rough idle

By diagnosing the cause. Potential suspects include a vacuum leak and worn spark plugs. If the Check Engine light has turned on, there will be at least one diagnostic fault code stored in the Engine Computer. That will get you into the circuit or system with the problem, not necessarily the defective part.

You might be able to hear the hiss of a vacuum leak under the hood with the engine running. Also, when it's still cold, use a spray bottle to spray water around the intake manifold gasket and vacuum hoses. If you see the water get sucked in or the engine speed suddenly changes, look closer in that area for a leak.

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