4 Common Engine Misfire Causes

The combustion engine is relatively complicated as it has a lot of parts that need to work in the correct order and at the right time for everything to function. Broken, stuck, worn, or dirty parts can all lead to issues. Some defects can cause all of the cylinders to misfire and some only cause one to misfire.

Causes of an engine misfire can be hard to find since there are so many engine components involved. Here are the most common causes to help you troubleshoot.

Problems with the Ignition System

Worn spark plugs or spark plugs with an incorrect gap are a common issue because the tip is delicate and spark plugs have a relatively short lifespan. If the spark plug gap is too big, the spark may not always be able to jump the gap between the center and side electrodes. No spark means no power on that stroke.

Ignition coils (and distributors) convert the relatively low voltage of the battery into the high voltage needed by the spark plugs. These components may wear out eventually due to the heat generated each time they fire. If the insulation on the ignition coil is worn, all that electricity may wander down a different path, instead of going into the spark plug or wire as intended. 

Distributors are spinning mechanical parts found on older cars, and as such are subject to wear. Sometimes carbon, debris, or water finds its way under the distributor cap, hindering the distributor’s ability to transfer spark to the plug wires. Water in the distributor cap can make it difficult or even impossible to start the vehicle.

If the ignition is overly advanced or retarded, the spark will fire at the wrong time and could cause incomplete combustion and a rough running engine. Check that your distributor is adjusted properly using a timing light.

Some older electronically fuel injected (EFI) engines also have a sensor that can be adjusted in this manner (such as the cam angle sensor on older Mazda MX-5 Miatas). Refer to a repair manual for vehicle-specific instructions.

Spark plug wires are a common failure mode, as they wear out even faster than ignition coils. A worn spark plug wire may fire intermittently, weakly, or not at all. These can be tested using a timing light or a multimeter with an inductive pickup.

The pickup is placed around the spark plug wire and the device gives a reading each time the wire fires. If the timing light never illuminates or the multimeter shows low or no voltage, you know you have a problem.

Don’t discount the obvious, though. Make sure the spark plug wires are firmly attached to the ignition coils and the spark plugs. You may want to remove and inspect each wire to look for corrosion. Corrosion can place a gap between the coils, wires, and spark plugs, causing the engine to miss on that stroke.

Problems with the Fuel

A tank of “bad gas” (incorrect octane or old gasoline) can cause a misfire. Weak or dirty fuel injectors can restrict the flow of fuel into the cylinder leading to the wrong air/fuel ratio. A clogged fuel filter leads to low fuel pressure which can also limit fuel flow.

Electrical Problems

Sometimes the problem is due to electrical issues outside of the ignition system, such as a failing mass airflow sensor. Other computer or wiring problems can block the signals for the amount of fuel to inject, the timing of fuel injection and spark, etc.

Spark plug wires generate quite a lot of electromagnetic frequencies (which is actually how inductive pickups work). The electromagnetic frequencies from the plug wires can sometimes cause electrical noise that interferes with other signals emitted from nearby sensors or the ECU, though it is not a common issue on stock vehicles.

Mechanical Problems

Since there are a lot of moving parts in the system, the engine should be checked for mechanical issues as well. Cracks can cause a vacuum leak, the timing belt or chain can slip and affect the timing of the opening and closing of the valve.

Moving parts such as the piston, rod, and crank bearing can break, and parts such as valve seals, valve springs, gaskets, and cylinder heads can wear out.

Valve problems are a common culprit. Carbon buildup around the valve seats of the cylinders can prevent them from closing all the way. A failing EGR valve could stick which puts exhaust gases back into the intake manifold or allows the air/fuel mixture to escape before being ignited.