Engine Misfire Causes – Fuel, Ignition, Coolant Or Compression Related

Misfires - What You Should Know About Engine Misfires
Misfires - What You Should Know About Engine Misfires

Engine misfire causes; reduced gas mileage and increase emissions; which can cause you to fail an emissions test.

More seriously, engine misfire causes damage other engine parts; like the oxygen sensors or catalytic converter.

While driving, pay close attention to how your engine is running. Let’s look at what to do when diagnosing engine misfire causes.

When a misfire occurs, take note of the circumstances such as whether the engine is cold or has warmed up; the speed at which it occurs (low speed or high speed); the frequency it occurs; and if it occurs only when accelerating or at a steady speed.

Keep a log of the problems; this information will help you pinpoint the source of the problem.

Engine Misfire Causes
Spark Plugs Ready For Inspection

The First Thing To Remember Is That The Spark Plug Is A Window Into Your Engine. So, paying close attention to their condition may help narrow down your diagnostic search.

So, you can find many engine misfire causes just from examining your spark plugs. Consequently, a visual inspection of the spark plugs will display symptoms and conditions of engine misfire causes.

Basically, There Are Three Main Things That Can Affect Engine Misfire Causes:

  • Loss of spark
  • Air/fuel mixture is too far out of balance to ignite
  • Loss of compression

Consequently, There Are Other More Dire Engine Misfire Causes:

  • Computer or wiring problems
  • Breakage in the rotating mass (pistons, rods, crank bearings)
  • Valves leaking
  • Cracked or warped cylinder head
  • Cooling difficulties might permit overheating
  • Blown head gaskets

If you have an engine misfire and have isolated it to one cylinder; the cause may be obvious when you remove the spark plug.

Ignition Related

An ignition system problem is one of the most common reasons for an engine to misfire. As a result, Spark plugs, ignition cables, distributor cap and rotor, and ignition coil wear over time. Hence, their ability to transfer the needed spark to ignite the air/fuel mixture inside the combustion chambers becomes compromised. In the early stages; the spark will only be weaker and the actual misfire will be subtle. You may also find a cracked or broken spark plug.

Insulator Cracked Or Broken
Insulator Cracked Or Broken

If the plug appears to be OK but is wet; inspect the plug wire and boots for damage. Measure the plug wire’s resistance; end to end, with an ohmmeter. As a rule, resistance should not exceed 8,000 ohms per foot. Finally, replace the wire if resistance exceeds specifications.

Compression And Oil Related

Oily deposits on the plug would tell you oil is being sucked into the combustion chamber. It may be going past worn valve guides or seals, or worn or broken piston rings. Furthermore, there may be carbon deposits holding valves part way open causing compression loss. There is no easy fix for this kind of problem short of a valve job or overhaul.

Oil Fouled Spark Plugs
Oil Fouled Spark Plugs

Installing a spark plug with a slightly hotter heat range may help resist fouling. The most likely cause is worn valve guide seals and/or guides. Consequently, worn rings and cylinders can also allow oil to enter the combustion chamber.

Replacing the spark plug will temporarily cure your misfire problem; but, until the oil consumption problem is fixed; the engine will continue to foul spark plugs.

A leakdown or compression test will help you determine if the oil is getting past the valve guides or rings. If the cylinder shows little leakdown or holds good compression when a little oil is squirted into the cylinder (wet compression test); it would tell you that the engine needs new valve guide seals and/or guide work. Most late model engines have positive valve guide seals.

Coolant Leak

So, a spark plug that shows heavy whitish to brown deposits may indicate a coolant leak. This type of problem will only get worse and lead to even greater problems if the leak isn’t fixed. Coolant makes a lousy lubricant and can cause piston ring; cylinder and bearing damage if it gets into a cylinder or the crankcase.

Coolant Leak
Coolant Leak

Loss of coolant can also lead to overheating; which may result in cracking or warping of aluminum cylinder heads. So, if you suspect this kind of problem; pressure test the cooling system to check for internal coolant leakage.

Spark plugs that show pre ignition or detonation damage may indicate a need to check timing; the operation of the cooling system and conditions that cause a lean air/fuel mixture. You might also want to switch to a colder heat range plug.

Fuel Related

Also, if the ignition components and compression in a misfiring cylinder are fine; that leaves a fuel-related problem as the only other possibility. You can start by checking for voltage at the injector. A good injector should also buzz while the engine is running. No buzzing would tell you the injector is dead; while a no-voltage reading would tell you it isn’t the injector’s fault but a wiring or computer driver problem.

Engine Misfire Causes
Fuel Injector Comparison

If the injector is buzzing and spraying fuel but the cylinder isn’t getting enough fuel; the injector is dirty or clogged. On-car cleaning may help remove the varnish deposits that are restricting the injector and restricting fuel delivery.


So, The best way to avoid an engine misfire condition is through following the scheduled maintenance in your manual. Finally, keep your vehicle’s engine tuned according to factory specs.

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