Oxygen sensor removal question - Vintage Mustang Forums

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post #1 of 6 (permalink) Old 05-19-2017, 08:18 PM Thread Starter
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Oxygen sensor removal question

I've got a 1990 Chevy 2500 with a 350 in it. The previous owner removed the catalytic converter and installed a glass pack. The oxygen sensor is just wrapped around the exhaust pipe. Besides the fact that the truck is really loud- it hesitates when cold and smells rich. I'm thinking the O2 sensor and missing cat might be my problem. I rebuilt the TBI and it runs a little better. Could the rich condition and rough cold engine stumble be because of the previous owners mods? I'm thinking of getting a high flow cat. Do the cats have the bung to install the O2 sensor or is it installed in the exhaust pipe? Any advice? (Good advise- not gettin rid of the Chevy!). Thanks
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post #2 of 6 (permalink) Old 05-19-2017, 10:06 PM
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It only has one converter? One O2 sensor? Could have 4, should have at least 2 sensors.

Anyway, an 02 sensor that's installed in the exhaust manifold *before* any cat is used by and is necessary for the computer to make it's fuel trim calculations. If this 02 (called the upstream sensor) is out of the exhaust stream, the computer needs to make a guess by using the balance of it's inputs. It could run rich, lean, or any combination based on it's other inputs and rpm.

Any O2 sensor that would ordinarily be installed *after* the cat has no effect on the performance of the engine. This downstream sensor's sole purpose is to feed information to the computer so the computer can monitor the effectiveness of the catalytic converter(s). If this sensor is out of the exhaust stream, no performance related issues will be shown, but it will set a cat efficiency code of this O2 is missing or misplaced.

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Last edited by John_Del; 05-19-2017 at 10:11 PM.
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post #3 of 6 (permalink) Old 05-20-2017, 03:05 AM
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Pre-OBDII vehicles often only used one sensor. That sensor is indeed needed to tune the EFI. Drill a hole and weld a bung on as close to the original factory configuration as you can. No Cat is needed for functionality. However you might need to have one to pass NC inspection. The secondary downstream O2 sensors weren't standard on everything until 1996 and later. And then all they really do is keep an eye on the efficiency of the converter.
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post #4 of 6 (permalink) Old 05-20-2017, 07:56 AM
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Quote:
Originally Posted by GypsyR View Post
Pre-OBDII vehicles often only used one sensor. .

You're definitely right on this one. I used Autozone's site when I first posted and they show both upstream and downstream for the 1990 C2500 5.7L V8. But I checked Rockauto just now and they show only upstream sensors. I guess Autozone hopes you buy both and forget to return the one it doesn't need.

Still, I thought any V engine would have at least two: one on each bank as close to the head as they could get it. If indeed uses one (and that seems to be the case with this 2500), they must put it after the Y pipe - a lousy place to put an O2 sensor.


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post #5 of 6 (permalink) Old 05-20-2017, 11:47 AM Thread Starter
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Thanks for the replies. So I guess the first step would be to reconnect the o2 sensor and see what ailments it cures. Since it's before the cat then I really shouldn't need to reinstall the cat unless I need it to clear up some exhaust smell, if it still exists after the O2 sensor has a chance to change the engines running characteristics. Luckily I don't need a cat for this truck to pass inspection. Thanks
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post #6 of 6 (permalink) Old 05-20-2017, 01:54 PM
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On my 1995 F150 351W the single O2 sensor was originally in a small bridge pipe where the the y-pipes first came close but didn't yet join. After putting on long tube headers on I basically had a choice of using the fitting in one or the other header collector. I wasn't real happy about having to pick a side but the ECU can only use a single input. But barring an odd failure of one side of the engine, it works OK. Doing the same might open some installation options for you.

With the o2 sensor disconnected you should have a "check engine" light on all the time. People have been known to take the bulbs out of those. If you don't see the light come on with the key on before you start the engine then you might want to look into that. Also if the bulb is out you can't check for codes with the "paperclip method".

My same F150 I mentioned above has no cat and the only thing I've noticed is that it tends to make more "steam" smoke out of the tailpipe on humid-cool/cold days than I recall it doing with the stock exhaust. I might put a cat on at a later date and the only thing I'd really expect to change because of it would be a very slightly milder exhaust note.
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