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Code P0300, multiple misfires

Tiny
TOTALMC
  • MEMBER
  • 2005 INFINITI FX35
  • 3.5L
  • V6
  • 2WD
  • AUTOMATIC
  • 125,000 MILES
Car listed above is a FX35. Customer changed spark plugs, then noticed not running well; stated may have mixed up coil connections.

I think I have a coil connection right, but still running rough at idle, and smokes at higher RPMs.

A p0300 comes up after about a minute.

Scanner shows STFT bank 2 pegged at 25%, bank 1 is normal.

New plugs 2, 4, and 6 seem dark and fuel fouled.

Tried a smoke test but could find no leaks (assuming an intake manifold gasket failure).

Not sure where to go next, any help or ideas would be greatly appreciated.

Thanks
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Monday, December 13th, 2021 AT 2:10 PM

6 Replies

Tiny
AL514
  • MECHANIC
  • 4,692 POSTS
Hello, does this car have two catalytic converters? And can you look at live data and see what all 4 oxygen sensors are doing?

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter
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Monday, December 13th, 2021 AT 3:40 PM
Tiny
TOTALMC
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Bank 1 was fluctuating normally
0 -.5.

Bank 2 was holding closer to.7.
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Monday, December 13th, 2021 AT 3:51 PM
Tiny
TOTALMC
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I did notice some rattling in the exhaust (somewhere near the cat) when the vehicle was first started. Not sure if it was a rattling heat shield, loose clamp, or the actual cat.

If the cat was faulty on Bank 2, wouldn't a p0420 code be present?
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Monday, December 13th, 2021 AT 4:00 PM
Tiny
AL514
  • MECHANIC
  • 4,692 POSTS
You would think so, but i would recommend doing a back pressure test on both cats to see if one banks cat is bad. Because this has a mass air flow sensor, sometimes because of low air flow on one bank the ECM may overcompensate. I mentioned this because there's a case study of this happening, no cat codes, and same situation. And also, because you have a whole bank affected by this issue. It was a case study by John Thorton, I think it was named fuel trim imbalance. The other shops couldn't figure out what was happening, they even pulled the cat down and inspected it but missed the melted cat somehow. So, checking the either the back pressure, or dropping the exhaust and inspecting them, but the back pressure test is easier, 1 PSI is the max on a converter back pressure. It's worth checking out, especially since you have a bank that is completely pegged out.

Here's the Coil wiring diagram to check that if needed:

it's also odd that the 02 is reading full rich at .7v and still 25% fuel trim. With that reading the ECM should be going completely the other way.

https://www.youtube.com/watch?v=hmFozmTf7xg&t=35s
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Monday, December 13th, 2021 AT 4:59 PM
Tiny
TOTALMC
  • MEMBER
  • 106 POSTS
Thanks, I'll try and recheck my live data to make sure I got the numbers right. Can't remember if I was reading the pre cat or after cat O2 sensor, and it may have been minus 25% on the STFT.
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Monday, December 13th, 2021 AT 5:13 PM
Tiny
AL514
  • MECHANIC
  • 4,692 POSTS
I wouldn't be surprised if it's minus, but still pull the front A/F sensors and do a back pressure test. It's just odd having a whole bank off like that unless there's a timing issue, but there's no code for VVT or anything like that. And those injectors are controlled separately, so it's not a bank fired injectors driver issue.
I'm not sure if this system has intentional injector shutdown either. Doesn't seem so with plugs that are that bad. I'm sure your smoke test would have showed an intake leak.
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Monday, December 13th, 2021 AT 5:35 PM

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