Bad Gas Milage after fixing exhaust manifold

Well I found the cause of the excessive noise when on the highway, a missing exhaust stud, went though a lot of trouble to fix it figuring it would help my gas millage on my 99 XJ 2.5 L 5sp. Well it got worse.

I keep very detailed logs on my fill ups.

You might have an intake air leak. In that case it would probably be a bad gasket but it could be a cracked manifold or maybe there's another missing bolt. If the leak's small, your motor might not misfire enough to set a code but still enough to affect your gas mileage. Just a thought...

how long has it been, that your now comparing too? by that I mean, if it was sucking a good amount of air into the exhuast could it have adjusted fuel/air so much that now that its fixed its too soon to get an accurate milege... that make any sense?

give it some more time maybe, to allow the computer time to calculate the O2sensor readings and mix it right again.

Well I know the intake was good, as i had to remove it to get to the exhaust. I checked it out, and put a new gasket in on it. As for the computer it has been about 1000 miles. I also reset the computer by removing battery power from it, it started to go fine at first but currently I think it is running rich since my idle is close to about 1000rpm when it is normally closer to about 750.

Get yourself a new O2 sensor...