I aquired a 91 integra GS from a cousin and the RPM raves back and forth sometime after starting and at complete stops. The car runs fine but with the raving of the car, it's not getting very good miles per gallon. My cousin told me it might be the oxygen sensor but I'm not sure. Any suggestion is appreciated.

Thanks in advance

 

It usually raves between .9 to 1.5 RPM. It usually does that for about the first 30 mins only.

 

my bros car is also doing that, check the idle air bypass valve, this was our problem