my plenum was leaking some, but not huge. what i noticed was severe pinging with hypertech, low power. mpg was pretty good at about 13 city and 15 hwy. i was not losing a lot of oil. after plenum repair i had no more pinging, better power. no change in mpg. the only reason that i changed mine was because the water pump went out and i had to get in there anyway. otherwise i was able to run premium gas to get rid of the pinging. the premium gas also fixed some of the low power that i was experiencing with 87.