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Start with cleaning the EGR valve and the passageways to the plenum. Since it idles and runs rich when it is warm this may be bad and causing it to run rich. 2. Clean the idle air control valve and passageway on the throttle body. This will cause idle problems if the IAC isn't working correctly. 3. Check the map sensor since it can cause issues and finally you may want to replace the O2 sensor. It sends signals to the ecm to tell it how much gas to send into the engine. If it is bad this will cause poor fuel economy and a rich condition.