What is misfiring and what are the causes of misfiring?

What is misfiring and what are the causes of misfiring?

In order for a cylinder to fire properly, it needs 4 things all working together:

  • Proper Fuel/Air Mixture
  • Good Compression
  • Proper Timing
  • Correct Spark

Common Misfire Causes

If one or more of the above factors aren’t in proper working condition, you’ll run the risk of misfiring. There are a variety of different misfire causes that can occur that can keep a cylinder from combusting properly. Sometimes it takes a combination of issues in order to make a cylinder misfire. Other times, misfiring isn’t consistent, and sometimes the cylinder will fire properly, while other times the cylinder will misfire. It depends on the type of issue and how severe the issues are.

Faulty or Fouled Spark Plugs,

Spark plugs provide the spark for the ignition of the fuel/air mixture in the combustion chamber. But spark plugs can go bad for a number of reasons and are one of the leading causes of misfiring. Spark plugs can get fouled from oil leaking into the combustion chamber, or from carbon buildup. They can also wear down more quickly from overheating and improper plug gap. The bottom line is that if you’re experiencing misfiring, one of the leading causes is bad spark plugs. It’s easy to check and easy to fix.

Cracked Distributor Cap,

The distributor is what controls the timing of the spark. When the distributor is functioning properly, the spark to each spark plug is sent at just the right time to ignite the fuel/air mixture right when the piston is ready. A cracked or damaged distributor cap can cause the signal to be lost to one or more of the spark plugs, resulting in a misfire.

Lean Fuel/Air Mixture,

The fuel/air mixture is a pretty delicate balance. If the mixture gets out of balance, it can cause a misfire. So what are some ways that the fuel/air mixture can get out of balance? If things like the fuel pump or the fuel injectors get clogged, then not enough fuel will be able to get to the combustion chamber and can readily cause a misfire.

Lack of Compression,

If a cylinder lacks proper compression, then the spark from the spark plug won’t ignite the fuel/air mixture.  Most of the time by leaky exhaust valves or a blown head gasket.

In either case, you’ve got a pretty major problem. One way to tell whether your head gasket is causing the problems or a leaky exhaust valve is to see if two cylinders next to each other are misfiring. If so, then you most likely are dealing with a blown head gasket instead of a leaky exhaust valve. In addition to this, if an engine overheats or runs hot, and if you’re noticing that you’re also mysteriously losing coolant, then you most likely are dealing with a blown head gasket.

Quick Diagnosis with a Misfire Code


On OBDII cars, the OBDII system will not only identify misfires but also coil and injector problems. Consequently, if the MIL lamp is on and you find a code for a misfiring cylinder and a second code indicating an injector fault for the same cylinder, bingo, the engine probably has a bad fuel injector. Likewise, if you find a misfire code for a cylinder and also a code indicating a coil fault for a multi-coil distributorless ignition system or coil-on-plug ignition, you can probably bet on a bad ignition coil.

In cases where there’s a cylinder misfire code but no other codes. The ignition or fuel delivery system may be borderline and not yet bad enough to set a code of its own. A shorted or open fuel injector solenoid, or a shorted or open coil will usually set a code. Consequently a dirty or weak fuel injector or a weak coil probably won’t set a code.

Bad spark plug wires are a common cause of misfire codes. After 50,000 or so miles, the OEM wires may be leaking current to ground or other wires, shorting the spark before it can reach the plug. Check resistance and if it exceeds specifications, replace the wire set.

A code ( P0300 – P0312 ) may mean that one or more of the following has happened:

  1. Bad spark plugs or wires
  2. Faulty coil (pack)
  3. Failed oxygen sensor(s)
  4. Faulty fuel injector(s)
  5. Burned exhaust valve
  6. Faulty catalytic converter(s)
  7. Stuck/blocked/leaking EGR valve / passages
  8. Faulty camshaft position sensor
  9. Defective computer

Help Finding The Misfire

In the case of a steady misfire, isolating the misfiring cylinder is the first step in diagnosing the problem. The old-fashioned method for finding a weak cylinder is to temporarily disconnect each of the spark plug wires, one at a time, while the engine is idling. When there’s no change in the idle speed, then you have pinpointed the weak cylinder.

