I'm getting horrible milage in the 760T, like 17 MPG with a relatively light foot. I've got a MBC installed and am running 13 lbs but sometimes drive for days without exceeding 10PSI. I've also got an air-fuel gauge and it's telling me that the 02 sensor is working properly. Just wondering If it can go out of calibration and make the car run slightly rich. There's black sooty stuff on the outside edge of the tailpipe. The wife gets over 25 MPG in her '91 740T. LH 2.4 wouldn't account for that much difference would it?

Thanks,

As your Oxygen sensor aproaches the end of it's life, it will probably have both voltage falloff and a slower reaction time. However, the car can not tell that something is wrong until the sensor is completely dead (incapable or producing more than .5 volts and it should trip a fault code at this time). As you get more and more voltage falloff, the car will enrichen the mixture to compensate, and believes itself to be running fine.

For the greatest sensor life expectancy, always buy Bosch. I've yet to have any non-Bosch sensor last more than 2 years. I typically change my Bosch sensors after about 5 years, just to keep the optimum fuel effeciency for my vehicle.

For more information, read the following posts:
http://www.brickboard.com/RWD/index.htm?id=721861
Cost effective 3-wire O2 sensor:
http://frwilk.com/early944/misc/oxygen.htm

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 231K

Sorry to barge in, but I checked out the links you provided and they led me to another volvo site where a guy converted a single wire O2 sensor to work on his 3-wire car.

Do you know what the guy did with the other 2 wires?

Mike

I have a limited understanding of this topic so I'll chime in, Perhaps someone will correct me if I'm wrong. An O2 sensor needs heat to operate properly. The "other" 2 wires go to a heating element which gets the O2 sensor hot earlier than it would in the normal warming up of the exhaust manifold, this improves emissions when cold. Once the exhaust manifold is up to temp, a 3-wire sends the exact same signal as a 1-wire sensor.

So they are interchangeable, to the slight detriment of emissions and possibly gas milage. If you wanted to switch to a one wire setup, I'd just terminate the other 2 wires somewhere where they couldn't be grounded.
--
Bob W.
16 Volvos ('58-'91)
445-544-122-240-740
Four turbos

Quote: "An O2 sensor needs heat to operate properly. The "other" 2 wires go to a heating element which gets the O2 sensor hot earlier than it would in the normal warming up of the exhaust manifold, this improves emissions when cold. Once the exhaust manifold is up to temp, a 3-wire sends the exact same signal as a 1-wire sensor."

Bob, you are correct. In a 3-wire O2 sensor, two of the wires are for a heater element within the sensor. When cold, I would guess that this element draws about 4 amps or so to rapidly warm up the sensor so it can begin to function. The O2 sensor needs to be hot to operate properly (a bit above 1,000 degrees F or so, IIRC). The heater element varies in resistance as the temperature changes, and it will hold at a constant temp after it's hasty warmup. With the sensor holding at a fixed temp, the computer can get a more reliable reading from the sensor than if it were not heated. I've also heard that heated O2 sensors tend to last twice as long as a non-heated sensor. This makes sense, but I have yet to own a vehicle using a 3-wire sensor long enough to verify this from personal experience.

If you are considering swapping a single wire O2 sensor (Bosch 11027) in place of your 3-wire sensor, take note of the following: Any Volvo with OBD (On-Board-Diagnostics) will trip the "Check Engine" light if it detects that the current draw for the O2 heater is missing. Therefore, there are only a few years ('85 through '87 or '88, I believe) that can get away with this swap since OBD had not yet been installed. However, with the falling costs of some of the more mass produced 3-wire sensors, it would be worthwhile and spend the extra $15 or $20 and pick up a generic 3-wire sensor (Bosch 13913, 13953).

God bless,
Fitz Fitzgerald.
--
'87 Blue 245, NA 231K

good call. i'd hate to see that 1 come in the shop for a running problem. and so it goes, chuck.

Sorry to barge in, but I checked out the links you provided and they led me to another volvo site where a guy converted a single wire O2 sensor to work on his 3-wire car.

Do you know what the guy did with the other 2 wires?

Mike

Thank you! Good reading there. Mine has unknown milage/age on it and now I've got a good reason to replace it. I've got a spare AMM but want to change one thing at a time and re check the milage so I know what I did that fixed it.
--
Bob W.
16 Volvos ('58-'91)
445-544-122-240-740
Four turbos

See the FAQ for tips on rich mixture, including fuel pressure regulator troubles and ECT failure.

you cannot tell mixture by looking at the tailpipe. the convertor changes things. don't know what the equipment you mention is, but i'd start with the basics, fuel pressure, ignition, etc. good luck, chuck.

mostly they just respond more slowly as years go by.