What Does a Misfire Mean Car Truck or SUV Engine Problems

What is an engine misfire and what does it mean? That's what we're going to talk about in this video.

You can see the engine's running really rough. We have a misfire, and it needs to be fixed. So here we have a P 300, which is random or multiple cylinders misfiring detected. And here we have a P0304, which the last number, whether it's a 304 or a 305, 306, 307, 308, however many cylinders you have, that determines which cylinder is actually misfiring.

Let's go over the basics of a four-stroke motor. Now the first stroke, when you have the piston up high and you have the intake valve opened up, you're gonna get an air-fuel mixture come in. As the piston goes down, it's going to suck in that air-fuel. That's the first stroke. Then the pistons going to start going up and it's going to compress that air-fuel mixture. That's the compression stroke.

Next you're going to have the power stroke. You're gonna have the spark plug fire, and that's gonna push the piston down, igniting all the fuel and all that air-fuel mixture has burned. And next you're gonna have the exhaust stroke when all that fuel has burned, and the piston is gonna go up. The exhaust valve is gonna open and it's gonna push all the exhausts and bad gasses out. And then the process is going to start over intake valve will open, and it will repeat.

So while this is happening, the crankshaft is turning, and all of these different cylinders and pistons are doing this all at a different time. Attached to the crankshaft, there'll be a reluctor wheel just like this and a sensor that will send a signal to the ECM. Now that signal will tell the ECM exactly where the engine is and how fast it's spinning.

Every time the spark plug actually ignites the fuel and sends the piston down, that's gonna speed up the reluctor ring just a little bit and the engine is going to monitor that and see that if there is not a sudden burst of energy, it's gonna know there's something wrong with that cylinder.

Let's say you were on compression stroke and the spark plug didn't fire. Whether the spark plug's faulty or the coil's faulty or something else happened, where there was no fire, you wouldn't get that sudden burst of speed and that would cause the misfire light to come on.

Also, if for some reason you didn't get the correct air-fuel ratio or you didn't get any fuel if one of the injectors were plugged, then once you go to compression stroke and it goes to fire, it's not gonna speed up the crankshaft and you'll get your code.

You could also have too much fuel. If you have way too much fuel here, it's not gonna let the spark plug do its job. It's not gonna fire and that'll cause a misfire as well. Or you could have a mechanical problem where the valve, either valve the intake or the exhaust valve doesn't open up. If the intake doesn't open up, you're not gonna get fuel into the cylinder. If the exhaust valve doesn't open up, you're just gonna have tons of burnt fuel and fresh fuel mixed together. It's not gonna go anywhere.

All right, so if you have no spark, no fuel or a mechanical issue, you're gonna get a misfire. A couple of easy ways to diagnose a misfire, if you have a scan tool where you can actually graph the misfire and see which cylinder is misfiring, then you can shut the vehicle off and you can take a coil, swap a coil with another one, the known good one with the bad one, the one that's misfiring, and then see if the cylinder moved, if the misfire moved. You can do that with the plugs as well.

If you don't have a scan tool that can graph it, you can do it individually. If you have a solid misfire, you can pull the injector one at a time. And if the engine vibration changes, then you know that that's probably not the cylinder that's misfiring. If you disconnect the injector and nothing changes, the RPM doesn't change, the vibration doesn't change, chances are, it could be that cylinder that's misfiring.

Doesn't mean that it's definitely the injector, but just means that that cylinder is the one that's misfiring. We could have a plug or a coil that's bad as well.

If you've narrowed it down, and it's not a fuel injector, it's not a coil, it's not a plug, it's nothing else electrical, it could be something mechanical. You're gonna want to do a compression test and also a cylinder leak test. You'll need those tools to do that.

If you have a misfire and the check engine light is flashing, it's important to know that you should not drive the vehicle. You could cause damage to the catalytic converter and you don't want that. It's really expensive. If you enjoyed this video, make sure you subscribe to our channel, ring that bell turn on all notifications, so you don't miss any of our videos.

Thanks for watching. Visit 1aauto.com for quality auto parts shipped to your door, the place for DIY auto repair. And if you enjoyed this video, please click the subscribe button.