In our last article, we discussed what engine misfires are and listed most of the noticeable symptoms of engine misfires. So, how are you supposed to diagnose what is causing the engine to misfire in the first place?
The causes of an engine misfire can be difficult to identify because there are so many aspects of the engine itself. Here are the common causes to help you in troubleshooting.
Top 4 Cylinder Misfire Causes
#1. Issues with the Ignition System.
Worn spark plugs or spark plugs with the wrong gap are both common because the tip is fragile, and spark plugs may have a relatively short life.
In the case where the spark plug gap is too large, the spark may not be able to jump the gap between the center electrode and the side electrode every time. Without a spark, there is no power on that stroke.
The ignition coil (and distributors) convert the low battery voltage into the high voltage that spark plugs need. These components can wear out after a time because of the heat generated every time they fire.
If the ignition coil has worn insulation, all that electricity could take an unwanted path instead of through the spark plug or wire.
Distributors are spinning mechanical parts on older cars and can be subject to wear. Sometimes, carbon or debris or water can end up under the distributor cap so that the distributor can not transfer spark to the plug wires.
Water in the distributor cap can make it difficult or impossible to start the vehicle. If the ignition is overly advanced or over-retarded, it will fire at the wrong moment and could create incomplete combustion and a rough-running engine.
Before moving on to another component, check that your distributor is in spec with a timing light. Some older electronically fuel-injected (EFI) engines also feature a sensor that allows you to make the same adjustments (for example, a cam angle sensor with older Mazda MX-5 Miatas).
Refer to a repair manual for vehicle-specific instructions, as spark plug wires are a typical failure point, as they wear out even more quickly than ignition coils.
A spark plug wire that is worn may fire weakly, intermittently, or not at all. Any spark plug wire can be tested with a timing light or a multimeter with an inductive pickup.
The pickup is placed around the spark plug wire, and then the device LED reads each time the wire fires by illuminating. If a timing light never illuminates or a multimeter shows no or low voltage, you have a problem.
Don’t forget the obvious. Double-check that you have good connections between the spark plug wires and the ignition coils, and the spark plugs.
You may want to pull and inspect each wire to check for corrosion. Corrosion could create a gap between the ignition coils, spark plug wires, and spark plugs, and this could cause the engine to miss on that stroke.
#2. Fuel Problems.
A tank of “bad gas” because of incorrect octane or old gasoline could create a misfire as well. Weak or dirty fuel injectors can restrict fuel, not allowing the correct amount to enter the cylinder, which affects the air/fuel ratio.
A clogged fuel filter will cause low fuel pressure, affecting the fuel flow as well.
#3. Electrical Problems.
Sometimes the issue can be from electrical problems outside of the ignition system (for example, a failing mass airflow sensor). Other computer or wiring issues could block the signals for the amount of fuel to inject, the timing of the injection of fuel and spark, etc.
We know that spark plug wires make a whole bunch of electromagnetic frequencies (this is how inductive pickups work). The electromagnetic frequencies from the spark plug wires could create some electrical interference, which could confuse output signals from nearby sensors or the ECU, but this is not a common issue on stock vehicles.
#4. Mechanical Problems.
Considering all the moving parts, there should be no shortage of potential mechanical problems with the engine as well. Cracks could cause vacuum leaks, and timing belts or chains could slip, which could change the timing of when the valves open or close.
Considering moving parts, pistons, rods, and crank bearings could break. While gaskets, valve seals, and valve springs generally their life expectancy is shorter, cylinder heads wear out. You could even suffer a spark plug blowout.
Failure due to the valves is are common culprit. Carbon build-up around the valve seats of each cylinder could be limiting the valves from closing completely.
A failing EGR valve could stick injecting of exhaust gases back into the intake manifold or allowing the air/fuel mixture to escape before it ignites.