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Discussion Starter #1
I removed and replaced the throttle body and did the idle relearn procedure several times, and I'm getting incredibly high idle when the engine is cold (2000-3000 RPM surging). After the engine warms up, the RPM bounce between 1000 and 2000. Unplugging the MAF sensor drops and stabilizes the idle RPM. I cleaned the MAF sensor and replaced the PCV valve, but nothing changed. I am not getting any faulty MAF codes; however, I am getting (intermittently) misfire codes on two cylinders. Previously, I got P0507, but that issue seems fixed with a new PCV valve.

There certainly could be many other issues with this abysmally maintained, 250k mi. 2.5L '06 Altima.
A bit of history:
  1. air filter never replaced in about 5 years
  2. spark plugs corroded, and coils never replaced ever
  3. oil not changed in about 20k mi
  4. valve cover replaced due to spark plug well gaskets leaking
  5. corroded radiator replaced
Could there be fuel injector issues? I do seem to smell fuel around the engine when it's running.
 

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First thing I would resolve is the misfire.
Inspect and if necessary replace the main ground cable. Very common to fall apart.
The first giveaway is misfires.
You may have to replace a few coils as well. But be sure the ground cable is good first.
When you go to do the idle relearn you may have to unplug one of the coils. Odd but usually works.
The fuel smell may be coming from a non firing cylinder, hopefully not a external leaking injector.
 

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Discussion Starter #3 (Edited)
I had replaced all the coils, but I started running into this idle stability issue after cleaning out the throttle body and vacuuming out the MAF. (It was idling high before, but stably.)

I discovered a piece of cylindar 4's fuel injector o-ring sitting right on the manifold. Brake cleaner also detected a leak there. I'm currently in the process of taking off the upper intake manifold, but I can't remove the fuel line quick connector. The FSM says there's a special tool, but wouldn't needle nose pliers work?

Apparently Nissans burn out fuel injectors more than any other car brand. Is that true?
 

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09 Altima 3.5SE 6MT Sedan
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The lifespan of Nissan injectors is good. Hundreds of thousands of miles and I have not replaced one yet.
A torn of missing ring sounds like it was apart at one time and was either not lubricated on install, or was installed improperly.
You must use the proper tool to remove the lines. Just like every manufacturer.
The oe injectors are Denso. I only recommend using the same.
I am going to put a set on my 05 in the spring. I believe one is worn and leaking after shutdown. This is at 320k miles though. Never been touched.
Priority 1 is to fix the fuel issue, once that is done make sure you do a idle relearn. Any time the throttle body is touched it must be relearned. Even the slightest change will make it idle erratically (usually high)
 

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I removed and replaced the throttle body and did the idle relearn procedure several times, and I'm getting incredibly high idle when the engine is cold (2000-3000 RPM surging). After the engine warms up, the RPM bounce between 1000 and 2000. Unplugging the MAF sensor drops and stabilizes the idle RPM. I cleaned the MAF sensor and replaced the PCV valve, but nothing changed. I am not getting any faulty MAF codes; however, I am getting (intermittently) misfire codes on two cylinders. Previously, I got P0507, but that issue seems fixed with a new PCV valve.

There certainly could be many other issues with this abysmally maintained, 250k mi. 2.5L '06 Altima.
A bit of history:
  1. air filter never replaced in about 5 years
  2. spark plugs corroded, and coils never replaced ever
  3. oil not changed in about 20k mi
  4. valve cover replaced due to spark plug well gaskets leaking
  5. corroded radiator replaced
Could there be fuel injector issues? I do seem to smell fuel around the engine when it's running.
From the sound of your problem, it's the idle air control valve, and/or a vacuum hose (that fuel injector sounds like a promising lead, too). The 5th gen Maxima's do the same thing when the IACV leaks, blowing a fuse and then fries a chip in the computer. No, not chips and French fries--fried transistor chip if I remember correctly. ;) You have to replace the fuse with a smaller capacity fuse, have a new chip put in the computer, and replace the IACV all at the same time. However, I doubt that has to be done with the Altima. It's probably a little different system. So the IACV and vacuum lines will probably do the trick. And fixing the misfire, of course, with some new coil packs and spark plugs.

I had to take off the upper intake manifold of my Maxima and use ATV on it to keep it from leaking when it's hot outside, causing the idle to go funky and idle out eventually. However, since it's not hot weather and your idle is going high, I'd say that's not your problem.

Good luck, man.

- Brian
 

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Discussion Starter #8
Could there be fuel injector issues?
The sides of the fuel injectors had tar stuck on them, and one injector had a glob of tar partially covering the nozzle. Their resistances ranged from 12.4 - 12.6 Ω. Is that acceptible? I'm thinking the ones on the misfiring cylinders were stuck on, leaking (not spraying) fuel.
 

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Discussion Starter #10
Update: I cleaned the gunk off the fuel injectors, tested them with a syringe and carb cleaner, replaced their o-rings, and re-installed them, and nothing changed. I still get the same surging issue.
Could the MAF sensor be bad? I don't get a code for that, though.

replace the IACV
Where is that on an Altima? The FSM doesn't mention it anywhere. Do 2006 and later Altimas have it?
 

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Discussion Starter #11 (Edited)
Nope, it wasn't the MAF sensor.

I'm think it's a throttle issue, or there's some vacuum leak I still haven't found.
My OBD program didn't register a throttle position change when revving. It went from 2.0 at rest to to about a constant 4½ V regardless the RPM being anywhere in the range 1000-3000.
 

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Discussion Starter #12
I replaced the throttle, and it made the engine sound smoother, but it's still doing the same surging.
I'm thinking it's an ECU issue. Tens of thousands of miles on restricted air-flow due to air filter not having been changed in 5 years seems to have messed up the ECU's learning process. There's something wrong with the air fuel mix, it seems. Doing the manual idle re-learns doesn't do anything.
I don't want to take it to the dealer. Can I rent a Consult-II somewhere?
 

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09 Altima 3.5SE 6MT Sedan
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Any codes? I wouldn't be surprised if the A/F O2 (upstream) sensor was contaminated/damaged by all that.
 

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Discussion Starter #14 (Edited)
Any codes? I wouldn't be surprised if the A/F O2 (upstream) sensor was contaminated/damaged by all that.
Yes, perhaps there is a bad air sensor, but I'm not getting any codes.
The O2 deltas were fluctuating in-sync with the surging, so I assume they're working OK.
 

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Discussion Starter #15
I did an air/fuel mixture relearn, and it helped make the surging less extreme, stabilizing a bit around a still-too-high ~2000 RPM idle. I followed it by an idle relearn, too, and it went back to extreme surging. The computer is thoroughly confused…
Is there anything else I should reset/relearn?
 

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09 Altima 3.5SE 6MT Sedan
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If you have the 3.5 did you unplug 2 injectors to do the idle relearn? It doesn't take if the initial idle is too high
 

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Discussion Starter #18
Any codes?
After having driven it about 60 mi., I get P0517 (high idle) and P0420 (catalyst efficiency below threshold // 3-way catalyst function).
I'm getting 37 MPG at 55 MPH, which is really good, so I'm thinking the high idle is still due to air volume or air-fuel mix learning not resetting (might need to go to dealer to have them reset this with their Consult device).
Does a bad catalytic converter cause surging?
 

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09 Altima 3.5SE 6MT Sedan
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No, a P0420 simply means your precat isn't behaving within parameters (or the 02 sensor isn't). It does not impact engine management in any other way. It's sole purpose is to turn on the CEL light to force you to fix it to pass your emissions test.
 
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