Originally Posted by shonanspeed

Is it throwing a CEL? I'm figuring you don't know whether you're hitting a rich spot in your fuel curve, right? Are you kicking up black smoke when you hit lag? When you put your exhaust system on did you put all the O2's back? I'm not a Mustang girl so I don't know how many your car has but leaving one off unresistored or having one go bad will seriously affect your fuel...but that's usually across the board and not rpm specific. Also, how do your plugs look?

Originally Posted by shonanspeed

Is it throwing a CEL? I'm figuring you don't know whether you're hitting a rich spot in your fuel curve, right? Are you kicking up black smoke when you hit lag? When you put your exhaust system on did you put all the O2's back? I'm not a Mustang girl so I don't know how many your car has but leaving one off unresistored or having one go bad will seriously affect your fuel...but that's usually across the board and not rpm specific. Also, how do your plugs look?

Originally Posted by sbdunkz757

brand new spark plugs, my friend put on the exhaust system he knew what he was doing he has been working on domestics for the last 5 years. and no black smoke when it lags, i started getting this problem ever since i got the gear ratio changed

Originally Posted by slo-180sx

I highly doubt the gears have anything to do with it. A tuner just makes sure your speedometer is correct. What does the exhaust system consist of? Do you still have cats? How much mileage is on your car? Fuel filter been changed lately? When the gears were done, was the diff rebuilt?

Originally Posted by sbdunkz757

Car:2000 Mustang GT 4.6 liter

mods: k&n ntake, 4.10 gear ratio. and flowmaster exhaust system

Originally Posted by sbdunkz757

magnaflow exhaust, yes i have the cats. 80k miles fuel filter has been changed, diff was not rebuilt