[v12-engine] Re: [xj-s] O2 sensor output

V12-Engine
1

WARNING: DUMB QUESTION ALERT

The best kind!

If an oxygen sensor senses oxygen (its off to a good start, eh?)

Nope. Actually, oxygen sensors sense the LACK of oxygen. They read
0.9V when there’s no oxygen in the exhaust, 0.1V when there is oxygen
in the exhaust.

and the sensors output varies from 0.1 Volt (lots) to 0.9 Volt (none)

Ah, so you knew it sensed lack of oxygen already!

then what reading is the ECU going for? (Attempting to adjust to
attain)

The ECU is constantly looking for the OTHER reading. If it’s reading
0.9, it’s trying to get to 0.1. If it’s reading 0.1, it’s trying to
get to 0.9. Hence, it’s trying to surf the break point between the
two.

Seems to me that if the car were perfect, there wouldn’t be any excess
oxygen (it would have been all used in combustion)…

Yeah, but you could get there by just pouring gobs of fuel to it –
which would be bad for many reasons. You want to burn all that
oxygen by providing JUST ENOUGH fuel to do the job. So, when the
sensor says you have enough fuel in there, the ECU starts cutting it
back until there ISN’T enough in there – then it starts adding
again. It’s a neurotic little control system.

I was expecting a midrange value of 0.5 Volts so the very high 0.87
Volt reading I get at hot or cold idle has me spooked. Should I be?

Richard Drozdowski
1992 XJS- 4.0L

Means it’s running rich. I have no idea what it should be running,
though.

PS. This occurred to me on the way to work this morning.
One of the secrets of asking dumb questions is to ask fast, before too
much thought can occur.

Good philosophy.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979From: Rick Drozdowski sssedm@compusmart.ab.ca

2

Kirbert wrote:

… So, when the
sensor says you have enough fuel in there, the ECU starts cutting it
back until there ISN’T enough in there – then it starts adding
again. It’s a neurotic little control system.

Bang-bang, but ‘nuerotic’ works too.

Walter

3

On my car which has an after market ECU - with an O2 sensor, it exhibits the following characteristics…

If the car is in open loop, then the O2 sensor reports .5-.6 volt for low to medium throttle settings and .86 volt at high throttle setting.

I am able to set different target settings for the car to aim towards with regard to the O2 sensor. So the settings are .6 volt at idle (about 1% CO) , .45 volt at light-medium throttle .70 volt at 3/4 throttle and .86 for WOT (Wide Open Throttle).

When the car goes into closed loop, at idle the voltage varies from .55-.65 volt. (so a nice smooth idle!) at cruise the voltage swings from .1-.6 (this is generally only a change in 1% fuel flow). at 3/4 throttle the voltage is .65-.75 and at WOT it records .84-.88.

As becomes apparent, it is easier for the ECU in the car to tune itself to richer conditions. What I have also noticed is the differences cooler temperatures make (to my car anyway) On a nice cold rainy day (in summer!), the ECU is able to lock onto the target settings a lot more accurately - for example: at cruise the voltage is more likely to be .3-.55. - This may or may not be a good thing…

Scott
1976 6.5 liter V12 XJ-S

“Kirbert” palmk@nettally.com 01/20/00 11:23 >>>

WARNING: DUMB QUESTION ALERT

The best kind!

If an oxygen sensor senses oxygen (its off to a good start, eh?)

Nope. Actually, oxygen sensors sense the LACK of oxygen. They read
0.9V when there’s no oxygen in the exhaust, 0.1V when there is oxygen
in the exhaust.

and the sensors output varies from 0.1 Volt (lots) to 0.9 Volt (none)

Ah, so you knew it sensed lack of oxygen already!

then what reading is the ECU going for? (Attempting to adjust to
attain)

The ECU is constantly looking for the OTHER reading. If it’s reading
0.9, it’s trying to get to 0.1. If it’s reading 0.1, it’s trying to
get to 0.9. Hence, it’s trying to surf the break point between the
two.

Seems to me that if the car were perfect, there wouldn’t be any excess
oxygen (it would have been all used in combustion)…

Yeah, but you could get there by just pouring gobs of fuel to it –
which would be bad for many reasons. You want to burn all that
oxygen by providing JUST ENOUGH fuel to do the job. So, when the
sensor says you have enough fuel in there, the ECU starts cutting it
back until there ISN’T enough in there – then it starts adding
again. It’s a neurotic little control system.

I was expecting a midrange value of 0.5 Volts so the very high 0.87
Volt reading I get at hot or cold idle has me spooked. Should I be?

Richard Drozdowski
1992 XJS- 4.0L

Means it’s running rich. I have no idea what it should be running,
though.

PS. This occurred to me on the way to work this morning.
One of the secrets of asking dumb questions is to ask fast, before too
much thought can occur.

Good philosophy.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979From: Rick Drozdowski sssedm@compusmart.ab.ca