Suppose you have a problem with the car-it has a misfire. Check engine light's on, maybe the check engine light's flashing; one thing's for sure, the car is sitting there and it's real rough at idle. So if you have exotic test equipment like an expensive Snap-On tool, well, fine. Hook it up, you can do the diagnostic work and so on, but not all of us are going to be lucky enough to have something like this.
So how do you go about doing it yourself? Well, first thing you do is you listen. You go back to the tailpipe and you listen for a "missing" sound, when instead of the gentle purr that an engine normally makes at the tailpipe, you're going to have-and going to be able to hear-the "missing" sound. Alright. Once you've done that, then you come up here and you use your own form of thermometer. This is a digital infrared thermometer. Now the engine is running, it's fully warmed up. These have a laser pointer on them, and you point the laser at each one of the runners of the exhaust manifold, one at a time. This is a four cylinder engine; you go down the line, you do all four of them. Six cylinder engine, do all six; eight, of course, do all eight. 
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