Diagnosis problem with the O2 patterns on a MODIS,

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  • Duncan
    Junior Member
    • Feb 2020
    • 12

    Diagnosis problem with the O2 patterns on a MODIS,

    Hello, I am working on a 1996 Jeep Grand Cherokee with a 4.0l in it, and I don't understand what I'm seeing on the )2 sensor graphs that the MODIS is producing. Here are the details of the vehicle:

    This Jeep has a "lower mile" 1992 4.0l that was recently swapped in, and ever since the Jeep has had poor mileage and less power. The replacement engine was pretty bare, so the Jeep still has all the wiring, sensors, manifolds, injectors, fuel pump, exhaust, and accessories that it had before the engine swap. It does not pull any codes now, nor has it ever pulled any codes.

    Both engines were/are stock and unmodified, except for having a throttle body with the restriction bored out, and 4-hole injectors to replace the OEM 1-hole injectors. The throttle body and injectors were changed years ago and they did not affect mileage or power noticeably at the time.

    A look at the O2 sensor output graph showed a strange pattern with a lot of latency and irregular crosscounts, so I installed a brand new O2 sensor of the same brand (NTK) and part number. Surprisingly, the O2 sensor output graph with the new O2 sensor installed is only marginally better, if at all! I have uploaded videos of the OLD sensor graphs and the NEW sensor graphs here: https://photos.app.goo.gl/AkQnQegdCZft1KvF7

    I should note that the STFT does hover around 0 (whether at idle or higher RPMs), and the LTFT ends up around 8%, which I don't think is too strange given that it doesn't have OEM injectors and that's less than 10% off. Perhaps I am wrong to dismiss this info though??

    What do these O2 graphs mean, or, what else should I look at to diagnose the problems with this engine? Thanks!
  • Duncan
    Junior Member
    • Feb 2020
    • 12

    #2
    I see that no one has had any ideas here, so I will post this on the General Discussion forum as well. I'll still check back in case someone can chime in though!!

    Cheers,

    Duncan

    Comment

    • tech25
      Senior Member
      • Oct 2017
      • 172

      #3
      hey there, not sure what your question is, but are you asking why the car goes rich and then lean constantly??

      If yes, then if you know, 14.7 is the best ratio for complete combustion for emissions.

      now. the question. why doesn't the engine just keep the air/fuel ratio at 14.7 constantly?? why adjust rich and then lean all the time??

      the answer is because going rich and then lean keeps the catalytic converters warm, thus working. imagine keeping a camp fire nice and warm, you need fuel ( wood ) and oxygen ( wind ). adding wood, and then combining a breeze keeps the fire nice and warm.

      14.7 is best for complete combustion, but will allow for the cats to cool down too much and limit their efficiency.

      Comment

      • Duncan
        Junior Member
        • Feb 2020
        • 12

        #4
        Hello Tech25! No, that wasn't what I was asking... I know about workiing around stoichiometric ratio to keep the best balance of a working cat and efficiency, but my question was why does the O2 graph from my MODIS Edge look so lazy even with a brand new, OEM brand/part # O2 sensor?

        When I look at the graphs I linked to in my previous post, I am expecting way more crosscounts and less latency for starters, but I am also expecting to see the O2 signal bounce back and forth from "RICH" to "LEAN" evenly, rather than favoring one side or another for a bunch of injector cycles.

        When I look at the O2 sensor graphs for this car, it sure seems like something's not working properly, but I need some help figuring out what is wrong! I'm still trying to figure it out but I've run out of ideas??

