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Oxygen sensor negative output

A

Adrian Tuddenham

Jan 1, 1970
0
I have been testing a Bosch automotive exhaust gas oxygen sensor of the
zirconium dioxide type, which I suspect is faulty. There are four
connections to it: two wires to a floating heater element, which require
12v of no particular polarity; one sensor output wire and one sensor
earthing connection to the body of the device.

The heater correctly takes about 1.7A when cold, falling below 1A as it
warms up. The sensor is supposed to give readings between +0.2V and
+0.8v when exposed to exhaust gases, the voltage becoming more positive
with decreasing oxygen level. Instead, it gives readings between -0.8v
and -0.2v, with the voltage becoming more positive with decreasing
oxygen levels. In other words, the readings are behaving correctly but
are displaced about 1.0v negative.

According to various websites, this is typical behaviour for a sensor
which has become contaminated with silicone residue, and the obvious
solution is to replace it. However, I can find no source for such a
contamination and some websites imply that the contamination might be on
the reference surface of the zirconium oxide 'thimble', not on the side
exposed to the exhaust gasses.

My question is: does anyone know which side of the sensor is the
contaminated one when the voltage is displaced negative?

I realise this is not exactly a 'design' problem, but I think that I
shall only get a proper answer, as opposed to hearsay and guesswork, by
asking designers who have actually worked with such devices.



[I have already eliminated bad earthing contact between the exhaust
system and the engine, which is the commonest cause of a spurious
voltage readings. The vehicle is not consuming large quantities of
coolant, so anti-freeze contamination is not very likely. To the best
of my knowledge no repairs have been done with silicone sealants]
 
I have been testing a Bosch automotive exhaust gas oxygen sensor of the

zirconium dioxide type, which I suspect is faulty. There are four

connections to it: two wires to a floating heater element, which require

12v of no particular polarity; one sensor output wire and one sensor

earthing connection to the body of the device.



The heater correctly takes about 1.7A when cold, falling below 1A as it

warms up. The sensor is supposed to give readings between +0.2V and

+0.8v when exposed to exhaust gases, the voltage becoming more positive

with decreasing oxygen level. Instead, it gives readings between -0.8v

and -0.2v, with the voltage becoming more positive with decreasing

oxygen levels. In other words, the readings are behaving correctly but

are displaced about 1.0v negative.



According to various websites, this is typical behaviour for a sensor

which has become contaminated with silicone residue, and the obvious

solution is to replace it. However, I can find no source for such a

contamination and some websites imply that the contamination might be on

the reference surface of the zirconium oxide 'thimble', not on the side

exposed to the exhaust gasses.



My question is: does anyone know which side of the sensor is the

contaminated one when the voltage is displaced negative?



I realise this is not exactly a 'design' problem, but I think that I

shall only get a proper answer, as opposed to hearsay and guesswork, by

asking designers who have actually worked with such devices.







[I have already eliminated bad earthing contact between the exhaust

system and the engine, which is the commonest cause of a spurious

voltage readings. The vehicle is not consuming large quantities of

coolant, so anti-freeze contamination is not very likely. To the best

of my knowledge no repairs have been done with silicone sealants]



--

~ Adrian Tuddenham ~

(Remove the ".invalid"s and add ".co.uk" to reply)

www.poppyrecords.co.uk

The theoretical output voltage is:
Cell output = (2.303RT/4F)x log ( P1/P2)

where:-
R = molar gas constant
T = absolute temperature of cell in Kelvin
F = Faraday constant
P1 = partial pressure of oxygen in the reference gas
(air in most cases)
P2 = partial pressure of oxygen in the sample

So negative voltages imply the partial pressure of oxygen in the exhaust stream exceeds that of the air at the sensor reference port.
 
M

miso

Jan 1, 1970
0
I have been testing a Bosch automotive exhaust gas oxygen sensor of the
zirconium dioxide type, which I suspect is faulty. There are four
connections to it: two wires to a floating heater element, which require
12v of no particular polarity; one sensor output wire and one sensor
earthing connection to the body of the device.

