What is an Engine Misfire?


Oct 21st, 2019

What is an Engine Misfire?

The automotive industry has come a long way - there is no denying that. Evolution has seen many advancements in engine sizes, transmissions, and designs. However, there’s one thing that has remained unchanged—the fact that we are yet to have a perfect engine.

On the bright side, diagnosing engine-related problems has become easier. Back in the day, the only way to figure out failures in the engine was through trial and error. These days, you can get information about engine system failure through a reader of fault codes from the Engine Control Unit (ECU).

Problems such as cylinder misfires are now easy to diagnose. Diagnostic tools can quickly narrow down and tell you exactly where the problem is. 

Everything you need to know about engine misfires 

The engine of any car operates in four steps: intake, compression, power, and exhaust. To put it in simpler terms: suck, squeeze, bang, and blow. An interruption or fail in any step in this sequence is what typically brings about engine misfiring. 

The affected cylinder(s) will not be able to carry out combustion well as a result. Some other factors may also cause engine misfire. Incorrect ignition timing, valve spring wear, cooling problems (overheating), and low fuel pressure can all lead to an engine misfire. The misfiring can happen to a single cylinder in the engine or the entire row.

An engine misfire reduces your car’s fuel efficiency and also increases emissions. If not fixed soon, the problem can spread to other engine components, including the catalytic converter or oxygen sensors. This situation will likely happen if you keep on driving your car and ignore the misfiring engine.

Engine misfires can be fixed quickly with no hassle. For vehicles built after 1996, an engine misfire will trigger the check engine light which will then turn on. The ECU will record a diagnostic trouble code. Unfortunately for vehicles built before 1996, the computer most likely won’t pick up on the misfire.

Symptoms of an Engine Misfire

An engine misfire results in part from the air-fuel mixture coming out of the engine unburnt. Modern cars come equipped with catalytic converters to help reduce emissions. When the unburnt mixture comes in contact with the catalytic converter, it will cause an explosion. 

That means not only will you have to replace the damaged or faulty parts causing the misfire but also the catalytic converter, which is not exactly cheap.

It is crucial to notice an engine misfire, especially when it's happening for the first time. If you ignore the first sign and continue driving, you risk more damage to your car. Just because the engine can operate without the cylinder that is misfiring doesn't mean that you should keep on driving the car nonetheless. 

Here are some of the symptoms to check out for:

  • Rough Acceleration - an engine misfire can manifest itself as light or sudden jerk from the engine
  • Uneven idle 'jumps' that can result in the engine shutting off
  • Heavy vibrations when the car is accelerating or idle
  • 'Check Engine’ light turning on 
  • Lowered accelerations that may even put the car in a limp mode
  • Strange engine sounds when you are driving

While different issues may cause these symptoms, it is still worth having your car checked so that you can be sure. 

The Engine Control Unit (ECU) can detect a misfire, but it is not a guarantee. When the crankshaft sensor notices a failure in the engine, say a misfire, it will send the information to the ECU. The crankshaft sensors make use of the camshaft sensor to determine which cylinder, in particular, is affected by the misfire.

If the ECU cannot narrow down the misfire, it will send a DTC code P0300 to show that it is a random misfire. 

Diagnosing and Fixing an Engine Misfire

A misfire generally occurs when either the ignition, air-fuel mixture or compression process fails to happen or happens at the wrong time (incorrect timing). To check the exact cause of the misfire, it is then necessary to go through all the steps and determine if all the components involved in each step work properly.

To save time, however, you can directly start by looking at the common faults that usually cause engine misfire.

First, read the DTC code in the DTC memory. Once you have it, you can go ahead and determine what it represents. For instance, here is what the following codes mean:

  • P0171 or P0172 – implies lean or rich fuel conditions
  • P0300 – random misfire
  • P0301, P0302, P0303 – several misfires 

You can also check the ignition because a failing ignition can cause a misfire. Next, check the leaks, especially inlet gasket leaks. Lastly, check the compression and fuel because, at this point, the remaining likely cause is the engine not injecting fuel on the affected cylinder(s).

Here is what to do when diagnosing a misfire:

When it’s the spark 

The parts that control spark to an engine are prone to wear in the course of operation. As these ignition parts wear, they won't be capable of performing efficiently. Their impedance will increase to a point where no electricity gets to the spark plug to cause an ignition.

This failure doesn't take place suddenly; instead, it happens over time, which explains why some engine misfires are intermittent. To prevent it from getting any worse, you can replace the spark plugs. It is simple and quickly done in a few minutes. While at it, you can also replace the ignition wires.

When it’s the fuel

Components involved in the fuel system don’t get worn out easily. Two causes of misfires that stem from the fuel system are clogging of the fuel filter and dirt on the fuel injectors. The problem could also be the fuel pump or airflow sensor failing.

These causes are often easily noticeable when idling as opposed to when accelerating. So, if the engine chugs when idling but runs smoothly when moving, then it probably is a fuel system issue. 

You can replace it if it is failing or clean the injectors to solve the misfiring issue. 

When it's a mechanical issue

Sometimes the cause of engine misfire is a complicated factor. It could be due to some cracks or to problems with the vacuum lines to the intake manifold. It is worth checking the throttle body and the condition of the gasket manifold. Also, take time to analyze the valve train.

To correct such issues, get your car checked and handled by an automobile repair expert. Do not wait for the engine misfire to worsen before going to an automotive repair shop.

Sam O