Tech Thread Only?

[smgmn]

Would it be possible guys , if we could use this thread just for tech stuff ? As long as its ok with Barry? I am kind of burned out on the political stuff. I smog, diagnose, and sometimes actually fix stuff, lol.

Thougts ? Discuss ? Questions?

Mark

[scanner]

smgmn wrote

Would it be possible guys , if we could use this thread just for tech stuff ?

Let me start with a "Check Engine" light and driveability failure.   It was a 2001 Nissan Sentra.    It would misfire and die at idle.   The consumer had already thrown a gang of parts, including the PCM at the vehicle in an attempt to rectify the problem.  After hooking up the scanner, I knew exactly what was wrong.   For those that don't know what A/F Alpha is, it's the fuel trim measurement.   100% is equal to 0% in OBDII terms, or 128 in GM Block Learn and Integrator terms.  

I've posted this on another forum, so those that already know the answer, please let the other techs here show their diagnostic chops.   When you answer, please give a reason for your diagnosis.   I don't want to see, "It's the MAF", or "It's the IAC" without an explanation.  

[Moderator]

I'm pretty sure that I know, but I won't ruin it for the rest.  I'll e-mail my "guess" to scanner, so it can be registered.

[Just the Facts]

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[Moderator]

I'm still not sure, but I believe that you're over-thinking it JTF ... my answer was based on my previous experiences with shade-tree repairs, not normal wear, tear, deterioration, and clogging.

[Edit:  I didn't see scanner's post on the other forum, but the data capture looked familiar, because I've actually seen very similar data first-hand at least three times in the past.  One of them was a mid-eighties 3.8L Thunderbird (with the high-pressure CFI), and the other was a mid-nineties Vanden Plas ... I can't remember the third.  I'm a bit ashamed to admit that I punted the Jag, and the owner came back later to tell us what the dealer found.  It was a forehead slapper.]

[justa]

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[Moderator]

You're on the right track, Justa ... more than half-way to the correct answer (scanner e-mailed back to say that I nailed it).

What kind of bone-headed "repair" would cause the symptom that you correctly interpreted from the data?

Being an amateur drummer, I am highly aware of irregular cranking rhythms.  However, being an amateur drummer, my hearing has been permanently damaged, so that's about the only thing I can hear.

[Just the Facts]

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[Moderator]

Is that your final answer, JTF?

[smgmn]

Thanks scanner for posting this, is the fuel control not totaly out of whack? and if so why? lol good stuff :)


Mark

[smgmn]

I used to play the drums also, before I got married lol, dieng to get a roland electronic set. There goes my thread lol! awesome picture.

Mark

[smgmn]

I hate to say I have seen this done, but it was by someone else and he is a my boss, just a kid who thinks he knows everything. Always on a Honda lol Pretty lucky he didn't fry computer.

Mark

[Just the Facts]

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[scanner]

Just to let everyone know, there was a fix, and you can see the results in this scan.  




Michael nailed the problem.  I'll post the fix Monday around noon.  In the mean time, I'd like to hear the opinion of the rest of the group for this case study.  

[909 Smog]

Does the bank-1 A/F Alpha reading of 125 mean fuel is being added to or subtracted from that bank?  I'm looking at the graphs as if an Alpha above 100 is adding fuel and below 100 is subtracting fuel.  Am I wrong?  I expect to see fuel being added to bank-1 with H02S1 reading 0, which it is.  If the opposite is correct, then the theory of crossed bank-1 and bank-2 sensors makes more sense.  I guess I'll sit back and wait for the answer.

[scanner]

909 Smog wrote

Does the bank-1 A/F Alpha reading of 125 mean fuel is being added to or subtracted from that bank?  

Added.  

I'm looking at the graphs as if an Alpha above 100 is adding fuel and below 100 is subtracting fuel.  Am I wrong?  

You are correct.  

I expect to see fuel being added to bank-1 with H02S1 reading 0, which it is.  

Yes, your observation is correct.

If the opposite is correct, then the theory of crossed bank-1 and bank-2 sensors makes more sense.  I guess I'll sit back and wait for the answer.

I'll let everyone know the answer tomorrow afternoon.

[dmlusmc]

I saw an ase question recently with limited PID data to work off of. So it showed 19hg vacuum at cruise speed about 2500 rpm no fueltrim numbers or inj ms pids. and the symptom was a Po300 random misfire code which appeared at cruise. and there were 3 choices that I remember 1. bad cam sensor, 2. fuel problem,3. ignition problem. and 4th choice I cant remember. I think I put fuel problem cause high vacuum at cruise would mean its lean and lacking fuel but then again a secondary ignition problem causes cruise misfires as well.

[scanner]

Michael emailed me the correct answer from the get go.   The B1S1 and B2S1 inputs to the PCM were switched.    Here's a pic of the connectors on top of the exhaust manifold.   The connectors were the same and would interchange with no extra effort on anyone's part.    How they came to be switched is a question I never got an answer.  



You can right click on the image and "View Image" to see the whole pic if you can't scroll over.

[smgmn]

Great job again scanner, how Nissan allowed that to happen is interesting, but that car was made 14 years ago. Again great stuff everyone and kind of have to agree with Justa cause im kind of an old fart and not overlooking the basics is something i try not to do but sometimes we can outsmart ourselves if we go into a diagnosis and overlook this practice, wonder who's gonna pay for all the parts that where not needed? LOL.

Mark

[909 Smog]

That was smart of Nissan to use the same color connectors with the ability to plug into each other.  One question I still don't understand.  If the connectors are crossed and and Bank 2 oxygen sensor measures .8, why is the Bank 1 Alpha adding fuel with a reading of 125?  The same with Bank 1 oxygen sensor measuring 0 and bank 2 Alpha subtracting fuel?  Seems backwards to me.  Please explain Scanner.