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#791677 - 12/19/06 08:19 AM Benefits of Changing O2 Sensors>?
ULVER Offline


Registered: 09/21/04
Posts: 502
Loc: Atlanta, Ga.
Just curious...

I have close to 165K on a Ford Crown Vic, which I took out of service at 78K. I know for sure, since I have had the vehicle, the O2 sensors have never been changed.

Is there any immediate/obvious positives I could expect, if I change them now?

Any thoughts appreciated muchly!
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Men and their motor oil, are as bad as women and their shoes.

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#791678 - 12/19/06 08:38 AM Re: Benefits of Changing O2 Sensors>? [Re: ULVER]
Quest Offline


Registered: 12/19/04
Posts: 6551
Loc: beaver land EH?
If your O2 sensor is rendered ineffective due to poisoning (very common due to various additives, such as Manganese, MMT, etc.) replacing it with a new one shall restore your car's usual responsiveness, peak fuel economy, proper combustion efficiency (more precise fuel control during close loop, etc.) and reduced emissions.

If your O2 is simply lazy, you shall be able to get back some of your fuel efficiency by have a much more responsive O2 sensor (quicker in entering the close loop).

If your O2 is still functional, replacing it with a new one does little....

Hope that helps.
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#791679 - 12/19/06 09:08 AM Re: Benefits of Changing O2 Sensors>? [Re: Quest]
Axe Man Offline


Registered: 10/17/05
Posts: 355
Loc: Highland, NY
If your Ford is 1996 or newer my suggestion is to leave the O2 sensors alone. The OBD-II system in 1996 and newer vehicles monitor the O2 sensors very closely. If it gets "lazy" or fails outright the ECU will quickly and persistently notify you with a "check engine" light.

1995 and older is a different story. "Lazy" O2 sensors typically won't throw a check engine light until they totally #@$%! out. So there could be a benefit of preventative replacement.

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#791680 - 12/19/06 05:17 PM Re: Benefits of Changing O2 Sensors>? [Re: Axe Man]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Change them if you are going to keep the vehicle. Those high miles are really stretching it.
The rear O2 sensor only monitors that the front one and cat is working. It is not that precise of a system.
It will not be worse , but only better with the new ones in there. How much? Who knows?

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#791681 - 12/19/06 05:58 PM Re: Benefits of Changing O2 Sensors>? [Re: mechtech2]
swalve Offline


Registered: 06/03/05
Posts: 588
Loc: Chicago, IL
The computer can only tell if the O2 is out of range or fails to change. But if it's just wrong, but still functional, it can't show a code.

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#791682 - 12/19/06 06:47 PM Re: Benefits of Changing O2 Sensors>? [Re: swalve]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
The O2 sensor degrades in a linear manner.

The check engine light is a digital (binary) device.

See the problem here?


Edited by brianl703 (12/19/06 06:48 PM)

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#791683 - 12/19/06 09:46 PM Re: Benefits of Changing O2 Sensors>? [Re: brianl703]
andyd Offline


Registered: 09/25/04
Posts: 3897
Loc: Marshfield , MA
They can be tested with a meter. Google up Bosch O2 sensors.
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#791684 - 12/19/06 10:15 PM Re: Benefits of Changing O2 Sensors>? [Re: andyd]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
You really need a digital multimeter with a bargraph fuction if you're going to measure the output of the O2 sensor in-car to see how fast it switches. Analog meters don't have a high enough impedance unless they're one of those electronic ones (either a vacuum tube voltmeter or the solid-state equivalent).

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#791685 - 12/19/06 10:23 PM Re: Benefits of Changing O2 Sensors>? [Re: brianl703]
Quickbeam Offline


Registered: 08/27/06
Posts: 300
Loc: Out there somewhere
Quote:

The O2 sensor degrades in a linear manner.

The check engine light is a digital (binary) device.

See the problem here?




Well, having an A/D converter followed by digital processing in between them makes that point sort of moot


O2 sensors are easily one of the most needlessly changed parts on vehicles however if you have a decent meter (a scope would be better) you can test them:

1) A good sensor will cross count a minimum of 8 times in 10 seconds and reach at least 200 mv on the lean side and 800 mv on the rich side while doing it. The midpoint should be right around 450 mv which is stoichiometric.

2) A good sensor will respond within 100 ms when driven from lean to rich or vice versa.

3) You can check them off the engine with a propane torch but it's much easier check them on the engine.

4) If it's OBD-II what Axe Man said.

All that said it wouldn't hurt to change it as it's no doubt near the end of it's useful life.
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#791686 - 12/19/06 11:40 PM Re: Benefits of Changing O2 Sensors>? [Re: Quickbeam]
eljefino Offline


Registered: 06/15/03
Posts: 24663
Loc: ME
The post-cat o2 sensors only monitor the cats and do nothing for performance or economy. I would only replace those if the car threw a code relating to 'em.

Front ones, what others have said. Just replaced one on my saturn with a bosch universal and the "bog" is gone.

Decide if you want universal (wiring splicing required) or OE fit then shop around... Find a Ford forum to see how well different brands do and if you can get away with a universal one.

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#791687 - 12/20/06 12:28 AM Re: Benefits of Changing O2 Sensors>? [Re: eljefino]
ULVER Offline


Registered: 09/21/04
Posts: 502
Loc: Atlanta, Ga.
Thanks guys... I think... A lot of this is over my head. I do know a couple things:
CEL comes and goes a LOT, but this became sooooo common in some cruisers, the shop just put black electrical tape over the light.