A power balance test will tell you the same thing, but this requires some hookups and an engine analyzer. A power balance test is preferable to pulling plug wires, because it keeps you away from the voltage and prevents the voltage from causing any damage to the electronics in the ignition system.

When a plug wire is physically disconnected from a spark plug, the high voltage surge from the coil cannot follow its normal path to ground through the plug wire and spark plug, so it passes back through the coil. Most ignition systems are robust enough to withstand such voltage backups intermittently but not on a prolonged basis. If the coil or ignition module is already weak, it may push the component over the brink causing it to fail.

Compression And Ignition Misfires

Spark Plugs
Spark Plugs

If you have a misfire and have isolated it to one cylinder, the cause will be obvious when you remove the spark plug. If the plug’s insulator is cracked or broken, you’ve found the problem. When 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. Refer to the vehicle manufacturer’s specifications, but, as a rule, resistance should not exceed 8,000 ohms per foot. Replace the wire if resistance exceeds specifications.

You have found the source of the misfire if the plug is fouled. You still have to determine what caused the plug to foul. Heavy black oily carbon deposits would tell you that the engine is burning oil. Worn valve guide seals and/or guides could be the problem. Also worn rings and cylinders can also allow oil to enter the combustion chamber. Replacing the spark plug will temporarily cure your misfire problem.

Cylinder Leak Down Test
Cylinder Leak Down Test

A leakdown test or compression test will help you determine if the oil is getting past the valve guides or the rings.

If the cylinder shows little leak down 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.

A spark plug that shows heavy whitish to brown deposits may indicate a coolant leak.  It could be past the head gasket or through a crack in the combustion chamber. Coolant makes a lousy lubricant and can cause ring, cylinder and bearing damage.

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

Spark plugs that show preignition 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.

Short trip stop-and-go driving can cause a rapid buildup of normal deposits on plugs. The cure here might be to switch to a one-step hotter spark plug.

Compression Test
Compression Test

If the spark plug and plug wire are OK but the cylinder is weak do a leakdown or compression test. You may find out there is a compression problem.

The exhaust valves are the ones most likely to lose their seal and leak compression, so, if you find unusually low compression, follow up with a wet compression test to determine if the problem lies with the valves or rings.

No change in compression with a wet test would tell you the problem is valve related. A blown head gasket may be another cause. But, if the compression readings are significantly higher with a wet compression test, it would tell you the piston rings and/or cylinder walls are worn.

Either way, your looking at major repairs. The only cure for a leaky valve is a valve job. The only cure for a leaky head gasket is to replace the gasket. Likewise, the only cure for worn rings and cylinders is to overhaul or replace the engine.

A rounded cam lobe may also cause low compression. If the valve doesn’t open, the cylinder can’t breathe normally and compression will be low. A visual inspection of the valvetrain and cam will be necessary if you suspect this kind of problem.

Fuel Related Misfires

Fuel Injector Spray Patterns
Fuel Injector Spray Patterns

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.

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. Replace the injector if it causes a steady misfire.

You can also observe injector performance on a scope, and check its response to changes in the air/fuel mixture.

The injector scope pattern will tell you how long the injector is on.

If you make the air/fuel mixture artificially lean by momentarily pulling off a vacuum hose, and/or artificially rich by feeding some propane into the manifold, you should see a corresponding change in the injector on time as the computer responds to input from the oxygen sensor. No change would tell you either the O2 sensor is dead or there’s a problem in the computer.

A good injector should produce a cone-shaped mist of fuel vapor. If you see solid streamers in the spray pattern or a solid stream of fuel, the injector needs attention.

You should also check fuel pressure to see if the pump is weak or the pressure regulator is defective. A plugged fuel filter can reduce fuel pressure. If fuel pressure is within specifications, check the intake vacuum to see if there is an air leak that’s upsetting the overall air/fuel mixture. A couple of overlooked causes here may be a leaky EGR valve or a leaky power brake booster.


The best way to avoid an engine misfire condition is through following the scheduled maintenance in your manual. Keep your vehicle’s engine tuned according to factory specs. Plus, a yearly trouble-code scan by a well-equipped shop will uncover any potential problems before they become major faults.


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