I finally straight piped my car (still has mufflers just no cat). Immediately I noticed a huge loss of fuel economy, about 15mpg average instead of 20mpg average. I always drive about half freeway half city and I didnt change my driving habits. I did get on it kinda hard when I first got it back, but even the next 3 fill ups, I still noticed that Im not getting anywhere near the same gas mileage.

I wouldn't mind a little loss of mileage but it seems like too much of a loss. What are you supposed to do with the air tube that goes to the cat? Im pretty sure the muffler shop just left it there open.

Also the car still seems to run a little rich still even with the new o2 sensor (hard to tell with no cat but it seems to smell more than my friends 69 chevelle with a 350). I think its a vacuum leak because im still having intermittant problems at full throttle (made a thread about that earlier)

So anyway did anyone else notice the loss of fuel economy after removing their cat? Anything that can be done?

 

Removing the cat should not reduce your fuel economy at all. Something else it going on.

 

My car smells rich after removing the cats too. I think that's just a byproduct of removing them. Have you pulled some plugs to get a visual on it possibly running rich?

 

Check for exhaust leaks before the O2 sensor or after...that could impact the readings. Also, what brand of O2 sensor did you use? Better be Delco for proper readings to ECM.

Do you have headers? If so, then a heated O2 sensor should be installed.

 

I used AC Delco o2 sensor. I do have headers and I retightened the bolts. I dont have a heated o2 sensor though

 

No I haven't yet. I was running with a bad o2 sensor for a while though

 

With headers, you typically need a heated O2 sensor, so the ECM will go into closed loop. The AFS-74 is one that I have used...do a search on heated O2 sensor on this site for more info.