Testing CO Level Using O2 Sensor

[LifterEyes]

Hey guys,

I just read FR Wilk's article on testing the engine's CO Level using the oxygen sensor. I don't have a CO tester and was just wondering if this is an accurate enough way of testing the CO level? Also, does the DME sense that the sensor is disconnected and automatically go into closed loop mode? If it doesn't, wouldn't it just cause the DME to think the engine is running too lean and effect the CO level that you are trying to measure by disconnecting the sensor in the first place?

[krystar]

u mean open loop. open loop is when it ignores the sensor. closed loop is when there is a sensor.

[HondaDustR]

The oxygen sensor doesn't measure CO directly as far as I know. It measures excess oxygen, which indicates the presence of complete combustion or not. With the correct 14.7:1 air:fuel ratio, there shouldn't be much CO. CO should go down with a lean mxture, but then NOx will go up. CO and HC are the result of too rich a mixture, so yeah, you could get it pretty close if the O2 sensor is in good shape. Just get it to read 0.5v on a voltmeter. You can't take too long or heat soaking will change the mixture characteristics and throw off the reading from what it would be under normal driving. If it's a heated sensor, be sure to jumper the supply wires for the heater (two white ones, polarity doesn't matter), or the sensor will stop reading accurately at idle once unplugged. If it's a non-heated sensor, you have to rev the engine to heat it up enough to get an accurate reading at idle, since at idle, the exhaust is too cold for optimum sensor operating temp. The goal of adjusting is to get it close on it's own so the DME doesn't have to correct much. Don't mix up the wires when jumpering the heater, like shorting them to ground, to each other or to the signal wire or you could burn something out in the DME.

[LifterEyes]

Originally Posted by HondaDustR

The oxygen sensor doesn't measure CO directly as far as I know. It measures excess oxygen, which indicates the presence of complete combustion or not. With the correct 14.7:1 air:fuel ratio, there shouldn't be much CO. CO should go down with a lean mxture, but then NOx will go up. CO and HC are the result of too rich a mixture, so yeah, you could get it pretty close if the O2 sensor is in good shape. Just get it to read 0.5v on a voltmeter. You can't take too long or heat soaking will change the mixture characteristics and throw off the reading from what it would be under normal driving. If it's a heated sensor, be sure to jumper the supply wires for the heater (two white ones, polarity doesn't matter), or the sensor will stop reading accurately at idle once unplugged. If it's a non-heated sensor, you have to rev the engine to heat it up enough to get an accurate reading at idle, since at idle, the exhaust is too cold for optimum sensor operating temp. The goal of adjusting is to get it close on it's own so the DME doesn't have to correct much. Don't mix up the wires when jumpering the heater, like shorting them to ground, to each other or to the signal wire or you could burn something out in the DME.

Thanks HondaDustR! So, from what you said, this procedure seems to get the CO level close enough and the DME will correct for any small discrepancies.

With that in mind, how much can the CO adjustment have an effect on Idle quality? Is it pretty dramatic?

[HondaDustR]

What you are adjusting is the AFM equivalent of the idle air bypass adjustment screw on the throttle body. It allows some air through without being metered by the AFM, enabling you to richen or lean out the idle mixture. It's indirectly chemically adjusting CO along with everything else related, rather than directly. I imagine it will also affect the low end and low throttle possitions somewhat as well, but to a decreasing extent, most likely exponentially, as airflow increases. From there, it's just the rich/lean scenario as explained above. You want to set it so you get as close to stoichiometric ratio of 14.7:1 to minimize emissions on it's own. The O2 sensor will then only need to compensate when it's really cold, hot, etc.

