Engine Running Rough
If the engine is running rougher than usual, there is a problem with the fuel delivery or ignition systems. A likely cause is a cylinder is firing at the wrong time or misfiring. The firing order is designed to minimize vibration from the engine. If the order is upset, the engine will not have the proper balance to counter the force of ignition. This will produce a vibration.
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The firing order is designed to balance out the forces of the pistons. If the engine is running rough, the balance will be upset, causing vibration throughout the vehicle. This can be more apparent during idling.
Faulty Fuel Pump
A faulty fuel pump will not deliver the correct amount of fuel to the engine. This can cause variations of the fuel/air mixture in the combustion chamber. Some cylinders may get more fuel than others, causing more power in certain pistons.
Faulty Spark Plugs
Faulty spark plugs may fire later than they're supposed to or not at all. This will distort or abolish the power in the cylinder to drive the piston.