Decreased gas mileage with new exhaust...help

I originally bought the K&N cone filter for my Jeep and that gave me 1-2 extra mpg. I then bought a high flow catalytic converter and a free flow muffler and that dropped my gas mileage 3-4 mpg. Everything I've ever read says a less restrictive exhaust gets you better gas mileage. I noticed performance gains with the K&N filter but nothing or little change with the new exhaust.

I checked the ECU and got no code readouts so I assume that all of the sensors are good. I've had the exhaust for 3-4 months at least and reset the computer when I did get it.

The sound of the exhaust is really loud at 1500 rpms and when I'm hard on the throttle. Hard on the throttle I consider normal but the low rpms noise gets me and I don't understand it. Could there be a leak somewhere that is causing a problem? Would a small leak cause the huge decrease in gas mileage as well?

Any help or recommendations are appreciated because I'm stumped and the low gas mileage is killing me.

It might be possible that something was disturbed or broken during installation. I would have a good look at the oxygen sensor. Don't rely on the ecm to give you a code. Get it checked out and see if the electrical resistance reading it is generating is correct. It may take a dyno and a sophisticated diagnostic to find out.

It seems that sometimes a much better flowing exaust can cause a lean situation and the ECU will overcompensate for you with too much fuel causing your engine to run rich. This will cause the loss of mileage and power.

Have someone press the gas pedal while you look at the exaust to see if any black smoke will come out. If it does its a sign of your car running too rich.

Or of you want to be 100% sure you can take it to the dyno run a wideband and you can find out preciselly what your A/F is

One thing you can try for free is disconect the battery to reset the ECU and maybe starting fron the default maps it will learn better how to cope with your exaust.

Well I've never seen black smoke come out of the exhaust pipe but I did notice that my nice clean exhaust pipe was caked with soot before I had driven it 75 miles. Is that normal though or do you think it actually is running really rich?

Well I've never seen black smoke come out of the exhaust pipe but I did notice that my nice clean exhaust pipe was caked with soot before I had driven it 75 miles. Is that normal though or do you think it actually is running really rich?


unusualy large acumulation of black deposits on the exaust tip are often signs of running rich. its probably happened as i mentioned in my firts post.


try reseting the ECU and see if it fixes it.

umm well also dont mean to sound like i'm a total dick are you driving a bit harder? ECU reset would be worth a try.

No I'm not driving harder. Got over that in the first week or two. It's not a dick question at all. Most people ask it when talking about bad gas mileage.

I also might have to say that. Since you free more restricted power to your car, and you feel the difference in power. There are times that you tend to drive it harder than usual because of the power difference. This might be a reason. Other than that, I have no clue. Improving the breathing system of your car (I/H/E) should generally give you better gas mileage.

Improving the breathing system of your car (I/H/E) should generally give you better gas mileage.


that is not necesarily true. ECU always have to compensate for them and IMO they will often ERR on the side of rich since its much safer than a lean situation.

that is not necesarily true. ECU always have to compensate for them and IMO they will often ERR on the side of rich since its much safer than a lean situation.

Not necesarily true? I always thought that it would have better gas mileage.

Not necesarily true? I always thought that it would have better gas mileage.


as i mentioned in my first post mods like that can introduce a lean situation and the ECU has to compensate. And often enough its overcompensates and makes the car run rich killing your power and gas mileage.

and i belive this to be the case here....the fact that he found quite a bit of soot caked on his exaust is a sign poiting to a rich mixture

this is why all the best upgrade parts in the world are useless without tuning.