While idling at a stop light, does your engine tremble harder than a San Andreas paint mixer? Does it send tremors through the steering wheel or into the rest of the vehicle, resulting in a noticeable loss of power? If so, misfiring cylinders could be to blame -- again [sources: B&B; O'Reilly].
If a misfire is your culprit, other signs will soon present themselves. These might include trouble starting, stalling during idle (especially if accessories like the air conditioning, headlights and/or defroster are running) and bad gas mileage. The hydrocarbons choking your exhaust could also botch your annual emissions test [sources: B&B; O'Reilly].
Engines typically misfire for three reasons: spark loss, lost compression or a way-out-of-whack air-to-fuel ratio. Bad spark plugs, fouled-up plug wires or a cracked distributor cap can cause spark loss, while compression loss -- in which too much of the air-fuel mixture flees a cylinder before going bang -- commonly arises from a leaky exhaust valve or a blown head gasket [sources: B&B; O'Reilly].
An air-fuel mix that is too thin to burn could mean a gunked-up fuel injector, an air leak, a weak fuel pump, a choked-off filter or a compromised pressure regulator. It's also possible, though less common, for the mix to be too rich (which we mentioned before), but this will tend to affect all the cylinders, not just one [sources: B&B; O'Reilly].
A diagnostic scanner can pick out the misbehaving cylinder, but that's just the first step in figuring out what's wrong [source: O'Reilly]. Once you've narrowed down the problem, decide if it's something you feel comfortable tackling yourself.