What Causes Intermittent Engine Misfire?
3 Answers
Engine misfire can be caused by various factors including the mechanical system, fuel system, intake system, ignition system, and electronic control system of the engine. Symptoms of engine misfire: Abnormal engine operation or insufficient power, uneven engine performance with rhythmic vibration at high, medium, and low speeds, black smoke and backfiring from the muffler. Hazards of engine misfire: Running an engine with misfire can cause significant damage. Firstly, it increases fuel consumption and emission pollution. Secondly, fuel may enter the oil pan, contaminating the lubricating oil and shortening its service life, while accelerating wear on moving parts. Additionally, unburned fuel may combust in the exhaust pipe, damaging the three-way catalytic converter.
I've encountered this issue too. The engine misfires sometimes but not always, which is really annoying when driving. It's most likely a problem with the ignition coil or spark plugs. These components tend to wear out over time, especially when the internal wiring of the ignition coil develops poor contact. It may work fine when the engine is cold, but as the engine heats up and the wires expand, problems can occur. Another possibility is worn high-voltage wiring causing a short circuit, which is more likely to happen on humid or rainy days. Insufficient cylinder pressure could also be a reason, such as poor valve sealing or worn piston rings leading to pressure fluctuations in a particular cylinder. This requires professional equipment to measure cylinder pressure for confirmation. Lastly, a clogged fuel injector might cause these intermittent symptoms, especially if there are impurities in the gasoline. I recommend first checking the condition of the spark plugs—replacing them with new ones is inexpensive and worth a try. If that doesn't solve the problem, then test the ignition coil. Don't delay too long, as it could damage the engine.
I have experience with this issue. Intermittent engine misfires are often related to wiring problems. For example, if the main harness connector is aged or loose, the connection may become intermittent when the car bumps around—this is quite common in older models. Excessive spark plug gap or severe carbon buildup can also cause unstable ignition, especially noticeable during cold starts in winter. Additionally, a dirty or faulty crankshaft position sensor can lead to inaccurate readings, causing the ECU to send incorrect ignition commands—this can be identified by scanning for trouble codes with an OBD scanner. Don’t overlook the fuel system either; unstable fuel pump pressure or a partially clogged filter causing uneven fuel delivery can also trigger misfires. If it’s just an occasional issue, be sure to check before a long highway drive, as breaking down midway is too dangerous. If you’re handy, you can inspect wiring connectors for corrosion yourself, but for other cases, it’s best to visit a professional repair shop.
