took off the upper plenum the other day to clean it because it looked gross. was black from dirt. ever since i took it off and put in back on the car seems to be runnin a lot richer and my gas mileage has gone down a bit. i also get a gas smell at random times. what could this be?

 

i'm guessing you knocked something outta place

 

did you clean off the gaskets really good? recheck egr connection, & the t-fitting in the back of the manifold.

might bean air leak.

 

Well we're not there
Your problems are not too common with what ya did.
What does sound like what happened is vacuum leaks
The Manifold is full of dirt due to routing of vacuum lines into the intake for emissions reasons.
IF your car ran sh!ttty before the end result is what ya now see. Carbon buildup.
Pull back off, check your work, lines, etc.
Take top and flip it over toward passenger side to avoid disrupting any lines/setting of TB & such.
IF ya already upset the lines & TB setting due to movement of upper intake, you have to start all over again.
Not there but yeah things can happen when ya disturb the sleeping bear!
Hoses patch by using 5/16" windshield washer line. Works perfect. I've spent whole day doing that and worth it.

 

Did you knock off the vacuum line to the fuel pressure regulator?