I always thought that when you turn off your car after a day of driving and you get that ping ping ping uder the hood, that meant that the gas in the cylinders was not all totally ignited and that was caused by bad gas or bad plugs. I recently did a tune up on my car and i'm still having the same ping, what is the prob? Also where is the car supposed to idle, Mine idles at the 600 rpm level(not 6000)

 

it's usually just due to the fact that when the engine was hot, it expanded slightly and when you turn it off, it starts to contract and that causes the noises

so tune-up wont change that