Testing CO Level Using O2 Sensor
 

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?

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

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?

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.

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?

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.

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!

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.

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?

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.
http://forums.pelicanparts.com/uploa...1260323325.gif

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.

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.

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.

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?

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 ??

FQS - Fuel Quality Switch Here ya go.

And there it was - Thanks HondaDustr -

Merry Christmas

Happy New Year

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.

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.

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.

Yeah, I would just replace it and go from there if it was me.

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/514367-test-results-new-universal-o2-sensor.html

I think I'll just go ahead and give that a try. I plan on ordering the 15725 sensor soon and will update once I get it all set up.

Thank you HondaDustR, you have been a great help. I really do appreciate it.

Rich Running Condition
 

So I had a chance to replace the o2 sensor with the Bosch 15725 unit and got very similar results to what I was getting before.

When I measured the voltage coming off of the sensor with only the heater wires jumped the voltage would rise to about .9V (Where it was showing about .8V with the old sensor) and it would stay about in that area as I raised the RPMs. So this seems to me that the engine is running rich enough that the AFR is just out of range of a typical Lambda sensor.

With that said, what could cause it to be running this rich? I have checked the FQS and it is in the stock "0" position, the altitude sensor is connected correctly, and there is no jumper on the coding plug. These three things are the only things I can think of that would change the fuel mapping in any way. Also, I checked the fuel pressure some time ago and I remember it being around 36 Psi at Idle and would raise as I opened the throttle. As far as the Ignition goes, I swapped the coil with a known good one and saw no Improvement either. What else could I be missing?

Check out the AFM. Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk

How's the fuel injector spray patterns? Do any of them leak if the fuel pump runs when none of them fire. Either jumper the DME relay or disconnect all of the injector connectors and crank the engine to check. How about the fuel pressure?

How do the ignition parts look? (cap, rotor, plugs, wires)

What O2 sensor signal do you get at the DME itself, grounding the meter at the DME case. Is it very different from the voltage at the sensor plug? The DME connector comes apart to allow you to stick a test lead through the back of the pin block while it's plugged in. This is also the best way to test the AFM.

Looking back on previous posts, does the voltage still swing up and down the way it used to? The update rate should increase at higher rpm from idle. Slower update at idle than higher speeds is normal.

Well the fuel injectors, Ignition Cables, Rotor and Cap are all new with no cracks, leaks, or anything like that.

Some time ago I checked the fuel pressure and I remember it reading about 36 Psi at Idle and it would rise as I opened the throttle but I don’t remember where the pressure would top out at.

I seem to be getting the same O2 signal at the DME terminal itself. I tested the AFM signal from the DME terminal as well and got about a .98 V signal at Idle and it seemed to rise and fall appropriately with changes is throttle. However, I was having difficulty testing the resistance of the two temperature sensors this way. I’m guessing this might be because the DME was sill connected at this point.

With the new sensor the voltage does noticeably swing up and down much faster then it did with the old sensor. It is hard to tell how much faster it swings at higher RPMs due to the refresh speed of my meter. In any case, it seemed to read about .7 then jump to about .3 and back at higher RPMs.

I seem to be getting the same O2 signal at the DME terminal itself. I tested the AFM signal from the DME terminal as well and got about a .98 V signal at Idle and it seemed to rise and fall appropriately with changes is throttle. However, I was having difficulty testing the resistance of the two temperature sensors this way. I’m guessing this might be because the DME was sill connected at this point.

The voltage doesn't matter so much as how smooth the response is. That is checked with the DME plugged in and the engine off. Measure the signal return to the DME and slowly move the AFM flap from closed to fully open and watch for a smooth increase in voltage. Full open voltage should be at or just shy of 5v if it's a late DME. There should be no glitches or jumping around on the readout while openning and closing it. You can find how the condition of the wiring factors in by testing the voltage out from the DME to the AFM first at the DME connector and then at the AFM. Repeat for the signal to the DME. Test at the AFM at full open and at closed and then re-test at the DME connector to see if there's a voltage drop. I found it easiest to just pop the cover off the AFM for this and then you could also get a visual on the condition of the resistor track.

