Floating O2 sensor voltage...

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Nov 1, 2007 | 07:32 PM

ninetyfivevette

Thread Starter

Burning Brakes

Joined: Jan 2002

Posts: 1,228

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From: Dallas Texas

Floating O2 sensor voltage...

So I'm breaking in my new 383 LT1 ('95) and I'm having a problem with the right O2 sensor hovering around 450mV (new Delco sensor). This of course totally messes with the fueling (108/160 fuel trims). At that point the engine is running so bad, testing things is difficult. I tried disconnecting both O2 sensors to make the PCM stay in open loop and throw codes- however, DataMaster is now reporting a steading 450mV, PCM does not go into closed loop, but no codes are thrown (there might be a timer I am not exceeding for the O2 sensors to become active).

Shouldn't PCM report 0mV for O2 sensor measurement when disconnected?

I've verified the wiring to the sensor to the PCM is good. The O2 sensor heater wire has proper voltage when the ignition key is on. Ground connection from sensor harness to engine resistance is sane.

Any thoughts or suggestions?

Nov 1, 2007 | 10:00 PM

malc

Instructor

Joined: Aug 2007

Posts: 141

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From: Western Suffolk, Long Island NY

Quote:

Originally Posted by ninetyfivevette

From all that I have read, 450mv is the voltage that is considered the crossing voltage. The voltage of an active sensor will bounce above and below that if the AF mixture hovers around 14.7. I would suspect that the input to the computer is matched to this with high a resistance votage divider (or regulator) producing a constant 450mv. This would keep the input from giving erroneous crossing events due to ignition noise during a disconnected sensor condition.

With the sensor disconnected, you should get no new crossing events. Maybe someone could chime in on what a normal number of crossing events is for a good sensor.

http://www.freeengineinfo.com/o2-sensor.htm

Excellent article on wikipedia quote:
The ideal point is 0.45 V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, called the stoichiometric point, and the exhaust output will mainly consist of fully oxidized CO2.
http://en.wikipedia.org/wiki/Oxygen_sensor

Last edited by malc; Nov 1, 2007 at 10:11 PM.

Nov 7, 2007 | 08:19 AM

ZylaRace

Drifting

Joined: Mar 1999

Posts: 1,931

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From: Norcross GA

You did swap sensors to see if the problem swapped banks, right?

Nov 7, 2007 | 09:26 AM

mseven

Le Mans Master

Joined: Feb 2005

Posts: 5,146

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From: The Motor City

Quote:

Originally Posted by malc

Excellent article on wikipedia quote:
The ideal point is 0.45 V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, called the stoichiometric point, and the exhaust output will mainly consist of fully oxidized CO2

not exactly, it is only ideal because that is where an 02 is most accurate, and does not mean that the car is truly operating at 14.73. An 02 sensor measures the product of combustion.
Usually if AIR is Incorporated the crossover point is bumped up to .550mv.to comp.

Quote:

The voltage of an active sensor will bounce above and below that if the AF mixture hovers around 14.7. I would suspect that the input to the computer is matched to this with high a resistance voltage divider (or regulator) producing a constant 450mv.

It 'bounces' because it acts as an oscillator, the crossover point is programmed in the ecm's prom.

Quote:

Originally Posted by ninetyfivevette

If this is a freshly modded motor and no changes have been made to the tune, you will need to address the fueling. With an 02 reading of .450mv. would mean it is not adding fuel, not warmed up or 'ready', or not 'seeing' closed loop (bad sensor, grounds etc.) or possibly seeing oxygen (exhaust leak).

Last edited by mseven; Nov 7, 2007 at 09:48 AM.

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