I’m seeing this happen a lot more at shops. Customer comes in, they have a complaint about a rough running engine, but it’s only during certain conditions. Let’s say pulling away from a light. They notice a little stumble when they’re going from, let’s say, 2,000 to 3,000 RPM under load. When this happens, well, it’s what’s known as a fish-bite or a general stumble. You scan the vehicle, there are no codes. You look underneath the hood, nothing’s wrong. So you don’t feel too confident pulling the trigger on ordering parts to solve the problem. So go for a test drive.

There are two tests with your scan tool that you can use. You can use a test called a Power Balance Test. Think of this as a visualization on the screen of the crankshaft position sensor and seeing how smooth it is. If you have any cylinder that’s higher or lower, that’s the sign that it’s misfiring. The other one that you can use is the Misfire Monitor. If you use the Misfire Monitor, this tracks the misfires for specific cylinders.

And back to our point about this one having no codes, a Misfire Monitor goes for a certain amount of engine revolutions. In other words, if that engine does not rack up enough misfires from that cylinder, well, it’s not going to set a code. But you can see with that Misfire Monitor where the cylinder is misfiring and which ones are suspect. From there, you can do further diagnostics with a scope to look at the primary and secondary sides of the coil. With this, you can confidently order a new coil and not worry about having to return it or not solving the customer’s problem. I’m Andrew Markel, thank you very much.

This video is sponsored by Auto Value and Bumper to Bumper.