With the new sensor the voltage does noticeably swing up and down much faster then it did with the old sensor. It is hard to tell how much faster it swings at higher RPMs due to the refresh speed of my meter. In any case, it seemed to read about .7 then jump to about .3 and back at higher RPMs.

If it's noticably faster at idle, that's good for sure. Rapidly swinging between .3 and .7 is exactly what's supposed to happen, since the DME does not constantly monitor the O2 sensor signal, but only responds to a too rich or too lean condition.
Lambda Inside
Not sure what the reason for just hanging out on the rich side is with the sensor unplugged...and .8-.9v is very rich. You can do the same checks for both temp sensors as before, test the sensor itself and then plug them in and test at the DME connector. This should be done with the DME connector unplugged. Higher resistance on the AFM from bad wiring, tracking, or whatever should theoretically cause a lean condition. The temp sensors trigger a richer mixture whith higher resistance, so it could very well be some bad wiring there or a temp sensor out of spec. Check that AFM fix link for the table of values for the AFM air temp sensor, and here's the values for the engine temp sensor.

http://forums.pelicanparts.com/uploa...1262022660.jpg


The fuel pressure may be a little high if it was ~36psi at idle. idle spec is ~29 psi, or 2 bar and should raise to 33-39 psi, 2.3-2.7 bar, when the vacuum line to the regulator is removed, according to the factory manual. I have no idea if that could make it run that rich.

Fuel Pressure is too high.

I'll go ahead and check the AFM signal, temp sensors, and the fuel pressure again since I may be remembering wrong and I I'll update after that.

Thanks again

Ok so I went ahead and tested both the AFM and signal again and the fuel pressure. I found that the AFM signal looked smooth and the resistor track looked clean. However when I tested the fuel pressure again, the pressure at Idle was about 33 Psi and when I removed the vacuum line it jumped to a little over 40 Psi.

Is 33 Psi at idle far enough from spec to potentially cause problems?

If so, Is there a way I can alter the fuel pressure and see how these changes effect the O2 sensor signal?

I don't know if 4 psi could throw off the mixture that far on the O2 sensor, but then again, it's not really like super rich, being a narrow band sensor. Without the O2 sensor to compensate, the injectors will time themselves to the pre-programmed injection time value out of the map, determined by rpm, air volume, temp, etc. assuming all the other variables such as fuel pressure remain at nominal values. Obviously, when the fuel is under more pressure, more will squirt out during a given "open" duration on the injectors, richenning the mixture, since the O2 sensor is the only way the DME can know what's going on beyond the intake ports. With the O2 sensor plugged in, it senses rich and reduces the injector time. It might be time for a new regulator if you want a naturally correct mixture, or you might be able to just open up the bypass jet far enough on the AFM to get 0.5v on the sensor and be done with it, since it seems that the sensor is correcting for the difference ok. It would have to be re-adjusted if you eventually replaced the regulator and it dropped the fuel pressure.

Cheap or perfect? Your call...

I actually had a chance to swap the fuel pressure regulator with a spare from an engine that didn't have a similar problem so I'm assuming it is good.

What I found was: I tested the pressure again before swapping in the spare regulator and found the gauge reading about 36 Psi like I thought I had remembered the first time. This was higher then the 33 Psi pressure I read the last time I posted. So I swapped in the second regulator and the gauge read about 36 Psi, same as the original. The weird part was when I put the original regulator back in the pressure was back to ~33 Psi again.

The vacuum hoses that go to the regulator and damper seem OK but I think I may replace them to eliminate all doubt.

With that in mind, can a bad pressure damper cause higher fuel pressure as well?

I have no idea. This is kind of out of my league now.

Well I guess having the O2 sensor compensate will have to do for now. Since the engine will run really rich until the point where the DME enters closed loop mode and compensates, should I be concerned about the new O2 sensor being fouled because of this?

Unless it's like loping and slobbering rich until it warms up, I wouldn't worry about it, since all cold engines need to run rich, but I've noticed mine seems to switch over fairly quickly (a few blocks) unless it's like 20 degrees or lower. The heater helps get and keep the sensor ceramic up to operating temp faster and more consistently, which is what helps keep it from fouling up...mostly why non heated sensors are supposed to last only half as long as heated sensors.