The check engine light will flash as the engine misfires, but it may stop if the misfire stops as well. If your check engine light is not flashing but you see other signs of a misfire, the engine may still be misfiring.
The scanner will give you a code that’s made up of numbers and letters. If it doesn’t provide an english description, you can find them in a vehicle specific repair manual or on the manufacturer’s website. The scanner will either give you an error specific to one cylinder misfiring, or a generic misfire error across all cylinders.
Misfires don’t always happen consistently, so the vibration may come and go under different driving conditions. If it feels like the engine is misfiring, make sure to take note of what kind of driving you were doing at the time (sitting at a stoplight, driving on the highway, etc).
Sputtering alone can mean other issues aside from a misfire, including a loss of fuel or airflow into the engine, so look for other signs of a misfire as well.
Reset the trip odometer on your dashboard when you fill up your gas tank to see how many miles you make it before you need another fill up. Divide that number by the number of gallons you put in to get your mileage. Compare that mileage to your vehicle’s mileage rating in the owner’s manual if you’re unsure of what it normally is.
There’s a wide range of acceptable temperature readings for this test, so what matters most is identifying the cylinder that stands out as not as hot as the others. For instance, if three cylinders read 190 °F (88 °C) and one shows as 80 °F (27 °C), the low one is the issue. This will only work while the engine is misfiring. If your misfire comes and goes, make sure to do this test while it’s happening.
If the misfire is not specific to one cylinder, it’s likely because the engine is not getting enough air or fuel to run properly. That could be because of a failing part of the fuel system. If the mass air flow sensor or oxygen sensor fail, they could give incorrect data to the engine’s computer, causing a misfire. Take note of any error codes to help you as you move forward in diagnosing the problem.
Replacing a bad vacuum line may resolve the misfire, or it may just make the engine run better.
If the engine starts to run worse with one injector disconnected, reconnect it and move on to the next one. If you disconnect a fuel injector and the engine’s behavior doesn’t change at all, it means that cylinder wasn’t firing and is the source of your issue.
If the fuel pressure is low or inconsistent, the fuel system prior to the fuel rail is causing the misfire. You’ll need to either replace the fuel filter or the fuel pump if this is the case. Replacing the fuel pump might require removing it from the fuel tank, so you may want to seek assistance from a professional mechanic.
You may be able to clean your fuel injectors instead of replacing them by pouring fuel system cleaner in with a full tank of gas.
Remove the mass air flow sensor by removing the two screws that hold it in place on the vehicle’s intake and disconnecting the wiring pigtail that leads into it. You can remove the oxygen sensor by disconnecting the wires and unscrewing it with an oxygen sensor socket. Make sure to connect the new sensors using the wires you unplugged from the old ones, then secure them in place using the same mounting hardware.
A spark plug that looks black or carbon fouled at the end means the engine was running rich (too much fuel). A plug that is wet with gasoline or oil means the fuel regulator may have failed, or that there are serious internal issues inside the engine block. If the plug looks fine, check the gap between the metal sticking out of the end of the plug and the base. Compare that gap to the specified gap in the vehicle’s repair manual. If the gap is too big, it may be preventing the air/fuel mixture from firing. You may also need to replace the wire that delivers the spark from the ignition coil to the spark plug. [14] X Expert Source Hovig ManouchekianAuto Repair & Design Specialist Expert Interview. 23 February 2021.
You can find the correct resistance rating in your vehicle’s repair manual. Find the coil packs by running your hands along the spark plug wires moving away from the spark plugs. If the coil needs to be replaced, simply disconnect the rest of the wiring and unbolt it from the bracket. Insert a new coil and reconnect it the same way the old one was. [16] X Expert Source Hovig ManouchekianAuto Repair & Design Specialist Expert Interview. 23 February 2021.
Repeat this process for each cylinder. Make sure to reinsert the spark plugs after you remove the gauge every time. Like the temperature test, all of the cylinders should have similar figures except one, if the misfire is being caused by a lack of compression. If the numbers are the same across the board, the problem is not compression related. If numbers are low across two cylinders near each other, it likely means the head gasket is bad in that area. You will need to have the the cylinder head removed from the engine to replace the head gasket.
Replacing a head gasket is a pretty involved job that requires specialized tools in many applications. If you believe your cylinder head gasket has failed, you may want to take the vehicle to a certified repair technician.
Rebuilding the bottom end of an engine is a complicated and difficult process best left to professionals.
