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  #1  
Old 05-16-2003, 07:20 AM
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Question cats and gas mileage

A guy here at work who was a mechanic for some time was talking to me about catalytic converters and gas mileage. It started with him saying that I should be careful with removing cats because it messes up the fuel air mixutre. He said that normally, the heat from the cats burns up excess hydrocarbons and your O2 sensor gets the "correct" reading that way. Once you cut the cats, the hydrocarbons go to the O2 sensor and plug it up and give it a bad reading. This doesn't really make sense to me, though, because our O2 sensors are upstream from the cats. Does anybody else know why cutting the cats would hurt gas mileage?
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  #2  
Old 05-16-2003, 02:14 PM
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You are correct. On pre-OBD-II SVXes (before 1996), the two O2 sensors are in front of the cats. Their purpose is to monitor the amount of unburnt fuel in the exhaust stream, and relay this info the the ECU. On OBD-II SVXes, there is a third O2 sensor in or just after the third cat. This O2 sensor is used for monitoring the condition of the emissions equipment.
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Old 05-16-2003, 03:20 PM
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Cat-astrophic

I have said it before on the boards, Yahoo! and here, it is a bad thing to remove the cats.

The cats do away with unburnt hydrocarbons, as your friend said. Your unleaded fuel has a loads of complex hydrocarbons added to do the job lead salts used to do before, stop your fuel from pre-detonating or "pinking" as we call it, and damaging your engine.

These complex chemicals have many known carcinogens in them, and the reason they are allowed in gasoline is because it is presumed your cat will burn them all up. Which is why your car needs to pass emissions testing.

You take away the cats between tests, and your engine is pumping unburnt carcinogens into the street. Does not display much concern for the health of your fellow man, woman or child in the street.

To anyone contemplating removing the cats, I say, grow up, get your few extra horsepower some other way besides poisoning people.


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Old 05-16-2003, 03:39 PM
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I still get 25-26mpg with no cats on highway. My wife can get 27 pretty consistently in my car but she has a lighter foot than me. I get 19-22 around town. My mileage did not change at all with the exhaust change. If I had known it wouldn't change I'd probably have left them on. Either the stock system flows very well or my butchered system isn't very efficient.
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Old 05-16-2003, 09:02 PM
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my obs, has had the post cat O2 sensor sheared off for quite some time, and the milage hasn't gone down at all, and it's not sensing anything at all.. (except the lack of o2 sensor and cel)

as to passing emmissions

removing the 3rd cat from the svx and leaving the 2 initial cats will probably still pass emmissions

IF you live in one of those crappy places that requires emmissions, i, do not
i passed inspection on the obs w/o the o2 sensor at all
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Old 05-17-2003, 07:17 PM
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First off, just because you don't have cats dosn't mean your car won't pass emissions. My SVX passed emissions with no cats and my freinds 302ci Ranger passed with no cats, on the dyno! As for the 3rd O2sensor it's only there to make sure the cats are working, it dosn't effect the way the vehicle runs at all, and with a lot of the new emissions tests OBD II cars will just be checked by pluging into them with a scan tool and checking for codes. So if you CEL is on it's going to fail.
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Old 05-19-2003, 09:07 AM
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you can hook an resistor inline with the 3rd o2 sensor to fool it into thinking it is working
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