I just bought an 08 g35S sedan. It started making a rattling noise at low idle that keeps getting worse. It goes away when I increase rpms above low idle. But it’s there when idling once the engine settles off high idle when first starting. Then it seems to get worse the longer it runs. It has also thrown a P0420 code now.

Here’s what I have done to try and diagnose: sound is clearly low and aft engine area mostly on left side. It rattles the cats and so I put a “stethoscope” on the left exhaust and followed it up all the way to the manifold from where it seems to be originating. That is where it is loudest, at the left manifold, though I can also hear it some on the right manifold and that side has some of the same vibrating knock in it. I have done the rag in exhaust check when cold to see if there is a clear leak somewhere in the manifold. But I don’t see anything. Putting the stethoscope on the front of the engine sounded normal so I think that rules out timing chain area? Also top of engine sounds fine. When you come off rev it clatters in that area pretty badly then settles in with the rhythmic knock. Car runs good otherwise.

I would appreciate any help or ideas about what it may be. Was thinking cats bad at first but since sound is worse in manifold than cats I figure it must be up there. Is there anything in that area of the engine that could be causing this or is it most likely an exhaust manifold leak of some sort?

Regards.

Mark

Sounds like pieces of the honeycomb inside the cats broke off and are rattling around inside the housings. It's not all that uncommon on high-perf N/I engines, we see it pretty frequently on Z's.

Thank you for taking time to provide some feedback. I really appreciated it. That was my original thought but why would the vibration be stronger/louder upstream from the cat at the manifold if the problem was in the cat?

If it's a piece of front honeycomb that broke off, the back-and-forth exhaust flow from the IVT's Miller-cycling will keep the debris going back and forth. Even if you don't want to replace the cat, I'd drop it and remove whatever is in there ASAP, you don't want the engine potentially inhaling any of that stuff.

This seems like a good spot for a brief discussion of Miller cycling, for those who may not know. Many of you N/I owners may have noticed your engine has no Exhaust Gas Recirculation (EGR) Valve. The purpose of EGR is to meter a bit of "dead" exhaust gas back into the intake stream. Although the gas is hot, because it has almost no oxygen, the effect is actually to lower combustion temperatures. This prevents the formation of nitrous oxides (NOx), which form only at very high temps. To avoid needing an EGR, most N/I engines instead use a technique called Miller cycling, where the ECM uses the cam phasers to close the exhaust valves late and open the intakes early, making the engine "re-inhale" a bit of the exhaust gas it just forced out. It has the same good effect on NOx, but without all the hideous plumbing that accompanies EGR systems.

The drawback to Miller cycling is when a catalytic converter breaks apart, or worse, partially melts and vaporizes. If any of the honeycomb debris gets sucked back through the exhaust valve into a cylinder, the cylinder is usually instant toast. Anything that comes out of a cat will be very abrasive, and certainly scored cylinder walls are the least you can expect. Most engines that are victims of cat inhalation are waste-cases that head straight to the junkyard.

Here are two YouTube uploads that will give you and idea of what it sounds like and where it comes from. The first is at idle. The second is under engine rev.

Idle: https://youtu.be/UdAhrtZ2WL8
Rev: https://youtu.be/MKd7U6x2PbA

One mechanic thinks it’s in the engine and another thinks manifold and some on here thanks it’s the cat based on my description.

Irrelevant. You don't want to risk that it's the cat. If it is, step 2 is a new engine.

Thanks for your help and the explanation of the Miller-cycling. Very informative Just trying to do a bit more diagnosing to see if I can get some consensus among different experts before I start throwing money at it. But I get your point.

You're most welcome. You obviously like the car, I don't want to see you lose it over a "death rattle". Cat inhalation from folks ignoring cat rattles or HO2S engine lights is probably the single biggest cause of premature N/I engine death.

How hard is it to drop the cat and check? I’m decently skilled and can figure out most things but not worked on exhaust before. Can it be done on jackstands or will I need it on a lift?

Also, any particular recommendation for cat replacement if that should be needed?

Thanks again

I'm in a Nissan dealership and not Infiniti, but if it's the same as the Z's then the top stud needs to be got from the top with a very long 1/2" extension.