I have found a rich idle is better than a lean idle as far as idle quality goes, and a stoichiometric idle is pretty much just as good. I also found a closed loop idle that is jumping around to maintain 0.5v from the sensor is a bit rougher than an open loop idle with the correct mixture. I also noticed that the closed loop idle (ie O2 sensor plugged in) when the open loop idle was really lean was noticably more unstable than when the open loop idle was really rich (done with FQS set to -3% and +3% fuel respectively). I did some testing with the O2 sensor signals and FQS a while back. I've never messed with the air bypass valve on the AFM, but it probably does need adjusting. I haven't had any sigificant enough problems with it and it passed emissions, so...
The only thing is, I've heard often times the o-ring is in bad shape and will start leaking vacuum, so once you start messing with it, you can't get a consistent setting unless you replace the o-ring. I have no idea where to get a new one from. It's not a separate part in PET.

[LifterEyes]

Thanks again. It seems like my engine will run fine at anything above 1200 RPM then anything below that and it starts to get rougher as is gets down to idle. If the AFM bypass valve (CO Level adjustment) is the culprit my theory is that, below that 1200 rpm point, the DME can no longer compensate for the bypass valve being out or range and things get rough. Or so I hope... Haha! Besides, It wont hurt making sure everything is dialed in correctly.

Also, would anyone happen to know where to get a replacement o-ring for the AFM adjusting screw?

[HondaDustR]

Yeah, once the engine approaches idle, the ratio of unmetered air through the bypass to metered air through the AFM increases quite a bit. If it's set wrong, it will idle badly if it's far enough off. Higher airflow levels will drown out the effect of the bypass setting, since it's a relatively small passageway, which would explain improved running from 1200 onward. Keep in mind it's essentially an adjustable vacuum leak, so be sure there's no real vacuum leaks lurking around somewhere first that might be throwing it off.

[LifterEyes]

Just a few more questions. What is the best way to do this procedure? You said you can't take too long to make the adjustment because heat soak will change the mixture characteristics so I'm assuming start with a cold engine. Will it make much of a difference if the ambient temperature outside is very cold? The car already runs horribly when it's very cold outside, I'm assuming because of the aforementioned issue, I am wondering if the winter weather will effect the mixture characteristics in the same way that heat soak would and I should wait for warmer weather before I go messing with that.

Thanks again!

[HondaDustR]

No, it definitely needs to be at normal operating temp. By heat soaking, I mean letting it sit and idle for like 10 minutes or longer after it has reached operating temp. Sitting and idling heats up the intake manifold and fuel rail/injectors more than normal driving, since normal driving flows more air through and around the intake which keeps it and the air inside of it cooler. Hotter air will throw off the mixture since it is less dense and atomizes the fuel differently, especially since the air temp sensor is before the intake manifold. Somebody a while back actually found that polishing the outside of the intake smooth actually increased intake temps a couple degrees, so even the texture helps keep it a little cooler. The engine temp fluctuates as well with the temps climbing high before the fans kick on. If you can't get it set within like 5 minutes, just drive around to settle everything back to normal and try some more. In fact it would be best to warm it up by driving rather than idling in the driveway, since it will keep the intake at normal temp.

[LifterEyes]

Today I did a quick little test and discovered that when I read the voltage coming off of the O2 sensor the meter would start at zero then slowly rise. Just the sensor heating up I'm assuming? It would continue to rise until I got a reading of .80 volts. I tried disconnecting the vacuum line from the fuel pressure regulator then tried creating a vacuum leak into the intake to see If the voltage would change and it remained steady at .80 volts. The only time I would observe the voltage change was if i revved up the engine and at the time I released the throttle the voltage would drop to near zero then come back to .80 once the engine reached idle. Does this sound like the sensor may be operating properly but the engine is running too rich to observe any changes on the sensor other then the fuel cut off when i release the throttle? Or does it sound more like the sensor needs to be replaced?

[HondaDustR]

Well, how old is the sensor, or how many miles on it? Going to 0 or near 0 when the engine is decelerating is normal, since the closed throttle switch cuts fuel above a certain rpm as you know, something like 1500 or so. The narrow band oxygen sensor that most emissions systems are equiped with are pretty sensitive right near 14.7:1 air fuel ratio, but their response drops off pretty significantly much above or below that.

Here's a typical response curve.


