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Trailsearcher

Mystery Rattle VG33

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I've been dealing with this for weeks! Ripped off all the heatshields except the left bank one and both cat shields. Its a faint rattling noise between 1-2k rpm when I floor it (lug it) in 2nd, 3rd and 4th. Yes I know lugging is bad but, the sound doesnt sound normal. Goes away instantly between 1.8 and 2k rpm and on. Also, it only makes the noise under load, seems to be a resonance thing. I originally thought it was the tranny but it was rebuilt only 40k miles ago. Also, if it was the tranny wouldn't it be constant? Since my I already ripped off the shields my next guess would be cats? My theory is that the honeycomb inside is corroded and clogging the system causing rattling until the pressure buildup holds everything down. How common is cat rattle? The guy who sold it to me did say it failed smog once and passed after changing the o2 sensor. They look original. Also, I've never driven a vg33 before but I have driven a vg30 in a 300zx. This seems sluggish compared to it. But also I'm running 32" tires and this tranny wasnt geared for acceleration. Do the larger tires make that big of a difference? I feel like the engine loses torque after 3k rpm. Is this the normal powerband? Removing those cats look like a pita, please tell me I'm wrong.

 

My second guess would be pinging? But, I would assume detonation would sound more violent.

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I couldn't tell you which one it is but predetonation does not need to sound violent. I could get my VQ to softly ping, like a regular cadence rattle, in a manual by flooring it in 2nd or 3rd gear from around 1,000 RPM. I figured this was because the knock sensor had pulled out all timing. In some engines it can sound more like a hard knock than a ping, but I haven't heard one I'd call violent.

As a test have you tried a tank of premium to see if it's better? Could also try an octane booster bottle but I don't know if I trust them to raise the octane as much as they say.

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I havent tried premium fuel yet. I'll try it and see what happens.

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I would check to see if you are getting the knock sensor code or if the previous owner installed a resistor in lieu of a knock sensor (you would not get any codes but then the ignition would not be retarded either) to get it to pass the smog test.

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16 hours ago, CALPATHY said:

I would check to see if you are getting the knock sensor code or if the previous owner installed a resistor in lieu of a knock sensor (you would not get any codes but then the ignition would not be retarded either) to get it to pass the smog test.

 

I already checked for codes, nothing so far.

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If it's a no longer functioning knock sensor many times you won't get a code. Premium gas should substantially reduce the pinging but may not eliminate it. If it helps, I would test the knock sensor or just replace if it is cheap. If premium doesn't do anything then I'd probably look elsewhere.

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I thought the knock sensor on the vg33 doesn’t actually pull time but just throws a code when it detects knock.

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I don't know. I've never heard of a car do that. And I don't have the service manual for a VG to look it up.

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Roll under the truck with the exhaust cold and inspect the clamps, whack various parts of it with your fist, see if anything rattles. That's how I found a loose exhaust clamp on mine that was only rattling under load. I assume the engine was rocking on its mounts just enough to unload that clamp. It's also how I found a bad cat on my friend's Blazer. The matrix inside had broken free of the walls and beat itself undersized, and it was rattling around. That was a much more expensive fix than mine.

 

Big tires don't help acceleration. If you've got a manual trans, you have a 4.3 final drive, which you could upgrade to 4.6 with parts from an automatic to offset the tire size increase. The 33 also has less aggressive cams than the 30.

 

You can download the service manual here.

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Thanks for the advice everyone. I threw some injector cleaner and premium fuel in there. I only drove a few miles so far so I don't expect any results yet. I have an original hard copy of the FSM so when I have time I'll do the electrical checks for the knock sensor. It seems the previous owner bookmarked the knock sensor electrical check, ecm and different obd codes. Unfortunately no notes. I wonder if he was chasing this problem before me or if the sensor has been changed. Lastly I'll look under the truck for any loose clamps or a bad cat. I tried banging on the cats but the mounts were too solid and nothing moved. Any suggestions? I heard a jackhammer to the frame works but who keeps a jackhammer in their garage? Don't answer that... At this point I'm really leaning towards the cat being corroded or the transmission being damaged but I will try to eliminate the knock sensor first. The fact that the rattle goes away after 2k rpm is really throwing my diagnosis off though. 

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Quite possible you have cats that are rattling. If one or both of the upstream cats are starting to rattle, they are not going to last much longer and will start throwing P0420 and P0430 codes. 

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I have a buzz in my '99 that happens between  1,200ish to 1,800ish. I haven't looked and just ignore it when I can't WOT and then just idle... :shrug:

(Yes, I an certain it is from the exhaust)

 

B

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Trailsearcher, is your truck a manual trans? If so, it might be the clutch damper that is mounted between the clutch master cyl and the slave on the firewall. A few years ago when I was working at a Nissan dealership, one of my coworkers was going insane trying to track down a similar rattle in an Xterra. It took weeks for him to find it and he was going through everything until he somehow found it to be the damper. 

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4 hours ago, Mr_Reverse said:

Trailsearcher, is your truck a manual trans? If so, it might be the clutch damper that is mounted between the clutch master cyl and the slave on the firewall. A few years ago when I was working at a Nissan dealership, one of my coworkers was going insane trying to track down a similar rattle in an Xterra. It took weeks for him to find it and he was going through everything until he somehow found it to be the damper. 

It is a manual... I'm checking this first thing in the morning.

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I think Sam unbolted it from the firewall and out a pad between it and the firewall or wrapped it in a rag to help isolate to see if it was the source of the noise. 

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Knock Sensor is good, all electrical checks passed. I had 91 in there for the past week. Nothing. Tried to bang on the exhaust while cold, no rattle anywhere. The motor mounts and headers are so solid I can't shake anything to get a rattle out of it. Srsly considering renting a small jackhammer to put to the frame and reproduce the rattle. But since it only makes the noise under load who knows it that'll work? I also cleaned the afm and threw some injector cleaner in there. Nothing. Checked the damper, looks solid. I banged on it and nothing. I guess I can unseat it and wrap it up like you said, but I doubt it's the culprit. I'm leaning towards tranny or cats. The sound goes away so it's resonant, I'm guessing it's the cats. It would explain the sluggishness and inconsistent acceleration once in a while when I'm at WOT. I'd call it a very slight hesitation every 5-10 pulls in the powerband between 2-4k. Gotta wait a couple weeks before take it to my guy so he can fab up some test pipes. It looks pretty tight in there, anyone ever change cats? I wonder if it's straightforward or if things need removing. Extra theory... Exhaust leak at a flange causing the gasket to flap around at low backpressure. Couldn't see anything though, but that doesn't mean it's not leaking.

Edited by Trailsearcher

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Side note, exhaust smells really rich sometimes at startup for a min or 2.

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