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Unread 03-06-2015, 06:51 PM   #1
MrRoundel
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OBDII Fuel-trim, O2 Sensor Questions

I just received my OBDlink OBDII connection setup. It is used with the company's proprietary software. Overall, it seems to work pretty well. At first it wouldn't connect, but that was because it configured for using a different USB port. I just changed locations and voila!

I'm now able to see such things as long-term fuel-trim, and some data for the 02 sensors as well. I've experienced a worsening of gas mileage of late, and wanted to see if it was something that might show up in the fuel-trim readings. This stuff is pretty new to me. Since the setup I got only works with Windows OS, and my Windows laptop has a bad battery, I'm not able to drive the car with it attached. I just run an extension cord through the window, which doesn't get me far. My readings at warmed up idle are as follows:
Long-term FT is just under 6%. Short-term is about -2%. Neither seems too bad. However, I don't know how the data stays in the OBD system, so I don't know what the LTFT is able to tell me. Does it use data going back from the last time the car was actually driven, or is everything based on what has happened since the last start-up? I.E., are the numbers at idle telling me a lot of nothing?

My 02 sensors are both showing around .8V. Does that sound about right? It seems that normal was closer to .5V or so. Thoughts? Thanks for any input offered.

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Unread 03-06-2015, 07:09 PM   #2
Timo_90xj
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PCM stores a number of parameters and adjusts them according to your driving style. Jeep PCM is a "learning" unit. Occasionally, if a component is slightly misbehaving but not actually broken, weird things may happen. Worsening MPG can be a sign of an issue, but a minor one that doesn't light up the CEL.

I just installed a new OEM/ Mopar 46RE transmission speed sensor to replace the broken one. I haven't had any issues regarding transmission operation even with the broken TSS. Now that I changed it and cleared the TSS fault code, my transmission went haywire. It would lock-up the TC at 40 mph, wouldn't release lock-up until well over 50% throttle, etc... I had to reset my PCM for it to actually work right.
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Unread 03-06-2015, 08:46 PM   #3
MrRoundel
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Quote:
Originally Posted by Timo_90xj View Post
I just installed a new OEM/ Mopar 46RE transmission speed sensor to replace the broken one. I haven't had any issues regarding transmission operation even with the broken TSS. Now that I changed it and cleared the TSS fault code, my transmission went haywire. It would lock-up the TC at 40 mph, wouldn't release lock-up until well over 50% throttle, etc...
Sounds like a bit of the old, can't teach an old dog, new tricks, routine. I hope your PCM has learned its newest lesson well.

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Unread 03-06-2015, 09:25 PM   #4
Uniblurb
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Here's a pretty good thread on o2 sensors along with some info on fuel trims MrR. In a perfect world you're upstream o2 sensor should read right around .45V. There's also some info on using laptop software at the end of the thread.

http://www.jeepforum.com/forum/f13/o...y-want-508460/
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Unread 03-07-2015, 05:03 AM   #5
coralman
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Look in my blue link under FUEL INJECTORS FUEL TRIM, SENSORS, there are about ten links in it dealing with this stuff.
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Unread 03-07-2015, 09:20 AM   #6
MrRoundel
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Thanks, Uni and coralman. I appreciate the help. I also found this helpful article.

Excerpt: The LTFT calcu-
lation is kept in memory on most vehi-
cles, so the PCM does not need to re-
learn the fuel trim calculation the next
time the vehicle is started.


http://www.motor.com/magazine/pdfs/092006_11.pdf
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Unread 03-07-2015, 09:57 AM   #7
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Hmm..after reading a few posts, articles, etc., including one by Uni, regarding possible exhaust leaks at the "donut" seal, I'm flashing on something. When I replaced my exhaust manifold a few years back, and again when I had to open the flange to get drop the oil-pan, I noted that there was no gasket between the exhaust pipe and the manifold flange. Since I didn't hear any excess noise, I figured it must be adequately sealed. Now I'm thinking not. I think that it might be a good idea for me to get one in there, as it might be causing some of my problems. That said, the mileage decrease is only a recent phenomenon.

