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Old 01-01-2014, 09:18 PM   #1
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Question Weird O2 Sensor Reading on my Motor.... HELP?!

Hey Yall... Since I rebuilt this 1994 3.4 SFI motor I have been trying to tune it so that everything is within spec but have run in to a couple issues that are puzzling me and I am hoping one of you will be able to offer up sugegstions on what is causing this weird O2 reading on the left O2.

So when I got this rig there was no check engine light wired up off the ecm nor was there an ODB1 port wired up. Both of those are done now so that I can see whats going on with the motor. The only codes being thrown are trans codes due to the fact I'm not running the 4L60E stock trans that the ecm was programmed for.

None the less have a look at the video below and pay attention to the O2 sensor Mv readings. From what I know your O2s should swing voltage from 0 to a peek of about 900 Mv. You can see the right O2 is reacting as it should but the left O2 is not. You can see the differences in the video at idle and through some higher RPMs....

Through the process of elimination I have swapped the O2 sensors from side to side and the left still reacts the same so it is not the sensor itself.

I'm thinking along the lines I have a plugged or a leaky injector on one or more of the left side injectors. The low Mv readings indicate a rich situation on the left side... As far as I know.

Thoughts? Ideas? I could really use some input on this from you EFI guys.

Thanks in advance!

VIDEO: http://www.youtube.com/watch?v=enYQNtd_tCM
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Old 01-01-2014, 09:42 PM   #2
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Well the left side is running rich and the PCM appears to be giving more fuel to the left side also based on the BLM. Since you've already swapped the sensors side to side and get the same results its safe to say its not a sensor problem. I would start with checking any vacuum lines on the left side of the intake manifold, from there a fuel pressure test would be good to make sure the fuel system (mainly the injectors) are holding pressure as they should. Also check the fuel pressure regulator, if the diaphragm fails it will draw fuel into the vacuum line .
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Old 01-01-2014, 09:50 PM   #3
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Originally Posted by 94 T/A View Post
Well the left side is running rich and the PCM appears to be giving more fuel to the left side also based on the BLM. Since you've already swapped the sensors side to side and get the same results its safe to say its not a sensor problem. I would start with checking any vacuum lines on the left side of the intake manifold, from there a fuel pressure test would be good to make sure the fuel system (mainly the injectors) are holding pressure as they should. Also check the fuel pressure regulator, if the diaphragm fails it will draw fuel into the vacuum line .
I am picking up a used fuel rail and a set of used injectors this weekend from the wrecker so I will put that assembly on and see if that curs it.

Its not an intake or vacumm leak, I just rebuil the whole motor and freshly sealed everything really good.

I'm gonna make another video tonight showing all the other data from my scanner and I'll post it up later this evening.

Thanks for your reply.
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Old 01-01-2014, 11:20 PM   #4
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As promised here is a new video showing all of the results in my scanner. Should give you a better idea as to what is going on?

Hope I can figure this out.

Video: Second Test Through All Settings 1994 3. 4 sfi - YouTube
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Old 01-02-2014, 08:53 PM   #5
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Old 01-02-2014, 09:09 PM   #6
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I haven't had time to look at your last video I'll look at it when I get off work tonight.
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Old 01-08-2014, 01:33 AM   #7
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Old 01-10-2014, 12:55 AM   #8
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Replaced injectors, new O2 sensors, swapped map sensors, throttle position sensor, IAC sensor.... Same results on the scan.

I am at a complete loss as to why the left side of the motor is over fueling once in closed loop.

I really need help with this guys!!!! I am sure I'm not the only one in the world that has had this problem.

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