loss of mileage after intake replace

I had my intake gaskets done at a G.M dealer a few months ago and I have lost on average 50 to 75 KLM. per tank of gas . Is this the norm or is there a sensor gone or ? When the rebuild was done they only replaced one sp. plug The engine runs fine as far as I can tell, is it possible that when the heads done that what is shaved off enough to make that much of a difference? All help will be greatly appreciated.Thanx..Thom

Wow, that is about 46 miles of difference. Why only one spark plug? If the intakes where leaking then your vehicle would have been running an improper fuel/air mixture, which would have damaged the spark plugs. That would have been corrected by replacment, and eventually the computer "learning" again. So if any thing your KLM should have increased, especially since it has been two months since the repair. So this also has me a bit clueless.

I have the same problem, picked up my car from dealer Thursday. I'm losing about 5 to 6 mpg

had my intake gasket replaced as well as the ac condenser, fuel system maintenance and the plugs. probably drove 5 tank fulls at lousy mpg before a cylinder 2 misfire code showed up. replaced plug wires (no. 2 snapped in half as I pulled it off) and mpg back to 24 mpg mixed driving (with 150k or so) on my 2002. go figure.