my friend bought a 5th gen civic ex coupe a few days ago. i went over to check it out and when starting the car i noticed the rpm's would jump up and down between 1500 and 2000 for a few seconds. i revved it a little and it still did it. then after a few minutes sitting at idle it would settle down to 1k. my friends said this was normal, but im thinking something's wrong.

 

Yeah....that's not normal.

I always suggest this, but it's definitely a possibility... vacuum leak. If a hose is cracked, or not attached properly, the idle can vary slightly like that.

There are tons of possibilities here. Clogged injectors (?!), fuel system in general, electric issue....etc... I guess we'd need to know more symptoms, unless anyone has had this exact issue before and resolved it.

 

From the symptom that you describe where it settles down after it has warmed up, It sounds like the IAC (Idle Air Control) which is used to adjust the rpm's while the motor is cold and until it warms up... generally the IAC will make your motor rev slightly higher than usual to help warm up the motor. Once the motor is warm, the rpm's return to their normal state. A typical indicator of a bad IAC is the rpms jupping around.... but as the previous post mentions, a vacuum leak could also cause this but my money is on the IAC

 

I had that happen with a 1984 Rex...

When i got it the idle was set at around 2k, and when i tried to set it to idle around 700, a similar trouble had happened. It idled fine around 7, but once it warmed up it jumped up to 1500 and back down to almost shutting off and then back up again, no one knew wtf, so i just put the idle back up until it didn't "jump". My girl totalled the car in the snow before i could really try to fix it...