misfire) and a P0171 (system too lean bank 1) this indicates a lean condition that is affecting the entire engine across the board.
Find out why there is not enough fuel first…
2. Inspect spark plugs and ignition coils: Remove spark plugs and inspect them for signs of wear or carbon tracking. Check the ignition coils for any visible damage or signs of failure.
Don’t forget to check the wiring and connections to the coils.
3. Confirm fuel trim readings: Use your scanner to read the fuel trim values on both cylinder banks. If the fuel trim is lean this indicates a problem with either a vacuum leak or fuel delivery in most cases. If it is extremely rich the opposite applies.
In upcoming lessons I will cover fuel trim in depth.
3. Perform a compression test: A compression test is the best way to know if the engine has internal wear or other mechanical issue. Poor compression indicates a mechanical problem.
If cylinder compression is low do a leak-down test to find out where the leak is.
Identifying Faulty Spark Plugs and Ignition Coils
To inspect spark plugs, remove them from the engine making sure to keep them in order. Look for signs of wear or fouling. Worn spark plugs may have a rounded electrode or excessive gap, while fouled spark plugs may have a buildup of carbon deposits or oil on the electrode.
Ignition coils can be tested using an ohmmeter to measure their resistance. If the resistance is outside of the manufacturer’s specifications, the ignition coil may need to be replaced. A coil output test can also reveal how strong the coil is.
While running the car carefully pull the coil up away from the spark plug while wearing insulated gloves. You should hear a strong snapping sound as the voltage jumps the gap you have created to the spark plug. This needs to be done VERY carefully and is one of those “old school” techniques.
The last and more accurate way to test ignition coils requires a lab scope to monitor the peak and drop of the coil. This is advanced and requires lab scope know-how for accurate diagnosis and repair of misfires, however, it is the most accurate.
Addressing Fuel System Issues that Cause Engine Misfire
Fuel system issues can disrupt the proper delivery of fuel to the combustion chamber, leading to engine misfires. Common fuel system issues include clogged fuel injectors, a malfunctioning fuel pump, or a restricted fuel filter.
Earlier direct-injected vehicles had problems with injector pintles sticking open, literally flooding the cylinder and converter with fuel.
This is why knowing the baseline fuel trim is so important when diagnosing issues with engine performance. You can see whether the engine is running too rich or too lean immediately. The first thing I ask my guys when they call me over to help with a running issue is “Where’s the fuel trims?”
To pinpoint fuel system issues, install a fuel pressure gauge before anything else and record running fuel pressure. Also watch the gauge when the engine is turned off, if it drops rapidly it indicates a leaking injector or a bad fuel pump check valve.
A bad check valve will cause a longer cranking time before starting, a common customer complaint.
Inspect the fuel injectors for any obvious issues. If the vehicle is standard injection use a noid light to confirm the trigger signal to the injector is OK.
Clogged fuel injectors can be cleaned using a specialized cleaning solution or replaced if necessary. ALWAYS test the fuel pressure using a fuel pressure gauge to ensure it is within the manufacturer’s specifications. DO NOT be the guy who guesses what the pressure should be. You need to know the exact specifications and never rule out problems because the pressure is “only” 10 psi low.
If the fuel pressure is too low, it may indicate a problem with the fuel pump or a restricted fuel filter.
Resolving Vacuum Leaks and Air Intake Problems
Vacuum leaks and air intake problems are very common causes of misfires and other running issues. Vacuum leaks can occur due to cracked or disconnected vacuum hoses, while air intake problems can be caused by a clogged air filter or a malfunctioning mass airflow sensor.
Crankcase breather issues are another “internal source” of false air. Always confirm breather pressure using a CEM meter. Another overlooked cause of fuel trim issues on many cars is a bad evap purge valve, this can be very intermittent. At times the purge valve will cycle but will not seat 100% after, causing a vacuum leak as the valve continues pulling fuel vapor from the evap canister, after the vapor clears the vehicle will go lean.
To identify vacuum leaks, inspect all vacuum hoses for any visible signs of damage or disconnection. Use a smoke machine to locate any hard-to-find leaks. Air intake problems can be identified by inspecting the air filter for any clogs or damage, and testing the mass airflow sensor using a multimeter, scanner, or lab scope
Tackling Timing Belt and Camshaft Issues
Timing belt and camshaft issues disrupt the engine’s valve timing, leading to engine misfire. The timing belt is responsible for synchronizing the rotation of the crankshaft and camshaft, while the camshaft controls the opening and closing of the engine’s valves.
To identify timing issues, confirm camshaft timing using the procedure for the car you are working on. Inspect the timing belt for any signs of wear or damage, such as cracks or missing teeth, and check timing chain stretch as well.
Preventing Future Engine Misfire: Maintenance Tips and Best Practices
Regular maintenance and inspections are essential to prevent engine misfires and ensure optimal vehicle performance.
In conclusion, engine misfire is a common problem that can significantly impact the performance of a vehicle. Understanding the symptoms and causes of engine misfires is crucial for prompt diagnosis and resolution of the issue. By following a systematic approach to diagnosis and addressing the root cause of engine misfires, vehicle owners can ensure optimal engine performance and prevent further damage to their vehicles. Regular maintenance and inspections are also important to prevent future engine misfires.
