idle problem

hi all..i overhauled my 88 gt...rings bearings new heads off a 92 gt..didnt remove the engine..took off oil pan..that was fun..anyway..its all back together and ive set the timing correctly...it wont stay running unless u hold the gas...im gonna check for vaccum leaks today but what could i have missed...when u hold in the gas it runs great...but no idle..any info would be much appreciated..thanks

yeah its efi...it has headers, gears, 65mm throttle body, 3 inch exhuaust, msd and rev limiter, i miss placed my list.. but nothing else internally....
well i found a vaccum lead at the map sensor...the sensor doesnt seem to be performing...it runs great with it unhooked and the live plugged...then again limp mode...but i also got a few codes...#94 and 44 KOER and 34 KOEO..i have an air pump but its not hooked up...its the 34 im looking into...