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Discussion Starter #1
I have an 07 mustang gt and i just changed the stock exhaust to an o/r x pipe from bassani and frpp stingers axle back. My mpg has gone down significantly. I dont have a tuner to turn off the rear O2 sensors so my check engine light has come on. Could this cause my car to push extra fuel into the engine from the readings of the sensors or is it just me driving it harder since it sounds so good now. It seems that im driving it the same. Also my exhaust has a noticeable stronger smell now? This is from the cats being removed and the exhaust lubrication burning off from the install right? Any help would be greatly appreciated.
 

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ya idk either i've heard from alot of people that after you take the stocks off or modify them you tend to lose miles...idk about 4 miles tho... thats deff alot.

i honestly would think if you changed them you'd get better fuel economy lol.

i can smell it too when i start up my car. i think its just because the cats are less emission restricting.

oh... well if you have no cats... then you have nothing filtering the gases, which is a common thing so no worries... unless its ya know noticible like blue or white smoke, which that could be oil or antifreeze burnin up. i think. lol not for sure but someone else will correct me most likely haha.
 

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It sounds like something is making it run rich. If your O2 sensors are messed up, that could cause it, or something else making it rich the O2 sensors would pick that up. Running rich is why you are loosing mileage and smelling that smell.
 

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ive heard that if you dont turn the downstream o2's off when you remove the cats the car will run rich in attept to heat up the cats that do not exist therefore hurting your gas milage
 

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I have o/r H-pypes and FRPP GTA mufflers, and get 17.5 mgp with exclusively in town driving. That is with 4.10 gears. I was told that it was necessary to turn off the rear O2 sensors with the o/r pipes and had it done as part of the tune I got.
 

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The reear O2s have nothing to do with fueling. May I suggest you're foot is just a little heavier because you're enjoying the sound. :)
 

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Discussion Starter #9
Im confused im getting 2 seperate answers now :-0....not turning them off causing the engine to run rich and STL saying it doesnt affect anything....anyone else with a definitive answer?
 

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Rear O2 sensors do nothing but measure catalyst efficiency. They do not tell the PCM anything other than how the converters are performing. The front O2s control closed loop fueling. That check engine light is nothing more than an annoyance.
 

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I just put my SLP Loudmouths on today and i noticed a drop about ~2 mpg's. Best I can figure with my "oh so infinite car knowledge" (sarcasm) is the loss of back pressure.

I also just reset the meter from when i did alot of heavy highway driving (had it up to 22.8 MPG's at one point, but hovered around 21.0-21.5) to just around town so see how that plays out. Now i'm bouncing between 18.7~9 and 19.5~ depending on how heavy my foot is.
 
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