symptoms of a dying O2 sensor?

well, I've done a few tanks of gas now, and have come to the conclusion that I'm down about 25KM per $20CDN of gas in fuel economy. Nothing else has been changed on the truck, so I'm assuming somethings gone wrong. (91 K1500
305)
When I had a TBI problem (FPR), the truck ran disgustingly rich, black smoke, etc all the time. it was obvious. but this time the truck only smells slightly rich on startup, and after warming right up doesn't smell rich at all...
can a failing O2 sensor allow a /somewhat/ rich mixture, not bad enough to smell outright, but bad enough to hurt fuel economy in a measurable way? what other symptoms are there? the truck runs and drives perfect otherwise...it was changed to a Delco piece January of 2002. so its not even got 2 years on it....is this normal??
b
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B,
A failing/old 02 can cause these symptoms, except the rich smell on startup which is normal. The ECM runs in open loop (aka really rich) when she's first started anyways.
When's the last time you gave her a healthy tune-up?
Doc

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when you say a healthy tune up, can you elaborate? plugs, wires, cap, rotor for example? if so, then about 35,000KM ago when the engine was rebuilt.
I run a K+N filter which I just cleaned. what else can be done?? I didn't think that any of the above parts hurt fuel economy??
from my original post, the reason I noticed right away that something was a little off...I noticed power was up to a measurable degree....a huge fluke it was just after a filter cleaning (but that doesn't cause a noticeable power increase)....
??
b

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Fuel filter can cause poor fuel mileage. I had a Van in the shop last week, $20 to go 150 kms. His fuel filter was so plugged that I could not blow through it. Cam

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actually, $20CDN to go 150 km is about right for my truck!!! :) the fuel filter is about a year old....time for a change out of process I guess...
b

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$20 CDN to drive 150 km is about right for any small block Chevy. 305 or 350. I think that's right in the ball park for the mileage to expect.....

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