@MikeInNH, I live here in GA. The summer blend has extra additives and oxygenators to help reduce smog. That means less fuel per gallon. I get better performance and mileage in the winter than with summer blend fuels. Of course, temps don’t drop as far around here, so I don’t need to idle my cars in the winter like most Northerners do in below freezing temps.
I guess your winter blend is different then the winter gas here in NH. Here in NH…a 10% reduction in gas mileage and performance is not unheard of.
Like your area @BustedKnuckles, here in the San Francisco Bay area we tend to get worse smog problems in the summer than the winter. Yours could be an explanation for why I get reduced HC readings w/the winter blend.
@circuitsmith, my theory about why timing retarding reduces HC may be that the fuel and air in the cylinder have a little more time to mix before “bang”, so the combustion process is a little less chaotic and therefore more complete if the spark plug fires a little later, closer on the upstroke to top dead center.
With regard to the questions about the oxygen sensor, the OBDI system is not as sensitive to the condition of the oxygen sensor as an OBDII system, so you can run sensors longer. It will set a check engine light when the sensor fails to hit the required voltage or if it is failing to switch at all, so generally you can run your oxygen sensor until the computer starts to complain about it. That being said, OBDI oxygen sensors generally give up sometime between 200k and 300k, so you can be confident that you have gotten your money’s worth out of it if you decide to try a fresh one as an experiment…
As for checking the cat, checking the pressure is a way to see if it has overheated and gotten broken down inside and plugged up. You can make a pressure test kit for about $20 out of an old oxygen sensor, some brass plumbing fittings and a piece of copper water tubing from the hardware store, and a 0-10 PSIG fuel pressure gauge. Static pressure at the manifold should never exceed about 1 psig at idle or 8 psig at full throttle under load. An easier/cheaper way to check for obstructed exhaust is to put your hand by the exhaust opening and see if you can easily feel the individual bursts of exhaust at idle. Understand, however, that the catalyst can be poisoned and not working and still flow just fine.
If you have access to an infra-red thermometer gun, check the temperature of the pipes entering and leaving the cat. If the cat is working, I believe that you should find that the temperature exiting the cat will be hotter than the temperature entering the cat. Inexpensive infra-red guns are available on line for about $25, and even if the thermometer is not accurate, it will be adequate for most uses around the garage (like pointing at the exhaust manifold to figure out which cylinder has a misfire) and the laser light on the thermometer is perfect for harassing your wife’s (feline) cat.
Here, here, and here are three textbooks that explain how retarded timing reduces HC emissions.
And here someone not so scholarly measured the effect.
Thanks @circuitsmith. Your links provide info that is spot-on.
So you don’t care that some of us prefer to breathe clean air.
Even the dumbest animals know not to soil their own nest.
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So you’re an admitted criminal, along with other things I won’t post…
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I think that practice mentioned of replacing all O2 sensors at every major service interval or every 30-60k miles is severely misguided and is not necessary. Sounds like gouging to me…or armed robbery.
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This is a thread from April 2013. Whatever was the problem, the OP has either solved it by now or dumped the car. The only valuable info missing here is the solution he found.
I realized the thread was from '13 when I responded. I was simply addressing Revmachine’s comments which are misguided to say the least.
I agree completely. Why replace something the car will tell you is going bad before it sets a code? That just seems like a boat payment to an incompetent mechanic.
That is totally absurd. In the past 40+ years we’ve had to have one and only one O2 sensor replaced. And that was a mistake by the shop that replaced my Exhaust system so they replaced it for free. 5 of those vehicles had over 300k miles on them. And we’ve never had a problem with a catalytic converter.
I’d never use a mechanic who did that.
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I have only heard one time in my life where replacing O2 sensors was considered a maintenance chore and that recommendation did not last very long. SAAB at one time a long time ago, said to replace O2 sensors 275 bucks each) at every 15k miles service.
I told the SAAB rep “You do realize this is going to cause WW III with every 15k miles service don’t you?”.
He blew this off. Not long after they changed the recommendation to every 30k miles; which was still BS.
Not long after that the recommendation became replace them IF needed and “IF needed” was never needed.
Guess SAAB was probably deluged with customer complaints over that boneheaded policy so that’s why it died a quick death.
Nice …
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And I forgot to say ’ Welcome ’ .
Buh-bye! 
How come I did not get dissed? I feel kind of neglected… 
Regarding procedures, I would think there are better ways of checking O2 sensors rather than randomly swapping some around to verify a problem.
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Ahh, “swaptronics”, the diagnostic process of the future.