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  #1  
Old 02-14-2020, 06:54 PM
Duncan Duncan is offline
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Default 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!
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Old 02-15-2020, 03:29 PM
Duncan Duncan is offline
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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
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Old 02-17-2020, 02:55 PM
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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.
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Old 02-19-2020, 07:09 PM
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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??
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Old 02-20-2020, 06:33 AM
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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.
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File Type: jpg o2 sensor.jpg (71.0 KB, 34 views)
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Old 02-22-2020, 11:32 AM
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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.
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File Type: scm 22118004.SCM (701.2 KB, 5 views)

Last edited by Duncan; 02-24-2020 at 11:01 AM..
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Old 02-24-2020, 06:52 AM
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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.
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File Type: jpg post 02 sensor.jpg (261.7 KB, 19 views)
File Type: jpg evap. purge valve.jpg (263.9 KB, 19 views)
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Old 02-24-2020, 11:32 AM
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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!
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Old 02-24-2020, 12:03 PM
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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.
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