Well there are a few things that can be causing an engine to be shaking or in otherwords not running smoothly at idle.

0. Always use premium gas!!! Low octane gas like 87 or 89 will not burn or ignite correctly in your engine and cause the engine to shake. Use at least 91 or 92 octane.

1. Change your gas/fuel filter.

If it is dirty, your engine will not be getting proper flow of gas to the engine. Please beaware of which gas station you go to as well. Gas stations with dirty gas will make your gas filter work over time. Your gas filter is located in back between your rear wheels.

If your engine still shakes and vibrates then you got to solve this problem from the engine.

2. Go and get your fuel injectors cleaned.

The fuel injectors need to be cleaned from contaminants that may have slipped through the gas filter over a 100,000 to 150,000 mile period.

I had a similar problem with my engine shaking and vibrating, and I had to get my fuel injectors cleaned for about 150 U.S. dollars.

3. You may need to replace a fuel injector or more if the fuel injector is not spraying correctly.

This can be costly. At the Mercedes Benz dealer, I had to pay 220 U.S. dollars for one fuel injector!

4. Lastly, if all the above was done and your engine is still shaking and vibrating, then you will probably need new spark plugs.

Old spark plugs may not fire correctly or smoothly. You may not have replace all the spark plugs in your engine at once, but sooner or later, all the spark plugs will have to be replaced. It might be a better idea to just replace all your spark plugs at once since labor costs are expensive. The good news is that spark plugs are not expensive. The German brand Bosche which makes spark plugs, sells for just under ten U.S. dollars per spark plug on their website.

I hope the above helps to solve an engine's shaking.