My MPG seem low, even for a big car...

Anyway, I know from my histroy alone, they have not been done in 80K.

Couldn't hurt I would think...

FYI: The 94's only had ONE cat sensor a side...
_________________________
Men and their motor oil, are as bad as women and their shoes.

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#791688 - 12/20/06 12:55 AM Re: Benefits of Changing O2 Sensors>? [Re: Quickbeam]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
Quote:

Well, having an A/D converter followed by digital processing in between them makes that point sort of moot




Not really. The O2 sensor has already started to degrade long before that light comes on.

Analogy: It's a similar situation with the SMART feature that hard drives have. Yes, SMART will tell you when the drive is nearing failure. But did you know that with the proper software you can actually watch the parameters that are used to determine when the drive is going to fail start counting down from the first time you power it on? I replaced the hard drive in my laptop because one of the SMART values was very close to the failure threshold. I suppose I could've waited until SMART told me the drive has reached the failure threshold, but what if that happened when I was on a trip or something?

And so it is with oxygen sensors. Of course, the failure of an oxygen sensor isn't going to destroy all of your data and render your car unusable.



Edited by brianl703 (12/20/06 01:07 AM)

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#791689 - 12/20/06 04:32 AM Re: Benefits of Changing O2 Sensors>? [Re: brianl703]
Quickbeam Offline


Registered: 08/27/06
Posts: 300
Loc: Out there somewhere
If his car is OBD-II there is no way the O2 sensor could not be doing it's job and the MIL be off. For a computer type you don't seem to have much faith in the power of code. Nor do you seem aware of how precise OBD II monitoring of lambda is. As Axe Man and others pointed out there is nothing a lambda signal can do that isn't monitored by OBD-II with at least 12 bit precision on the input and a complex algorithm after that. And unlike your hard drive diags the sensitivity of this monitoring is EPA mandated.

The algorithm is left up to the automaker but what it takes to make the MIL come on is standardized and quite tight. Even in one of my older cars (a 1987 import OBD-I system) there isn't a situation in which the sensor isn't monitored for every form of signal fault that would result in emissions exceeding 1.5 times the federal limit, which is what the sensor is there for to begin with. Remember, lambda sensors are not installed to make the engine run "well", they exist solely to serve the catalyst. It's their only purpose in life.

While it's true the sensor will start to degrade from the day it's installed as long as the MIL stays off the sensor is doing it's job because the monitoring code is written specifically to detect otherwise. Since there are a very limited number of things a lambda signal can do in total and even less when operating correctly it's hardly rocket science to precisely monitor it. It's as simple as that.

Now if his engine is pre-OBD-II I'd tend to agree with you because as a rule early domestic OBD was inferior compared to most imports. However even then the MIL should come on well before the sensor degraded to the point where drivabilty would suffer because by then you'd be well past 1.5 times the federal limt.
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Thousands on women and cars. The rest I squandered

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#791690 - 12/20/06 04:49 AM Re: Benefits of Changing O2 Sensors>? [Re: Quickbeam]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
I recall a problem with some Honda OBD-II vehicles where they would not turn on the check engine light when they were supposed to.

I believe this resulted in a recall, and a 150,000 mile
extension of the warranty on the O2 sensors.

Fact is, code is only as good as the ethics and competence of the person who is writing it. (Which of the two was lacking in the Honda case?). EPA regulations, in fact any laws, are only as good as the ability and willingness to enforce it.


As far as domestic OBD goes, both GM and Ford had quite sophisticated systems. GM had a data stream output long before anyone knew what OBD-II is, and Ford's EEC-IV had extensive self-test diagnostics. As one example,, compare the number of possible codes for a typical 1987 Ford EEC-IV system to the number of possible codes for a typical 1987 import system. Then remember there are three types of EEC-IV codes--memory, key on engine running, and key on engine off and that a single code could mean three different things depending upon what type of code it is.

As a final note, I will point out that long-term fuel trim on OBD-II is allowed to go as much as 25% positive or 25% negative before the light comes on and an error code is stored. I think you ought to start being concerned about the problem at 10%, and in fact I became aware of the presence of an otherwise asymptomatic vacuum leak when I looked at the long-term fuel trim on my old car--it was at about 22% correction, as I recall.

Quote:

Remember, lambda sensors are not installed to make the engine run "well", they exist solely to serve the catalyst. It's their only purpose in life.




If it were not for the oxygen sensors, that vacuum leak I described above WOULD have made the engine run like #@$%!. (It has a MAF sensor, not a MAP sensor).

Quote:

Even in one of my older cars (a 1987 import OBD-I system) there isn't a situation in which the sensor isn't monitored for every form of signal fault that would result in emissions exceeding 1.5 times the federal limit




So how much extra fuel is a car using when it is spewing out 1.45 times the federal limit in emissions compared to a car that's only spewing out the federal limit, anyway? (And what's that doing to the cats?)


Edited by brianl703 (12/20/06 05:08 AM)

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#791691 - 12/20/06 05:04 AM Re: Benefits of Changing O2 Sensors>? [Re: brianl703]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
My general rule for replacing heated O2 sensors is every 100K or when O2 sensor related code is generated, whichever comes first.

For unheated sensors--I've never had a car without a heated sensor so I have no rule for those.

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