I used to be boosted a few months back, but decided I wanted to go back to stock and retain my lovely 23mpg in the city. Well, ever since I put my car back to stock, stock injectors, stock intake...but I notice, i'm only getting 18 or so MPG! I DO still have a 2.5 inch spipe back exhaust on. It has no cats and a Magnaflow muffler. Could my large exhaust be the cause of my 5mpg loss I am seeing in gas mileage? I plan on putting the stock exhaust back on, but havent' had the opportunity. Any ideas?

 

No cats, thats whats killin your gas miles

 

There is a Cat in the stock exhaust header which is on.

 

Do you have a different ECU, stand alone, or piggy back?

 

No I do not. Completly stock now. the only thing that is not stock, is the midpipe and axel back portion of my exhaust. I was under the impression that that shouldn't cause my decrease in MPG but i'm up for ideas!

 

no secondary cat, heavy foot, and the car is still getting used to no more boost

 

bad O2 sensor can do it

 

Cats -1,2 or none - aren't going to noticeably affect your mileage. If you had a bad O2 you'd have a CEL.

Is your s-pipe 2.5"? If so that's a full 2.5 exhaust, you have less low-end torque than when you were stock and maybe you're gassing it more off the line to compensate.

Otherwise, go easy on the throttle for a while, wait a couple more tank fulls, then see where your at.

 

your car hates you for taking away the boost so it's making you pay for it!

 

Try resetting your ecu... Just take of the battery negative terminal for a bit and let everything reset. Then let your car relearn the adaptation values for idle and load and see where your at.

 

Well my .02 did you change back you ecu as well? or did you do that in the process of changing back to stock already?

 

seriously lol its like pulling aside a star quarter back and be like sir we're sorry we need to remove your apandages, but we'd love you to keep playing. He wont be pleased.

 

yeah try doing this.. you ecu adapts and alters signal due to boost and piggyback..

 

Don't forget to check your tire pressure. Probably isn't costing you 5 mpg but it does make a difference.

 

The O2 sensor can be working but out of spec and producing a richer than normal mixture causing MPG loss. You could have also lost some compression from wear in the cylinders/rings when you were running boosted. Compression check would be the first thing to do and maybe an exhaust gas check with a CO2 meter and hydrocarbon check. Change the oil in the engine and tranny since they both got beat up when running boosted. Front end alignmant check and tire wear - all that torque may have bent something too. Did you run a lower temp thermostat?