[hendrix2333]

Can anyone one tell me why im getting such bad mpg i have a 2003 3.8l mustang with flowmaster exhaust and kn filter and all 4 cats removed. my car runs fine with no problems but im getting really bad gas mileage i dont have a check engine light on or anything and when i drive i never go pass 2000 rpms i checked my fuel lines for leaks and i havnt seen any leaks and my exhaust isnt burning rich so i dont understand why my mpg is so low im getting about 13 mpg so i was wondering can anybody tell me why might my car be getting such bad mpg or anything i can do to improve my mpg

[Carbon28]

everybody hates me when i post advice cuz its usually shit

But someone will chim in and give a better answer. My guess would be because of the cats, if you say everything else checks out. Because even having the cats removed might have been a good idea at the time, it could be actually hurting your engine and performance rather than helping it. Has to do with the dynamics of how the engine and exhaust flow. Pretty much from what i understand is your engine isnt making enough power (exhaust gasses) to need such a free flowing exhaust set up. Therefore resulting in bad performance and robbing MPGS

[Rich J]

god damn 13 mpg? Yea my best guess would be cats too.

[NorCalWhtHrse]

13MPG?

[Carbon28]

Hahaha

[mustang227]

maybe spark plugs?

[Carbon28]

how would spark plugs cause bad MPGS? maybe if they were fouled up, but still at that youd be miss firing, or bucking or some noticable problem. Is the performance still the same?

[walt1120]

Originally Posted by hendrix2333 Can anyone one tell me why im getting such bad mpg i have a 2003 3.8l mustang with flowmaster exhaust and kn filter and all 4 cats removed. my car runs fine with no problems but im getting really bad gas mileage i dont have a check engine light on or anything and when i drive i never go pass 2000 rpms i checked my fuel lines for leaks and i havnt seen any leaks and my exhaust isnt burning rich so i dont understand why my mpg is so low im getting about 13 mpg so i was wondering can anybody tell me why might my car be getting such bad mpg or anything i can do to improve my mpg

-check tire pressure
-last time you changed your oil?
-did you reset your cars computer after installing your mods?

your exhaust might be pulling fuel out of your engine before it burns

[Carbon28]

walt, the only thing that he could check would be the oil, that could be a problem that he might have missed. Tire pressure will cause bad MPGS but its pretty noticable if the tires low, who knows. And with the mods he has, at least in our year of cars you dont need to reset the computer for the mods.

but explain how "your exhaust might be pulling fuel out of your engine before it burns"

[Mr. Red Pony]

Probably the cats. Did you remove 02s?

[walt1120]

Originally Posted by Carbon28 walt, the only thing that he could check would be the oil, that could be a problem that he might have missed. Tire pressure will cause bad MPGS but its pretty noticable if the tires low, who knows. And with the mods he has, at least in our year of cars you dont need to reset the computer for the mods. but explain how "your exhaust might be pulling fuel out of your engine before it burns"

its what low backpressure is, when you dont have enough back pressure..that means that the pressure in your exhaust pipe is low enough where when the exhaust vavle opens air rushes out so fast( air moves from high pressure(inside the engine) to low pressure(the exhaust pipe) and we know that air moves from high to low pressures so if the pressure is too low(low backpressure) the air shoots out of the vavle and sometimes pulls fuel from the injector along with it.

and my tires are low profile and its very hard to tell when there low so thats why i have that problem sometimes

[NorCalWhtHrse]

Its your 02 sensors. do you have a check engine light on? or did you use MILS?

[Gandolf]

Is the engine going from open feedback to controlled feedback from your o2 sensors...

Prolly not.............. since that is now impossible...

Try taking the tires off next.......

Your engine is a very carefully controlled machine... But you went "too far"...

Massive engine damage and such is still to come...

Like running 'square' tires on the freeway, you have now passed the point of where we cannot help you

"WHY", on God's Earth, did you alone, just chose to just ruin your car???

[hendrix2333]

thanks for the advice and the shop that took my cats off put fake o2 sensors on to trick my computer so the check engine light dosnt come on

[Mr. Red Pony]

Well you need the front o2s but not the back. the back just say if the cats are working or not.