The heater correctly takes about 1.7A when cold, falling below 1A as it
warms up. The sensor is supposed to give readings between +0.2V and
+0.8v when exposed to exhaust gases, the voltage becoming more positive
with decreasing oxygen level. Instead, it gives readings between -0.8v
and -0.2v, with the voltage becoming more positive with decreasing
oxygen levels. In other words, the readings are behaving correctly but
are displaced about 1.0v negative.

According to various websites, this is typical behaviour for a sensor
which has become contaminated with silicone residue, and the obvious
solution is to replace it. However, I can find no source for such a
contamination and some websites imply that the contamination might be on
the reference surface of the zirconium oxide 'thimble', not on the side
exposed to the exhaust gasses.

My question is: does anyone know which side of the sensor is the
contaminated one when the voltage is displaced negative?

I realise this is not exactly a 'design' problem, but I think that I
shall only get a proper answer, as opposed to hearsay and guesswork, by
asking designers who have actually worked with such devices.



[I have already eliminated bad earthing contact between the exhaust
system and the engine, which is the commonest cause of a spurious
voltage readings. The vehicle is not consuming large quantities of
coolant, so anti-freeze contamination is not very likely. To the best
of my knowledge no repairs have been done with silicone sealants]

Just a FYI. If this problem is causing too rich of a mixture, just
replace the parts before they ruin the catalytic converter. I've seen
this in defective mass air flow sensors. i don't know how this O2 defect
will effect the fuel mixture. [Well, I rather not venture a guess since
I'd hate to be the person who said it caused the mix to be lean, which
is just a power issue, versus too rich which kills the catalytic converter.]

Basically raw fuel makes the catalytic converter run very hot.
 
A

Adrian Tuddenham

Jan 1, 1970
0
I have been testing a Bosch automotive exhaust gas oxygen sensor... [...]
My question is: does anyone know which side of the sensor is the
contaminated one when the voltage is displaced negative?
[..]

The theoretical output voltage is:
Cell output = (2.303RT/4F)x log ( P1/P2)

where:-
R = molar gas constant
T = absolute temperature of cell in Kelvin
F = Faraday constant
P1 = partial pressure of oxygen in the reference gas
(air in most cases)
P2 = partial pressure of oxygen in the sample

So negative voltages imply the partial pressure of oxygen in the exhaust
stream exceeds that of the air at the sensor reference port.

I had wondered if the reversed reading was caused by an exhaust leak
causing gasses to stream past the reference port of the sensor, but
there is no sign of a leak near of the sensor. (There is a slight leak a
few feet away in the back box.)

The negative reading continues with the engine switched off and the
sensor heater sustained by an external battery, so it seems as though
either the sensor port is blocked and contaminated or the sensor element
is poisoned. As the sensor has only been in use about 18 months, I
suspect the latter, but I will try flushing out the port with clean
Iso-propanol as a precaution.
 
A

Adrian Tuddenham

Jan 1, 1970
0
miso said:
I have been testing a Bosch automotive exhaust gas oxygen sensor [...]

Just a FYI. If this problem is causing too rich of a mixture, just
replace the parts before they ruin the catalytic converter. I've seen
this in defective mass air flow sensors. i don't know how this O2 defect
will effect the fuel mixture. [Well, I rather not venture a guess since
I'd hate to be the person who said it caused the mix to be lean, which
is just a power issue, versus too rich which kills the catalytic converter.]

Basically raw fuel makes the catalytic converter run very hot.

The mixture has been all over the place, with prolonged bouts of power
loss interspersed with intermittent surges. This only occurred after
driving for a mile or two, which is what made me suspect that it was
something to do with the fuel control system switching to 'closed-loop'
operation. Surprisingly there has been no visible smoke in the exhaust
gasses.

On occasions the exhaust gasses have seemed exceptionally hot, as far as
I could tell by placing my hand in the gas flow behind the tailpipe. On
other occasions they have seemed normal. The hottest (almost at flame
heat) occurred during idling immediately after a severe bout of
misfiring, so I suspect that some damage may have already taken place.

