Three things can cause a high idle speed the most common is a vacuum leak next is a sticking idle air control valve the least common cause is the engine computer is commanding that high idle speed in response to something if the idle speed comes down on its own it is most likely not due to a vacuum leak. I have a 99 civic lx had the same problem i got a bottle of fuel system cleaner which i put in my gas tank and then filled it up i drove the car around a few days and its all cleared up my civic is now idling 600 rpm i have also stopped buying the cheap gas and only use permium 91 93 strangely the car runs smoother and faster. How to fix idle on honda civic not idle air control valve my d16z6 had a high idle and a surge so by buddy zack came to show me how to diagnose and correct. This article applies to the honda civic 1992 2000 a honda with a rough idle isnt terribly uncommon especially in older civics generally a rough idle is caused from a difference in how much air the engine computer sees coming in and how much is actually there