Testing CO Level Using O2 Sensor

[HondaDustR]

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

Test results for new universal O2 sensor

[LifterEyes]

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.

[LifterEyes]

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?

[HondaDustR]

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.

[LifterEyes]

“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)”

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.

“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.”

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.

“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.”

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.

[HondaDustR]

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.




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.

[onZedge]

Fuel Pressure is too high.

[LifterEyes]

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

[LifterEyes]

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?

[HondaDustR]

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...

[LifterEyes]

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?

[HondaDustR]

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

[LifterEyes]

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?

[HondaDustR]

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.