What Causes an Engine to Misfire?
The variety of culprits can make diagnosis difficult.
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QuickTakes:
Of the many ways that a vehicle's internal-combustion engine can malfunction, the misfire can be among the most frustrating to diagnose and remedy. Misfires may occur in any internal-combustion engine, including rotaries and diesels, and might result in wasted fuel and emissions-test failures. They can even leave you stranded.
What an Engine Misfire Means
An engine misfire generally means something is off with your engine's combustion. An internal-combustion engine and its control systems are designed to mix a precise ratio of fuel and air in each combustion chamber and then start the burning of that mixture at exactly the right moment.
When that combustion doesn't take place correctly, that's a misfire. This can be due to a number of issues, including a lack of fuel-air ignition, a weak ignition that fails to get the fuel to burn completely, or an ignition attempt that takes place at the wrong instant in the engine's rotation cycle. All are bad engine news.
A severe misfiring problem tends to be easier to feel and hear. You might, for example, feel unusual vibrations and/or shaking — especially under heavy throttle application — and hear popping or coughing sounds from the exhaust. Engine power will be diminished and fuel economy will suffer. Starting the vehicle may become more difficult.
Milder misfires might be so intermittent or subtle that not much is regularly noticeable from the driver's seat. Modern vehicles usually display a check-engine light when this takes place.
Different Issues Can Cause an Engine Misfire
There are a few common culprits for an engine misfire.
Ignition system problems: In a typical spark-ignition gasoline engine, the fuel-air mixture is ignited in the combustion chamber by creating a high-voltage arc at the tip of the spark plug at the right moment. Potential problems with this process include fouled or worn spark plugs, bad spark plug wires, dead or weak ignition coil(s), and failures involving ignition-system sensors.
Fuel system: If an incorrect amount of fuel is being delivered to the combustion chambers, misfires can occur. Problems with the fuel pump and/or fuel filter, fuel injectors, or bad gas could be the culprits.
Air system: The amount of air being fed into the engine must be controlled with great accuracy. Vacuum leaks can cause havoc with the air system, as can something as simple as a dirty engine air filter.
Engine-control system: Modern internal-combustion engines are loaded with an orchestra of sensors, solenoids, and valves — and that orchestra is conducted by a sophisticated computer known as the engine-control module (ECM). If the ECM stops functioning correctly, misfires can result.
Mechanical: Misfires could result from any problem that reduces piston or valve sealing — bad valve seals or piston rings, for example — a leaking head gasket, or a timing belt or chain that has skipped some teeth.
Diagnosis Can Be Tricky and Fixes Can Be Cheap or Costly
If you're seeing a check-engine light, consider having the on-board diagnostics (OBD) trouble codes read with a scanner. OBD misfire codes can offer specific clues. In addition, look for obvious loose wires and vacuum leaks under the hood, and make sure fuel and air filters are still in good condition.
When it comes time to get a misfire solution performed at a reputable shop, the complexity and cost could be as easy as changing a spark plug or as intense and expensive as an engine rebuild.