0.8v is still in the range where you should see some variation if you change the mixture somehow. I saw my sensor respond pretty definitively from 0.1 to 0.9v and it was very quick to respond to changes in mixture. Does it still read 0.8v when you hold the rpm at say 2000? Try setting the FQS to position 2 and see if it's different. That will subtract 3% fuel amount across the board on the fuel maps and should definitely make a difference if the sensor is working correctly. One quick and dirty way to drastically change the mixture is to unplug a fuel injector. I rigged the meter up to mine so that I could see it while driving so I could observe how the mixture changed under different driving conditions. If it just hangs around 0.8v, the sensor may be done, but I always thought sensors failed on the low side creating a rich condition when the DME falsely compensates. Try taking it out and driving hard to really heat up the exhaust and test it again just in case it wasn't hot enough.

[LifterEyes]

Well, I have no record of how old the sensor is so it may be a good time to replace it anyway. The sensor seemed to read .8V at all times, even if i raised the rpm to about 2,000 Rpm, and the voltage would only change at all when I would release the throttle. This is when it would drop to zero the raise back up to .8v once it reached idle. But giving it gas or holding it at the same Rpm would all read the same.

At any rate, I'll mess around with it again and try the different FQS setting and keep you updated.

[LifterEyes]

Well, I went and did another test on the sensor and found that there was no difference in the voltage reading off of the sensor between different FQS switch. I would get a reading of about .83 volts in the stock FQS position then I would shut off the car, change the switch position, start it, and watch the meter rise back up to this same voltage. So my guess is the sensor is shot.

[HondaDustR]

Yeah, kind of sounds like it. The only thing left you could try is inducing propane into the intake, which should definitely raise the voltage, but when it's function and age are questionable, it would be worth $40 to just replace it.

Just out of curiosity, what does the voltage do when the signal is being sent to the DME, allowing it to try to compensate?

[Cocacolakidd]

Question:
With the FQS position "0" is normal operation
position "1" is Retarded 2* ???
Position "2" is Retarded 2* + 3% rich fuel ???

Are these the correct settings at the FQS ??

[HondaDustR]

FQS - Fuel Quality Switch Here ya go.

[Cocacolakidd]

And there it was - Thanks HondaDustr -

Merry Christmas

Happy New Year

[LifterEyes]

Originally Posted by HondaDustR

Just out of curiosity, what does the voltage do when the signal is being sent to the DME, allowing it to try to compensate?

HondaDustR,

Well this one was interesting. This time I did a test with the sensor jumpered such that the DME was still getting the O2 signal. Starting from a cold engine, the voltage would slowly rise from zero until it reached about .7 volts. At this time the DME looked like it entered closed loop mode and the voltage began to toggle between .2 and .7 volts. Also, the engine would idle somewhat rough as it always has but I noticed that when the voltage would toggle to the leaner end (Around .2 volts) the Idle would smooth out somewhat.

[HondaDustR]

How fast does it swing back and forth? That sounds about right, but should cycle fairly quickly, almost erraticly. The meter (assuming a standard issue digital multimeter) should never settle on any particular voltage for any length of time. If it is slow, that's a good sign the sensor is worn out. The peak voltage output as well as the response time are what deteriorates to make an O2 sensor "bad". It idles rough because the mixture is swinging all over the place to each extreme. That brings us back to either a vacuum leak (or weak fuel pump, clogged fule filter, bad fuel pressure reg, or other lean condition), leaking injectors (or bad ignitionperformance for whatever reason, fuel pressure reg, or other rich condition), and/or the O2 sensor is old. Also worth noting is if the wiring or connector for the O2 sensor is bad at all, that will mess with the mixture regulation as well. An old sensor will compound an existing mixture problem because of it's slow response. A new one will respond and initiate corrections much more quickly and will more effectively cover up any minor existing conditions, but it will also give you a more reliable signal to adjust the CO level to get a naturally correct idle mixture.

[LifterEyes]

It seemed to take about 1.5 or 2 seconds to go full swing. The meter wouldn't really settle on any particular voltage but I could watch it ramp up and down pretty clearly.