I can only estimate the temperature during idling, as my simple
temperature testing method obviously cannot be used whilst driving.
 
On 11/25/2012 12:51 PM, Adrian Tuddenham wrote:
I have been testing a Bosch automotive exhaust gas oxygen sensor
[...]

Just a FYI. If this problem is causing too rich of a mixture, just
replace the parts before they ruin the catalytic converter. I've seen
this in defective mass air flow sensors. i don't know how this O2 defect
will effect the fuel mixture. [Well, I rather not venture a guess since
I'd hate to be the person who said it caused the mix to be lean, which
is just a power issue, versus too rich which kills the catalytic converter.]
Basically raw fuel makes the catalytic converter run very hot.



The mixture has been all over the place, with prolonged bouts of power

loss interspersed with intermittent surges. This only occurred after

driving for a mile or two, which is what made me suspect that it was

something to do with the fuel control system switching to 'closed-loop'

operation. Surprisingly there has been no visible smoke in the exhaust

gasses.



On occasions the exhaust gasses have seemed exceptionally hot, as far as

I could tell by placing my hand in the gas flow behind the tailpipe. On

other occasions they have seemed normal. The hottest (almost at flame

heat) occurred during idling immediately after a severe bout of

misfiring, so I suspect that some damage may have already taken place.



I can only estimate the temperature during idling, as my simple

temperature testing method obviously cannot be used whilst driving.



--

~ Adrian Tuddenham ~

(Remove the ".invalid"s and add ".co.uk" to reply)

www.poppyrecords.co.uk

From what I can gather from online diagnostics manuals, most fuel controllers are aiming for +0.45V average out of the O2S, with the output swinging from +0.2 to 0.8V somewhere in the range of 0.5-5Hz rate. Excursions into negative output voltages are okay as long as the time average comes to +0.45V.. And you're right, it is the excessively hot exhaust that damages the converter.
 
M

miso

Jan 1, 1970
0
You really need the shop manuals for voltages. I have 4 O2 sensors, and
not all have the same voltage range.

Frying the catalytic converter is far more expensive than taking it to a
garage. This is one of those no win situations, where you pick the
lesser of two evils. Kind of like voting in a Republican primary. ;-)

This reminds me of the old radio show Car Talk, where the callers had
put black tape over the check engine light for a year before getting
around to fixing the problem.
 
A

Adrian Tuddenham

Jan 1, 1970
0
miso said:
You really need the shop manuals for voltages. I have 4 O2 sensors, and
not all have the same voltage range.

There are two basic types: a voltage-generating system base on Zirconium
and a variable resistance system based on Titanium. I have managed to
establish that this is a Zirconium-based sensor which means that the
voltages are determined entirely by the electrochemistry of the process
(see other replies to this thread).

Some vehicles have multiple sensors at different places in the exhaust
system and I suspect that their readings would differ according to what
they were sensing, which would account for your observations. However,
no Zirconium-based sensors are supposed to give negative readings, no
matter what the exhaust conditions, which is what mine has been doing.

Frying the catalytic converter is far more expensive than taking it to a
garage. This is one of those no win situations, where you pick the
lesser of two evils....

Luckily my vehicle was made in a year where a catalytic convertor was
not mandatory and the emissions limits fairly lax. As long as it passes
the appropriate emisssions test for a vehicle of that age, I may be able
to get away without a functioning convertor.

This reminds me of the old radio show Car Talk, where the callers had
put black tape over the check engine light for a year before getting
around to fixing the problem.

There would be a lot less of that sort of thing if the manufacturers
provided proper diagnostics which were easy to use without specialist
equipment. When pintable [pinball] machines first started using
microprocessors, there was a simple built-in diagnostic system which
allowed the mechanic to read the signal from every sensor and to
test-operate every solenoid. With the elaborate dashboard displays in
modern cars, a similar system could be added for the cost of a couple of
switches and a bit of software.
 
