confusion after getting my exhaust manifold replaced

[TheBlueOompa]

I recently replaced a broken exhaust manifold, passenger side, with a new oem one. After getting it fixed I am seeing a drastic drop in mpg's. Before the replacement i was getting an average of 16.4-16.9 mpg around town and going up mountain roads, and 22 highway. Now I am averaging 14.3 around town and unknown highway. I just haven't done a strict highway run just yet. I know my flex pipe is leaking and has been for sometime before the manifold replacement, but what is going on here? I thought after getting it replaced would help boost my mileage even just slightly but at least keep it where it has been and not just drop it as it has.

 

[KChurch86]

Have you checked/re-torqued the manifold studs/nuts?

Perhaps moving the exhaust system around (when the manifold was replaced) may have increased the leak size in the flex pipe, causing lower MPG's?

 

[TheBlueOompa]

possibly but when i get on the throttle it is not blowy as it was when the manifold was cracked. it revs up smoothly and doesnt sound like there is anything terribly wrong i can hear a little something coming from under my seat when i go wot in the upper rev range but other than that it sounds like a normal exhaust. im about to move back home to houston where there are some good shops and hopefully be able to get rid of the flex pipe and weld in a straight pipe. mine is missing a coupling and when i asked the shop that installed my manifold what they can do about the flex pipe and he said that is how the flex pipe is supposed to look like. he was an idiot and i should not have used him