What Causes Engine Misfire?

Engine misfiring is not something to dismiss. It points to a range of problems that, if left ignored, can lead to engine damage and serious repairs. What causes engine misfire? There’s no simple answer — several underlying issues could be responsible.

Identifying the exact source of the problem is something you can do with the help of your mechanic. Here are the signs that it’s time to take your car into the shop for an engine inspection.

A Whole Lotta Shakin’ Goin’ On

A shaking engine is never a good thing. Typically, when that happens, you feel a vibration that rattles the dashboard and sends tremors through the steering wheel. It can happen when you’re driving or idling at a traffic light.

Like any other abnormal sensation related to your car, this vibration is your vehicle warning you that something is wrong. As soon as possible, take your car in for service.

Engine misfiring sometimes reveals itself in other ways. Specifically, you may have difficulty starting your car. Normally, this shouldn’t be a problem in modern vehicles equipped with direct injection and other electronic enhancements. Other signs of trouble include stalling, black exhaust and a strong gas, oil, steam or coolant smell. Your fuel mileage may also take a hit.

Get to Know the Part-iculars

If you you have a newer OBD-equipped vehicle the check engine light may be on due to a misfire. Using a code reader may reveal a P0301, P0302, or similar P03XX code depending on the number of cylinders in your engine. A P0301 code means that there is a misfire detected on cylinder number one. A P0308 code means that there is a misfire detected on cylinder number eight. Narrowing down the location of the misfire by cylinder can save valuable diagnosis time. Unfortunately there is also a P0300  code which means random or multiple misfires have been detected in the engine. A P0300  code can a bit harder to diagnose.

When diagnosing engine problems, your mechanic will look for several trouble signs, including worn or incorrectly gapped spark plugs, a defective fuel injector or worn seal valves.

If none of these parts reveals a problem, your mechanic will look for vacuum leaks and examine the ignition coil. Sometimes, a vacuum leak will make itself obvious by whistling or hissing. Otherwise, your mechanic may need to use an aerosol carburetor cleaner or a bottle of propane and a connecting hose to search for leaks. You can also use a leak detector kit at home. If the solvent or propane is pulled in, the idle speed will immediately change and smooth out. That’s where the source of the leak is.

As for the ignition coil, testing its effectiveness is easy to do. Using a multimeter, simply connect the positive and negative leads to the respective positive and negative terminals on the coil. The primary resistance should fall between 0.4 and 2 ohms; the secondary should fall between 6,000 and 10,000 ohms. Those numbers can vary somewhat, so check your vehicle’s repair manual for the correct range for your vehicle. If either reading is outside the allowed parameters, then you’ve most likely identified the source of your engine misfire.

Identifying and replacing the part causing your engine to misfire will quickly rid your car of the shakes. Being proactive as soon as you notice the problem will also help you avoid more serious issues like a blown engine gasket, which is a much more significant repair job.

Check out all the ignition system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on what causes engine misfire, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

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