Ok so i have a 2004 chevy swb 2wd with a 4.8 and auto....
I took it to exhaust shop and they didn't put what i wanted, so ill just tell you what i got and what the problems are.....
I ended up getting exhaust cut infront of the flange and true duals put on with 40 series flows and turndowns up and over the axle.... NOT WHAT I ASKED FOR
And i cut 3 holes in air box and put a K&N in the same day...
Gas dropped from 19.2 miles per gallon to 14.2..... Fuel is being dumped so bad, their was fuel in my oil when i changed it... Chevy garage shows no codes for anything wrong with truck...
I went back and asked to have them put it back to stock, then go from behind stock muffler to a Y pipe and 40's with turn downs....
I ended up getting a messed up Y pipe in front, 3 1/2" pipe for almost 2 feet and Y back in to 40's with turndowns.... Millage is now 8.6 miles to the gallon... What is going on with my truck and how do i fix it?

 

lol holy crap dude.. that is crazy. I would probably forget the flowmasters and get the exhaust put back to factory. The air box should not effect anything as long as it was before the maf sensor (which im sure it was or you would have check engine light) I dont know. That is insane. I cant believe changing the exhaust made your gas mileage drop 11 mpg and there is not some other reason. how does the truck run compared to before (when it was stock)??

 

when exhaust was first done and i drove it home it ran like a scalded ape!!! it was awesome, but im guessing thats because of all the fuel being dumped in it. Now that they tried to fix it and failed it runs like crap and has an exhaust leak.... Sucks, i could have lived with first bad gas but not fuel in my oil.... Wish someone knew how to help me.....

 

Is the K&N filter oiled? If so if the filter has too much oil on it and the oil got on the MAF sensor it could cause all you problems. I do not think the exhaust is where your problem lies.

 

i 2nd mdtahoe. the oil from the filter could be sticking to the maf sensor sending false readings and the 02 sensors think the motor needs more fuel. i had an airraid cai a long time ago re oiled the filter and gas mileage dropped 4 miles a gallon. dealer told me to put it back to stock and did. i'd say throw a fram filter or aem dryflow filter, clean the maf sensor and see what happens.

 

good point mdtahoe.. didnt even cross my mind. and i agree completely.. it can cause all the problems you have listed.

 

Im going to check that today!! Thanks so much guys!!

 

it was the holes in the air box!!!! Too much cold air!!! I couldn't believe it, but now that they are patched, truck is back to normal!!! Crazy, just freaking crazy!!!!

 

makes sense cuz if it was bringing in enough cold air. the 02 sensors would read open loop just dumping fuel in. glad you got it figured out.

 

HHHHmmm. make sme think. My truck has been falling on It's face today.