A

Adrian Tuddenham

Jan 1, 1970
0
miso said:
I have been testing a Bosch automotive exhaust gas oxygen sensor of the
zirconium dioxide type, which I suspect is faulty. There are four
connections to it: two wires to a floating heater element, which require
12v of no particular polarity; one sensor output wire and one sensor
earthing connection to the body of the device.

The heater correctly takes about 1.7A when cold, falling below 1A as it
warms up. The sensor is supposed to give readings between +0.2V and
+0.8v when exposed to exhaust gases, the voltage becoming more positive
with decreasing oxygen level. Instead, it gives readings between -0.8v
and -0.2v, with the voltage becoming more positive with decreasing
oxygen levels. In other words, the readings are behaving correctly but
are displaced about 1.0v negative.

According to various websites, this is typical behaviour for a sensor
which has become contaminated with silicone residue, and the obvious
solution is to replace it. However, I can find no source for such a
contamination and some websites imply that the contamination might be on
the reference surface of the zirconium oxide 'thimble', not on the side
exposed to the exhaust gasses.

My question is: does anyone know which side of the sensor is the
contaminated one when the voltage is displaced negative?

I realise this is not exactly a 'design' problem, but I think that I
shall only get a proper answer, as opposed to hearsay and guesswork, by
asking designers who have actually worked with such devices.



[I have already eliminated bad earthing contact between the exhaust
system and the engine, which is the commonest cause of a spurious
voltage readings. The vehicle is not consuming large quantities of
coolant, so anti-freeze contamination is not very likely. To the best
of my knowledge no repairs have been done with silicone sealants]

Just a FYI. If this problem is causing too rich of a mixture, just
replace the parts before they ruin the catalytic converter. I've seen
this in defective mass air flow sensors. i don't know how this O2 defect
will effect the fuel mixture. [Well, I rather not venture a guess since
I'd hate to be the person who said it caused the mix to be lean, which
is just a power issue, versus too rich which kills the catalytic converter.]

Basically raw fuel makes the catalytic converter run very hot.


Many thanks for the advice. I think I have now tracked down most of the
causes of the problem. The sequence of events seems to have been:

1) The fuel filter was choked, also the pump may have been worn out.
This meant that, on heavy fuel demand such as climbing a hill, the
pressure at the injectors fell.

2) The engine management system compensated for the reduced fuel
pressure by lengthening the injection period. When the throttle was
shut at the top of the hill, the fuel pressure rose quickly and the
engine flooded.

3) The extra fuel burned off in the exhaust system and damaged the
oxygen sensor. Also, the engine management system either went off-scale
or got in a tangle, which caused it to lose track of what was really
going on.

Replacing the fuel filter, fuel pump and oxygen sensor has put things
right but revealed a trivial underlying fault, which must have been
confusing the situation even more: the throttle stop switch was oily and
only made intermittent contact.

Once again, thanks to all contributors on this and other threads for
helping to save the vehicle from the scrapyard (for now, at least).
 
miso said:
I have been testing a Bosch automotive exhaust gas oxygen sensor of the
zirconium dioxide type, which I suspect is faulty.  There are four
connections to it: two wires to a floating heater element, which require
12v of no particular polarity; one sensor output wire and one sensor
earthing connection to the body of the device.
The heater correctly takes about 1.7A when cold, falling below 1A as it
warms up.  The sensor is supposed to give readings between +0.2V and
+0.8v when exposed to exhaust gases, the voltage becoming more positive
with decreasing oxygen level.  Instead, it gives readings between -0.8v
and -0.2v, with the voltage becoming more positive with decreasing
oxygen levels.  In other words, the readings are behaving correctlybut
are displaced about 1.0v negative.
According to various websites, this is typical behaviour for a sensor
which has become contaminated with silicone residue, and the obvious
solution is to replace it.  However, I can find no source for such a
contamination and some websites imply that the contamination might beon
the reference surface of the zirconium oxide 'thimble', not on the side
exposed to the exhaust gasses.
My question is: does anyone know which side of the sensor is the
contaminated one when the voltage is displaced negative?
I realise this is not exactly a 'design' problem, but I think that I
shall only get a proper answer, as opposed to hearsay and guesswork, by
asking designers who have actually worked with such devices.
[I have already eliminated bad earthing contact between the exhaust
system and the engine, which is the commonest cause of a spurious
voltage readings.  The vehicle is not consuming large quantities of
coolant, so anti-freeze contamination is not very likely.  To the best
of my knowledge no repairs have been done with silicone sealants]
Just a FYI. If this problem is causing too rich of a mixture, just
replace the parts before they ruin the catalytic converter. I've seen
this in defective mass air flow sensors. i don't know how this O2 defect
will effect the fuel mixture. [Well, I rather not venture a guess since
I'd hate to be the person who said it caused the mix to be lean, which
is just a power issue, versus too rich which kills the catalytic converter.]
Basically raw fuel makes the catalytic converter run very hot.

