Basic Understanding Why Your Car’s Engine Misfires

Your vehicle’s engine goes through a combustion process thousands of times each minute. Air and fuel are sent to each cylinder’s combustion chamber where the mixture is compressed. Coil voltage travels through a spark plug that is located at the top of each chamber. The spark plug ignites the compressed mixture, which provides the necessary energy to move your vehicle down the road. When a misfire occurs, the event affects your car’s performance, efficiency, and overall drivability.

This article will explore the reasons your engine might suffer a misfire. We’ll take a look at problems involving a loss of spark, unbalanced air-fuel mixture, and compression leaks. I’ll explain the factors that can contribute to each of these issues.

Loss Of Spark

A loss of spark can be due to fouled spark plugs, bad wires, or a distributor cap that has developed a crack. Plugs should normally be replaced every 40,000 miles. Even those that are advertised as being capable of lasting 100,000 miles should be replaced long before that marker arrives. Besides expiring due to normal use, oil deposits can build on the electrodes, preventing voltage from jumping the gap.

Spark plug wires are critical because a fouled wire will prevent voltage from reaching the plug. If that happens, the plug will be unable to ignite the compressed air-fuel mix in the associated cylinder’s combustion chamber.

If the distributor cap is cracked, the voltage may be unable to travel properly between the rotor tip and the terminals. Here too, this can prevent the plug from receiving the voltage necessary for ignition.

Unbalanced Air-Fuel Mixture

For several reasons, the air-fuel mixture within the combustion chamber can be too lean. When this occurs, there is an insufficient amount of gasoline to provide an efficient burn. This might be due to a fuel pump that is failing, a fuel injector that has formed an obstruction in the nozzle, or even a leaking exhaust gas recirculation (EGR) valve. Each can prevent sufficient fuel from reaching the cylinder’s combustion chamber.

There may be also be cases in which the mixture is too rich. Rather than an insufficient amount of gasoline preventing an efficient burn, there is too much in the chamber. This problem is far less common than a lean mix. When it occurs, it is usually due to a leaking injector.

Compression Leak

If a given cylinder’s chamber is suffering from a loss of compression, that means it is losing a portion of the air-fuel mixture before it can be ignited. This problem can usually be narrowed down to two potential root causes: an exhaust valve that has formed a leak or a blown head gasket. If you’re able to identify misfiring within multiple cylinders (and you have confirmed your spark plugs are fine), the issue is likely the head gasket.

A loss of compression can be confirmed by performing a leakdown test. It is a simple test that will help you identify whether compression is being lost through an exhaust valve with a deteriorating ring. This is a test you can do on your own rather than hiring a mechanic for the job. Most auto supply stores sell a special gauge that is inserted into the suspected cylinder’s spark plug hole.

Misfires can be serious. If you’re driving a small 4-cylinder car, a single misfiring cylinder can reduce your engine’s power by 25 percent. You’ll feel it shaking at idle. If the problem is severe, your engine may even stall. Even if your engine has eight or more cylinders, a steady misfire can reduce its fuel efficiency and impact its overall performance. Moreover, your car will fail an emissions test.

If your engine is misfiring, test the spark, air-fuel mixture, and perform a leakdown test for compression leaks. With a little time and effort, you can successfully narrow down and fix the root cause.