Q: Q: Car shakes when idling

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My car shakes at idle, and a cold start is getting a p0300 code misfire.

I know these vehicle symptoms all too well. If your GMC Yukon runs better after it has warmed up then it is likely that it is experiencing a vacuum leak in the intake manifold gaskets. It is not uncommon for these gaskets to fail as they are sandwiched between the cylinder head and the intake manifold and because of their location, they are exposed to extreme heat. When these parts start to fail, unmetered air enters the combustion chamber through the leaks, in turn, disturbing the air/fuel mixture your computer is trying to achieve. However to be 100% certain that this is indeed the problem, I recommend enlisting the assistance of a certified mechanic to come by and inspect your car’s shaking issue. They will have the experience and tools necessary to confirm if this is the problem as well as make the repairs to restore your GMC to running smooth again.

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