WHy does my car do this? I know the tach isnt totally accurate but when in gear my car is usually at 550ish 500 warmed up. After i sit in traffic or at a light for more than 30 or so seconds it drops like 100 rpm at idle and doesnt die out or anything but just idles real low. Is this normal. I have replaced my IAC b/c it was acting up, and got a non-ACDelco part, but does that matter? I am just a little concerned?

 

Autos idle real low. Don't worry about it until it starts to die at idle.

 

thats about where my 93 idled at.