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What causes a car to vibrate when idle?


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2014-06-03 22:18:09
2014-06-03 22:18:09

If the engine mounts are worn, or the engine is not idling at the proper setting, that can cause vibration at idle speeds.

Also, your shocks/springs could be worn and that could be a contributing factor as well.

The smoothness of idling in a car depends on several aspects: first, it is very important to have the power pulses coming from every cylinder as equal as possible and produced at precise intervals. Therefore, if a given cylinder is firing poorly due to a worn, misadjusted or dirty spark plug, or a defective ignition cable, or even a defective distributor cap and rotor; will result in a shaky engine. The timing was established by the distributor in older cars, therefore a worn distributor shaft bearing can produce vibration at idle. But not only the ignition can produce unevenness, differences in carburation or injection also cause a shaky engine; thus, in a two (or four) barrel carburetor, you have to carefully adjust BOTH sides of the carburetor to avoid a shaking engine. A partially or totally clogged fuel injector also results in a shaking engine.

The valve train can also produce an uneven idle, if for example, a certain cylinder valve is sticking, or the valve lifter is not working properly, unevenness will result.

Lastly, there could be some other rare cases of rotational unbalance, like fitting a wrong type of rotational or torsional Damper. (Ford made their 302 CI engine in TWO different crankshaft configurations, so that there were TWO distinct dampers available, installing the wrong one caused vibrations specially noticeable at idle, same for the flywheels, there were two of them for this engines).

Now, with an 8 cylinder engine, almost perfect cylinder balance can be achieved, less so with 6 and 4 cylinders engines. In recent 6-Cyl engines, the idle quality is established by the proper filtering of the unbalanced pulses by the advanced design of the engine mounts, which are very important in these cases. In four Cylinder engines, some Chrysler engines use a couple of balancing shafts to counteract the unbalanced character of a 4-Cyl engine, achieving a smoother idle. If those shafts, which are driven by a chain are not properly assembled, shake will be produced.

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Fast idle motor is not working. This is allowing the idle to fall below the threshold of what it should be when the A/C is on.

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The most probable cause would be the idle control on the engine. When you put a car in gear, there is a "load" put on it (the transmission is using power) and the idle speed normally drops. The idle could be dropping to the point where it becomes unstable and causes the engine to vibrate.

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