So an engine misfire isn’t just annoying—it’s a warning sign.
Beyond the shaky idle and poor gas mileage, ignoring a misfire can lead to expensive repairs.
Consequently, misfires reduce gas mileage and increase emissions, which can cause you to fail an emissions test.
More seriously, engine misfiring can cause damage to other engine parts, like the (O2) Sensors or catalytic converters.
Misfires occur when these factors are wrong, missing, or not working together:
- First: A too lean or too rich air fuel ratio
- Second: Bad ignition spark / wrong timing of the ignition spark
- Third: Low compression / air fuel ratio is leaking
- Improper Valve Timing
The symptoms
Is your car shaking at red lights? Is the Check Engine Light (CEL) flashing? An engine misfire isn’t just annoying—it’s a warning sign.
Common Causes of Engine Misfiring
1. Faulty or Fouled Spark Plugs
Above all Spark plugs are the leading cause of engine misfiring. They provide the electricity needed to ignite the air-fuel mixture. Over time, however, they can wear down or become “fouled.” In addition to spark plugs, failed ignition coils or wires are common culprits.
- Oil leaks: Oil seeping into the combustion chamber.
- Carbon buildup: Often caused by frequent short trips.
- Overheating: Damaging the plug’s insulator.
2. Failed Ignition Coils or Spark Plug Wires
In modern cars (COP systems), each spark plug has its own coil. But in older cars, a distributor cap and wires send the spark. If a wire is leaking current or a coil has shorted out, the spark will never reach the plug. This can cause engine misfiring.
- Pro Tip: If your car has over 50,000 miles, OEM wires can begin “leaking” electricity to the engine block. Instead of the plug.
3. Lean Air-Fuel Mixture
Above all your engine requires a delicate balance of air and fuel. If there is too much air and not enough gas, it’s called a “lean” mixture. This is often caused by:
- Clogged fuel injectors.
- A failing fuel pump.
- A vacuum leak (unmetered air entering the engine).
4. Lack of Compression
If a cylinder can’t hold pressure, the spark won’t ignite the mixture properly. This is usually a “mechanical” misfire caused by:
- Leaky Exhaust Valves: Allowing pressure to escape.
- Blown Head Gasket: Consequently engine misfiring in two adjacent cylinders often signal a blown head gasket. Check for mysterious coolant loss too.
Diagnosing the Problem with OBD-II Codes
Above all the easiest way to find engine misfiring is by using an OBD-II scanner. If your “Check Engine” light is on, look for codes P0300 through P0312.
- P0300: Random/Multiple cylinder misfire.
- P0301–P0312: The last two digits tell you exactly which cylinder is failing.
- Specifically, code P0304 identifies a failure in Cylinder #4.
“Bingo”:
- A misfire code + an injector fault code = Bad Fuel Injector.
- A misfire code + a coil fault code = Bad Ignition Coil.
- Misfire code + Lean code (P0171/P0174) = Vacuum leak or fuel delivery issue.
- This combination is a very common scenario for DIYers.
How to Isolate a Engine Misfiring Cylinder
What if you don’t have a scanner. Then you can find a steady misfire by temporarily disconnecting one plug wire at a time while the engine idles.
- The Test: If you pull a plug wire with insulated pliers and the engine idle doesn’t change. Thenyou’ve found the dead cylinder. Getting shocked by a spark plug wire is a “rite of passage” most DIYers would prefer to skip!
- Caution: So this “old fashioned” method can be hard on modern electronics. However a professional Power Balance Test is safer. As it prevents high-voltage damage to your ignition module.
Reading Your Spark Plugs
When you pull the plug, its appearance tells a story:
- Wet/Oily: Oil is getting past the rings or valve guide seals.
- Whitish/Brown Deposits: This suggests a coolant leak (likely a head gasket or cracked head).
- Black Carbon: The engine is running “rich” or you’re doing too much stop-and-go driving.
When you pull the plug, its appearance tells a story. For example, a wet or oily plug means oil is getting past the rings. On the other hand, black carbon suggests the engine is running ‘rich.
In Summary: Engine Misfiring
So a misfire is your car’s way of asking for help. Start with the basics: check your spark plugs and wires first. If the problem persists, move on to compression and fuel delivery tests. Catching a misfire early won’t just save you money at the gas pump—it might just save your engine.
“Thanks For Reading!”
