heavy exhaust smell poor fuel economy

[eyezpinned]

ok, so my car never really got great mpg but now it's just pissing me off. i get this heavy exhaust smell... especially on startup. the smell isnt an egg smell, just a heavy exhaust... like exhaust on steroids.i recently pulled the codes on my engine after getting an engine light flash a few times and got a code for the o2 sensor not producing high enough voltage. i replaced the sensor, haven't had the light come on since, but the mpg are pretty much the same. i was thinking the sensor couldve fouled my cat, but could a bad cat give bad mpg? any help would be great

[Juans_93_MX3]

Do a complete tune up. Make sure the vacum hoses arent ripped.

Last thing I can think of is that your CAT is clogged but that cant be it

[Vanished]

Yes, a bad sensor will foul your cat. If your car is burning rich or burning things it shouldn't, it can melt or clog your cat. I'd say get it looked at. If it's clogged replace with with a resonator or a high flow cat (not sure on the laws in your area regarding this type of thing)

That should give you a nice sounding, smelling, and performing exhaust!

[tehbrookzorz]

Mine smells floral.

[eyezpinned]

would a fouled cat affect mpg though? i always thought a fouled cat was always a symptom of another problem. ive always heard: "cats dont just go bad, something makes them go bad" so in my case probably the o2 sensor. so now the cat might be bad and affect emissions, but fuel economy?

[SuperK]

A new cat will definitely cure your heavy exhaust smell. It'll make your exaust almost smell like purple ponies periodically perfumed per Pasha de Cartier.

As you know, our car's mpg suffers in the winter as well, so keep that in consideration.
The only other reason I can think of is that the fuel / air mixture isn't correct. I remember this happened when my distributor cap was cracked, I wasn't burning all the fuel that was being injected.

Also, dirty injectors will cause your mileage to drop.

[eyezpinned]

it hasnt been all that long, but i just purchased the tools for a full tuneup from rockauto.com. i kinda wanted to go back to ngk plugs anyway. its funny you should mention the dist cap, because i didnt replace that or my rotor during my last and initial new owner tuneup, and who knows the last time that's been changed... if at all. so i'm looking at new plugs and wires, dist cap and rotor, and a pcv valve for good measure. anybody looking to do a tuneup should look into rockauto closeouts on a lot of the stuff i bought ($43 shipped for all).

one last thing, my car never idled smooth at spec base idle. for that reason ive always had the idle screw open a bit, as it corrects it. so in gear my car idles at about 900, but out of gear the idle will go as high as 1500 before the iac takes it down to about 1100. could this affect my mpg. i try to drive the car at spec idle, but after awhile the rough idle just bugs me.