2008 having with a problem that can't be found.

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ShipFixer
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Location: Arlington, VA

Re: 2008 having with a problem that can't be found.

Postby ShipFixer » Tue Jun 30, 2020 7:49 pm

Your mechanic's line of thinking is what leads people down the road of chopping up the insides of their airboxes for no good reason and other stuff like that. I'm curious why he replaced the timing chain? Hopefully not for a lumpy idle?

The intake is not too small. If the intake were sonically choked, it would impact wide open throttle, not idle. Think about how the throttle works: closing off air to maintain stoichiometric combustion. That butterfly valve is pretty nearly closed at idle. I do believe the intake manifold has too little volume. But that impacts throttle transients, where there is a sudden increase in vacuum. That's why the spacer works, or why I think it works. Doesn't mean Nissan designed it wrong, as there are other characteristics in play. But that's different than the intake being too small.

Anyway, "rough idle" is subjective, but I believe my V-6 is really smooth at idle and I have a much older truck (182K miles) with polyurethane motor mounts.

I'd consider the MAF sensor as mentioned above. Another thing you can do is try the idle air re-learn yourself. It can be done with a pedal dance, but I have an easier way. Look for an app called Nissan DataScan II in the Android or iPhone app store, and get an OBD II Bluetooth dongle with an ELM327 chip in it. Aside from being able to read Nissan specific ECU data, it can also perform the idle air re-learn at the push of a button. That does smooth out the idle a bit, especially if you've changed a bunch of things like plugs lately. If this was a non-Nissan dealer, they probably didn't do it.


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ShipFixer
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Re: 2008 having with a problem that can't be found.

Postby ShipFixer » Tue Jun 30, 2020 7:51 pm

Just one other hunch, have you tried switching your A/C compressor on and off? As mine was dying slowly over years, when it was hot outside and the compressor was working hard, it was a pretty big load on the engine. I would notice that at idle, and would definitely feel like what you're describing. Since I just got mine fixed, I have to pay attention to notice the change in engine tone, etc. Big difference.

I don't know why, but Nissan A/C compressors seem to be more noticeable that way :?

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smj999smj
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Re: 2008 having with a problem that can't be found.

Postby smj999smj » Wed Jul 01, 2020 1:32 am

I went to a Nissan training class many moons ago and the instructor told be that Nissan sets their engines up to run slightly on the lean side at idle to help them pass emissions standards, which makes sense. To be honest, I think almost all of the Nissan engines run a little rough at idle. You can smooth them out a little if you bump the ignition timing advance an additional 3-5 degrees over stock and raise the base idle 50-75 RPM over spec. On the VQ40DE, you would need a performance tuner or a capable scan tool to adjust the base idle and ignition timing.

jlocal
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Re: 2008 having with a problem that can't be found.

Postby jlocal » Sat Jul 04, 2020 5:28 pm

Thanks for all the info. First, I would never cut or modify anything on this truck. I'm just trying to make it as dependable as I can, and yes, he did bump the idle up. So far so good.


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