What Causes the 'Bang' Sound When Starting a Car That Has Been Parked for a Long Time?

[SpencerRose]

Due to ignition delay, unburned gasoline is expelled into the exhaust pipe just as the spark plug ignites, causing the atomized gasoline to ignite inside the exhaust pipe and produce the sound. Here is some relevant information: The abnormal noise occurs only at idle or low-speed operation. Possible causes include: excessive clearance between the piston and cylinder wall; overly tight piston pin or connecting rod bearing assembly; excessive clearance between the tappet and its guide hole; wear on the camshaft profile; sometimes, a loose starter pulley causing noise (especially noticeable during speed changes). Brief sounds occurring successively during rapid deceleration: At certain speeds, the noise becomes chaotic, and brief sounds occur successively during rapid deceleration. Possible causes include: cracked camshaft timing gear or loose fixing nut; broken crankshaft; loose piston pin bushing; excessive axial clearance of the camshaft or loose bushing.