Sparesbox Blog

How To Diagnose An Engine Misfire

By Linda Quan

Mon May 11 2020

What is an Engine Misfire?

An engine misfire is characterised by a brief “stumble” or halt in power delivery during driving. Engines run on a sequence of combining fuel and air, compressing the mixture, combusting the mixture with a spark and releasing the exhaust gases. When one of these steps doesn’t happen as intended in one or more of the cylinders, you’ll notice a sudden decrease in power – this is an engine misfire.

Signs of Engine Misfires

  • Sudden loss of power
  • Difficult to start engine
  • Decreased fuel efficiency
  • Sudden increase in emissions
  • Popping sounds from the engine
  • Backfiring intake or exhaust manifold
  • Excessive vibrations while idling
  • Stalling engine


Why Should You Address an Engine Misfire Straight Away?

Leaving engine misfires unaddressed could lead to much bigger problems down the track. Not only will your fuel efficiency suffer and emission levels increase, unburnt fuel can build up in the catalytic converter, causing it to heat up and eventually fail. The rule of thumb is the longer you wait to fix the issue causing the engine misfire, the more expensive your repairs will be, so don’t leave it too late!


What Causes Engine Misfires?

The main causes are:

  • Ignition issues: Spark plugs wear out over time and are designed to be replaced when they are no longer providing their maximum performance. Damaged and worn out spark plugs or ignition coils create weaker sparks as they degrade, thus causing small misfires that gradually get worse over time. This problem is easily solved by replacing your old spark plugs, ignition coils or ignition leads with new ones.

  • Incorrect air/fuel ratio: Misfires that caused by a malfunctioning fuel system appear suddenly and are more noticeable when idling. These misfires can be caused by clogged or leaking fuel injectors or fuel filters, or degraded EGR valves.

  • Vacuum leak: Misfires caused by vacuum leaks could result from damaged vacuum hoses, which are often indicated by the presence of a loud hissing sound during idling. Luckily, vacuum hoses are cheap and easy to replace!

  • Mechanical misfires: These can be caused by a slipped timing belt or chain, burnt out valves or a multitude of other issues. If the problem can’t be easily identified, it’s time to bring it to a professional.

If your engine is misfiring, it’s always a good idea to take your car to the mechanic sooner rather than later. If you’re in need of replacement parts, Sparesbox has you covered – our simple-to-use fitment tool and 100% fitment guarantee means you’ll always have the right part for your car. Head over to the home page and check out our huge range of Engine, Cooling & Drivetrain replacement parts today!