1If the engine does not shake too much at idle, it's usually because the idle
speed is too low or it cannot hold the idle speed. However, there're too many
possibilities, which usually can be categorized into: dirty air intake system
(air filter, idle control valve, etc), dirty fuel system (fuel filter,
regulator, etc), bad engine control system (sensors, connections, ECU, etc).
Most common cause might be dirty idle control valve or fuel filter.