Anytime my car is in idle .. it hovers around 750rpm and every few seconds it stutters down to 700rpm kind of like it is about to die, then jumps back up to 750rpm. It is kind of subtle but irritating. Is this normal for our cars? Like the computer might be trying to use as little fuel as possible during idle? And another question ... there is sooty deposit already forming around the bottom/outside of my exhaust .... is it running too rich? It is just approaching the 600 mile mark.