        Comment

        • tech25
          Senior Member
          • Oct 2017
          • 172

          #5
          ok, it appears the pre o2 sensors are as follows:

          sensing material : Zirconium
          sensor operating range: 0 - 1 V
          sensor principle: voltage generating
          lean mixture: 0 V
          rich mixture: 1 V

          as you can see by the picture, a lambda of 1, is a 14.7 ratio. which is about .45 V

          when you are looking at live data, there are too many pids, please de-select all of them and only select the pre 02 sensors so you have a faster re-fresh rate, and then compare voltages from what i have given you.
          Attached Files

          Comment

          • Duncan
            Junior Member
            • Feb 2020
            • 12

            #6
            Hello again! Thanks fort the suggestion to decrease the number of PIDs, I didn't know that refresh rate would be a problem. In the meantime, I will upload a ShopStream file I saved yesterday, right after swapping in another brand new NTK O2 sensor, just to be sure that the lazy trace was not a result of a poor sensor right out of the box. Maybe it will provide some better info for the diagnosis of why the O2 trace looks so lazy.

            I did reset the MODIS after warming up so that the scale would reset, but it still appears to have a max of 4.88v in this ShopStream video, so watching it with the O2 voltage scale set to 0-.9v makes things easier to see! This is a pretty long recording of the Jeep warmed up and driving around the block.

            Also, zooming in allows me to see that there are usually about 8 frames of the O2 sensor reporting one state (say, RICH), before switching to the opposite state for another 8 frames. This is a lot slower than the O2 graphs of other vehicles I've been looking at, hence my confusion - especially with a brand new sensor and no other driveability symptoms.
            Attached Files
            Last edited by Duncan; 02-24-2020, 11:01 AM.

            Comment

            • tech25
              Senior Member
              • Oct 2017
              • 172

              #7
              looking at the snap shot. a couple things stand out..

              1: why is the post 02 sensor going rich then lean??? does this car have catalytic converters on ??

              2: if you look at the duty cycle for the evap. purge solenoid, it is somewhat in connection with the pre 02 sweeps. for instance, when its open, it will draw in fuel vapors from the charcoal canister and make the exhaust rich...

              try disconnecting the evap. purge line from the intake and plug it off, and retest.
              Attached Files

              Comment

              • Duncan
                Junior Member
                • Feb 2020
                • 12

                #8
                Thanks for your observations and suggestions so far. To answer your question, yes, this car has a catalytic converter in it, and IIRC the cat is about 5 years old. It isn't pulling any codes, so I figured that the downstream O2 sensor voltages were within Chrysler's "allowed" range, even though the voltage is definitely not flatlining anywhere near 0.45 volts!

                More importantly though for diagnosis, I've been pretty fixated on how slowly the upstream O2 sensor is switching. Regardless of the Evap duty cycle, or the cat function, isn't the latency of the upstream O2 sensor way too slow? All the O2 sensor traces I've looked at have switched a heck of a lot faster, and I have now tried three O2 sensors on this engine - 1 old, two brand new, all the same OEm replacement brand and part number!

                I will test again with the Evap line disconnected and plugged later today and see what happens, thanks for the suggestion!

                Comment

                • tech25
                  Senior Member
                  • Oct 2017
                  • 172

                  #9
                  also, since you have a scope in your modis, please scope the 02 signal wire directly at at the ecm, ( back probing ) at connector, should be pin 24, and is a black/orange wire.

                  this way you are seeing exactly, in real time what the ecm is seeing. and not relying on old 8 bit computers to turn analog signals into digital, and send to the scan tool on a slow baud rate data line.

                  Comment

                  • Duncan
                    Junior Member
                    • Feb 2020
                    • 12

                    #10
                    Originally posted by tech25
                    also, since you have a scope in your modis, please scope the 02 signal wire directly at at the ecm, ( back probing ) at connector, should be pin 24, and is a black/orange wire.

                    this way you are seeing exactly, in real time what the ecm is seeing. and not relying on old 8 bit computers to turn analog signals into digital, and send to the scan tool on a slow baud rate data line.
                    OK! I have finally cleared enough stuff off my plate that I'm getting back to this problem. The exhaust definitely has a different odor than it used to (sweet, not fuel, but stronger than it used to be), and the mileage is still not nearly what it should be, so I'm going to keep trying to figure it out. Tonight I'll collect the O2 pattern on the scope to see if it's switching any faster than it looks to be. I was thinking about this, I imagine I can simply backprobe the O2 sensor pigtail connector, no? I'll see which looks like it will work and give it a go. I hope you're still out there waiting for this update!