If there is an exhaust leak at the flange, would this really cause a problem with the O2 sensor's reactions? While I realize that the volume of exhaust is decreased if there is a leak upstream of the upstream O2 sensor, wouldn't the mixture/ratio in the exhaust gas being analyzed be the same? Or is it that fact that the release of the gases through the leak cools down the flow to the sensor, which in turn makes it react differently? Either way, since I'm pretty sure there's no gasket in there, I'd best get one installed, no? Maybe it will be my "trophy" of late? Doh!
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Unread 03-07-2015, 10:15 AM   #8
96zedjay
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Although we mostly think of an exhaust system being a restriction and pressurized this is not always true. At lower loads and RPMs between the
pulses from each cylinder there is a period when the pressure inside is less than the outside.... In effect sucking air into the Exhaust system.
This air mixes with and contaminates the combustion gases causing the O2 to read not representative of whats actually happening inside the engine.
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Unread 03-07-2015, 10:25 AM   #9
MrRoundel
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Quote:
Originally Posted by 96zedjay View Post
Although we mostly think of an exhaust system being a restriction and pressurized this is not always true. At lower loads and RPMs between the
pulses from each cylinder there is a period when the pressure inside is less than the outside.... In effect sucking air into the Exhaust system.
This air mixes with and contaminates the combustion gases causing the O2 to read not representative of whats actually happening inside the engine.
Ah...so that's what's really happening. I just ordered the gasket, and will be installing it mid-week. Thanks for your input, 96zedjay.
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Unread 03-07-2015, 10:31 AM   #10
96zedjay
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I'm not saying this is happening, just the mechanism that it can....
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Unread 03-07-2015, 10:50 AM   #11
MrRoundel
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Quote:
Originally Posted by 96zedjay View Post
I'm not saying this is happening, just the mechanism that it can....
Right. I get that. Thinking out loud, though, it does seem that unless my exhaust valve was leaking, I don't see how the air introduced into the exhaust manifold during an intake stroke, when vacuum would be most likely taking place, would get to where it would effect the mixture.
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Unread 03-07-2015, 11:09 AM   #12
Montreal300
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It's not a case of a leaking valve introducing ambient air. It's the exhaust pressure wave travelling down the exhaust pipe. Behind every pressure wave there is a low pressure zone that MAY be below ambient air pressure. It's part of the old scavenger exhaust design theory.
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Unread 03-07-2015, 12:04 PM   #13
MrRoundel
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Quote:
Originally Posted by Montreal300 View Post
It's not a case of a leaking valve introducing ambient air. It's the exhaust pressure wave travelling down the exhaust pipe. Behind every pressure wave there is a low pressure zone that MAY be below ambient air pressure. It's part of the old scavenger exhaust design theory.
After reading your post, I read up a little bit on the exhaust scavenging. The theory is a bit above my brain's pay-grade, so I'll take your word for it. As I said, I will be replacing the gasket. I've been running it for a long time this way. I hope that I haven't done any long-term damage to my "bullet-proof" 4.0L. Time will tell.

BTW: Manifold vacuum is a pretty steady 16.5", FWIW. I believe that this is OK. Not stellar maybe, but OK. No?

Anybody see anything notable in the accompanying screen pic? I'm learning about this stuff as I go along, but am not very experienced in diagnosing modern vehicles, with such data as LTFT, etc. The table contains the averages, etc. for idle, 1,500 RPM, and 2,500 RPM. Thanks for any comments.
dsc05016.jpg  
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Unread 03-07-2015, 12:23 PM   #14
Uniblurb
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Hmm.... wonder when in the cycle the upstream o2 sensor is reading a max voltage of 1.2V and the downstream/cat o2 sensor is reading 1.3V max voltage?? Having a hard time seeing the decimal points and hope I'm reading this correctly. It's my understanding 1.0+V on an o2 sensor could mean it's bad but something may be skewed with the figures? Didn't you just install a new OE/NTK o2 upstream sensor MrR?
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Unread 03-07-2015, 01:49 PM   #15
MrRoundel
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Yes, Uni, I did. The table is indeed showing 1.2V on the high side of the mean for the upstream sensor and 1.3V on the downstream. Both are new NTK sensors. Somehow I suspect there's something else that's causing those highs. My mileage did improve by 4 mpg (11.5 to 15.5) after replacing the sensors, FWIW. It as terrible before I replaced them, obviously. I am used to getting a couple of more MPG out that 4.0L, even more on a highway drive, maybe 20 mpg. That's not going to happen the way she's running now.

Do you think that there's any chance that the probable exhaust leak could be causing it? I do know that I have a large vacuum leak in the evaporative system. I'm sure that it's not helping anything. I've got to pull the header panel, etc., to bet to the canister, as I the leak is likely there. Before I do that, I'd better buy a new header-panel, as my ex-wife smacked into something years ago, and one headlight has been loose in the panel. I sure wish I had a smoke machine. Oh well...wish in one hand....

Thanks for your input.
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