What would cause my Jeep to have sudden HORRIBLE gas mileage?

I’m guessing the flanges helped improved gas mileage, and now you removed them. What else would it be?

Is the straight pipe the same diameter?

Are the welds rough on in the inside so that they create turbulence in the exhaust flow that was not present when you had the flange?

Is there a bend to the pipe that was not there before?

The least little change in air flow on either intake or exhaust will affect how the engine runs.

It is also possible that the only reason you had good MPG before was due to the same things that caused you to fail emissions.

Average miles per gallon before?

I think @jerv‘s right. The stock system was probably tuned rather precisely for your engine. Every turn and flare is considerred to create just the right back-pressure for optimal performance (i.e. every turn and flare affects performance).

I have a hard time believing that the flanges would really have made that big of a difference in your mileage.

To be honest, 12mpg in a 12 year old Cherokee… that sounds about right to me.

If you want to improve it… you could try lighter weight oil, take as much crap as possible out of the car (added weight stuff)... make sure the tires are inflated… And that’s about it. Reworking the exhaust system, maybe the air intake, stuff like that… could net you a mpg or 2… but I dunno what options are even available for a Cherokee, since usually 3rd party upgrades to those systems are made for sports cars.

something that bad could be as severe as a fuel line leak, could be a really dirty filter, maybe clogged injectors, any number of things…

@tedd That depends on the quality of the exhaust work. I have seen some sketchy stuff. However, you are correct that Jeeps are not known for good MPG.

Yeah I believe you have taken notice of the horrible gas mileage now more than ever, on top of that gas prices have risen so perhaps you aren’t filling up as hardcore as you once did? Also take into consideration what @gambitking has mentioned. Run the vehicle, jacked up if you can to look for leaks, as well as vast smell in the air of gasoline fumes. If you are correct in that you may have gotten about 17 mpg as opposed to 12mpg now, then it defintely is escaping somewhere.

What happen to the O2 sensor?

My jeep has it installed behind the Y , if that has been taken out of the system the on-board computer will be trying to increase the fuel going into the intake to compensate for too much O2 in the exhaust.

A properly running engine should have almost no O2 in the exhaust, and if the sensor is left to sense the outside atmosphere it will read there way too much O2 left over after combustion.

I go with DrBill’s answer.

Yep, O2 sensor

There are two O2 sensors. I replace one of those before we put the strait pipe in. (It’s still there now). If it was an O2 issue I probably wouldn’t have passed smog, right?

Also, my tire pressure was very low. I’m sure that wasn’t helping anything.

My MPG improved about 10–15% by inflating my tires to near the limit printed on the sidewalls, which is about 4–6 psi above factory recommendations. Then again, at recommended pressure, I tend to wear the sides of the tread more than the center (a sign of under-inflation), and that varies by car so you have to play around a little to find what works.

For my car and driving style, I do best at 34 front, 30 rear instead of the recommended 28/26, but it may be different for you.

@simone54 I’m not entirely familiar with smogging tests because I don’t live in an area that requires them, but .. O2 sensors typically work with your cars computer, telling it how much oxygen is going in (to correctly mix it with the proper fuel amount), or in the exhaust to make sure it’s flowing, etc, etc, etc. I don’t think the smog test would even check these, as if I’m not mistaken they just check the smog coming out of the tailpipe right?

@tedd They check the tail pipe and they also hook up to your computer.