Many thanks for the advice.  I think I have now tracked down most of the
causes of the problem.  The sequence of events seems to have been:

1)  The fuel filter was choked, also the pump may have been worn out.
This meant that, on heavy fuel demand such as climbing a hill, the
pressure at the injectors fell.

2) The engine management system compensated for the reduced fuel
pressure by lengthening the injection period.  When the throttle was
shut at the top of the hill, the fuel pressure rose quickly and the
engine flooded.

does it even measure fuel pressure? didn't think many petrol cars did
they just have a mechanical pressure regulator recirculating fuel to
the
tank

and I don't think it would explain flooding, when you shut the
trottle
the injector period go to zero almost instantly, low or high fuel
pressure
the engine would floor with full on injectors and no air
3)  The extra fuel burned off in the exhaust system and damaged the
oxygen sensor.  Also, the engine management system either went off-scale
or got in a tangle, which caused it to lose track of what was really
going on.

Generally the ECU won't adjust more than a limited percentage based
on
the sensor, it can run reasonably in open loop

sometime sensors just go bad, wear out or what ever, every second year
or so
one of the two on my car needs replacement
Replacing the fuel filter, fuel pump and oxygen sensor has put things
right but revealed a trivial underlying fault, which must have been
confusing the situation even more: the throttle stop switch was oily and
only made intermittent contact.

Once again, thanks to all contributors on this and other threads for
helping to save the vehicle from the scrapyard (for now, at least).

-Lasse
 
A

Adrian Tuddenham

Jan 1, 1970
0
[...]
2) The engine management system compensated for the reduced fuel
pressure by lengthening the injection period.  When the throttle was
shut at the top of the hill, the fuel pressure rose quickly and the
engine flooded.

does it even measure fuel pressure? didn't think many petrol cars did
they just have a mechanical pressure regulator recirculating fuel to
the
tank

It does have a mechanical pressure regulator. Perhaps I didn't explain
myself very well. I think the fuel pressure drop during acceleration
caused a weak mixture which the oxygen sensor picked up. The ECU then
attempted to compensate for that by lengthening the injector period.

and I don't think it would explain flooding, when you shut the
trottle
the injector period go to zero almost instantly, low or high fuel
pressure
the engine would floor with full on injectors and no air

The engine was cutting out and giving intermittent low power, but the
inertia of the car kept it turning. The faulty throttle-stop switch
meant that the air by-pass valve was fully open a lot of the time
anyway, so there was always some air coming through. Once the injectors
reacted and stopped flooding, the engine would return to a fast idling
condition which allowed me time to change gear and to keep the vehicle
moving.

Drivers behind me were often annoyed.

Generally the ECU won't adjust more than a limited percentage based
on
the sensor, it can run reasonably in open loop

It ran much better open-loop when I eventually disconnected the oxygen
sensor altogether.
sometime sensors just go bad, wear out or what ever, every second year
or so
one of the two on my car needs replacement

I will keep an eye on things in future, and treat even
recently-replaced items with suspicion.
 
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