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211 Posts
Vacuum leak, secondary air injector system is bad, bad fuel injector, bad pump...the ECU is seeing a lean condition, adding fuel and raising the rpm to compensate. Could be throttle bodies as well but your best bet, as others have mentioned, scan for codes. That'll at least get you pointed in the right direction. If it's really rough at cold startup your fuel trims could be way out of whack which could be an issue with an injector OR elsewhere.