                    Comment

                    • Duncan
                      Junior Member
                      • Feb 2020
                      • 12

                      #11
                      OK, with a needle in A24 and a ground clamped onto the negative terminal of the battery, I was able to get a trace.

                      The idle trace looks pretty ugly, but the 2000RPM trace looks really, really good, with 4 crosscounts a second, in a regular pattern. I've attached the files I pulled off the MODIS.

                      So then... the O2 sensor is working really well. In light of that, what else can I look at to figure out why the power and mileage are down, and the exhaust has that "engine warming up and the cat is working hard" smell??
                      Attached Files

                      Comment

                      • tech25
                        Senior Member
                        • Oct 2017
                        • 172

                        #12
                        Hey there, by any chance have you checked the back pressure on the exhaust system before the cat??

                        To rule out a clogged/faulty cat.. I would remove the pre 02 sensor and make a hollowed out old 02 sensor and attach and vacuum/pressure gauge. And measure the pressure.

                        Comment

                        • Crusty
                          Senior Member
                          • Dec 2007
                          • 2450

                          #13
                          The only thing I'm aware of that can make the exhaust smell "sweet" is the very occasional fuel additive, but more often and very common is antifreeze. The almost imperceptible odour from fuel additives is virtually undetectable and is usually only evident when the fuel additive container is close by and has just been introduced.
                          It's old enough for gasketing problems. Antifreeze will destroy O2 sensors and contaminate CATs as well.
                          See if you can find some case studies on antifreeze screwing up O2 sensors and compare the traces.
                          If the power and fuel mileage is down, and the exhaust smells sweet, and the O2 sensors look horrible....what is the ROOT CAUSE-?

                          Comment

                          • Duncan
                            Junior Member
                            • Feb 2020
                            • 12

                            #14
                            Originally posted by tech25
                            Hey there, by any chance have you checked the back pressure on the exhaust system before the cat??

                            To rule out a clogged/faulty cat.. I would remove the pre 02 sensor and make a hollowed out old 02 sensor and attach and vacuum/pressure gauge. And measure the pressure.
                            I've been thinking I should indeed be doing this next. I will hollow out a spark plug though, as I have a ton of them available but I don't have a pile of old O2 sensors to destroy! How much pressure should I be seeing? Ideally pretty close to zero, I'd imagine... A clogged cat would indeed cause poor mileage and low power, but I can't think how it would cause the OBDII O2 signal to look so much slower than the signal straight off the sensor itself. The backpressure test is a very good idea, but I'm wondering if there's still a problem with wiring or the PCM since the upstream O2 sensor has such great trace!

                            I should have time to make that adapter tomorrow.

                            Comment

                            • Duncan
                              Junior Member
                              • Feb 2020
                              • 12

                              #15
                              Originally posted by Crusty
                              The only thing I'm aware of that can make the exhaust smell "sweet" is the very occasional fuel additive, but more often and very common is antifreeze. The almost imperceptible odour from fuel additives is virtually undetectable and is usually only evident when the fuel additive container is close by and has just been introduced.
                              It's old enough for gasketing problems. Antifreeze will destroy O2 sensors and contaminate CATs as well.
                              See if you can find some case studies on antifreeze screwing up O2 sensors and compare the traces.
                              If the power and fuel mileage is down, and the exhaust smells sweet, and the O2 sensors look horrible....what is the ROOT CAUSE-?
                              Thanks for your ideas Crusty. I am having trouble explaining the smell of the exhaust, but it is NOT the smell of antifreeze in the exhaust, luckily. I'm familiar with this smell. I remember back when I had a carb'ed vehicle and my friend had an early EFI Toyota, his often smelled like this too.

                              Comment

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