ive had my truck for about 7months now and its starting to run rough. ive done the things i can cap rotor wires plugs filter as well as dumping some injector and fuel treatment/cleaner and not much change well the change i notice is that its running rich now and idle is getting rough. i got under and looked at the exhaust and noticed that it was what we call back home a 'Kimchee rig' in other words half A$$ job at putting the exhaust together. it looks like the previus owner found some extra pipes lying around and put together what looks like one of those pipe video games where you maze around going thru big and small pipes put together with tapeto get out. from manafold to the cat it looks solid and stock with all same size but once you get to the muffler pipe size change and there are a few leaks here and there. with a big pipe running into a smaller one would that cause back pressure in turn causing the O2 sen to read wrong and affecting the performance? i do plan on getting the pipes redone soon but would the current ones be causing my headaches?
hope you can help

 

The exhuast pipe behind the muffler will not affect the 02 sensor, but I would recommend getting the leaks repaired with correct size tubing, either 2.25" or 2.5" (all one piece ).