Have replaced all vaccum hoses and intake gaskets after receiving codes 300. 302 . 306 multiple misfires. checked for holding vaccum at 15 pounds but no power. swapped coils, still reading above codes. Also showing 1746 and 1750 codes. Any suggestions????? Should the secondary air pump run constantly or intermittenly? Could a bad oxygen sensors set cause this problem? Car has 140,000 and never changed oxygen sensors to our knowledge. It runs rough with weak power. Brand new spark plugs, all new vaccum lines, coils seem to be good. AT WITS END.