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What is this noise?


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#1 Milenko2121

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Posted 06 December 2016 - 06:09 PM

My car recently developed this noise after 2 laps on a tight autox track, (2nd gear entire time). At first I thought it was knock, but I pulled the plugs in pairs (1/6, 5/2, 3/4) and the noise didn't change at all. Car appears to idle and drive fine.

 

https://youtu.be/l2alL4qf-ho


Edited by Milenko2121, 06 December 2016 - 06:20 PM.

1977 280zt with '87 Z31t ECU, CLSD and S14 SR Transmission


#2 Zetsaz

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Posted 06 December 2016 - 07:46 PM

Sounds like you spun a connecting rod bearing? Possible oil starvation during the laps? Did you ever hear any light clicking noise or tapping noise prior to this? 

 

If it's not a knock, that's what's most likely, especially if it's matching engine acceleration, unless I'm missing something. If I'm right, the tapping is coming from the connecting rods (any 1 or more that spun a bearing) tapping back and forth on the crank. I would drop your oil pan and check for any metal, either in chunks or shavings, that would have come off the bearings. 



#3 McGruber21

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Posted 06 December 2016 - 07:50 PM

Particularly hard turns on this track? Did you happen to notice anything peculiar about your oil pressure while on the track, ie, low pressure than gradual build up after a  hard turn or a hard acceleration or deceleration? If you have a stock oil pan, the oil will slush around and could cause oil starvation to your oil pump pickup. although a couple laps shouldn't cause engine failure due to oil starvation but on a clapped out engine it could accelerate any existing engine problems. The exaggerated behavior on the track will shine light on any gremlins hiding in a daily driver Datsun, so the noise can originate anywhere on a 40year old car, just comes down to pin pointing the noise specifically. 

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#4 Milenko2121

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Posted 07 December 2016 - 10:04 AM

I haven't dropped the pan yet, but I did take a stethoscope to the motor and the noise sounds like something hitting the valve cover above cylinder 1. My timing chain had a decent amount of slack on the side opposite of tensioner, but I don't know how much is excessive, I could get a lot of deflection out of it(did not test to see if I could pull chain above sprocket, but will later)

 

Oil pressure at track (from what I paid attention to) was normal, and it was the normal at idle and driving it home.

 

As for rod knock, isn't a test for this to pull your plugs in pairs and see if the noise changes and if it doesn't, it's not the rods?


Edited by Milenko2121, 07 December 2016 - 10:10 AM.

1977 280zt with '87 Z31t ECU, CLSD and S14 SR Transmission


#5 Miles

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Posted 07 December 2016 - 03:48 PM

I have had exhaust header leaks that sound just like that.  But then I have also had a wrist pin pull out of the piston and make the same sound.  Hope it is just the former.


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#6 Leon

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Posted 10 December 2016 - 01:06 PM

Pull your valve cover and inspect. Also send your used oil in for analysis, I use Blackstone Labs.



#7 rundwark

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Posted 18 December 2016 - 12:52 AM

Could still be rod knock.

I had a very similar noise on a built L16 in my 510. Also didn't go away with various plugs disconnected, but on closer inspection, did change very slightly with plug #3 disconnected.

It still made a bad noise without the plug because the piston was moving further up than usual (due to the pulverized rod bearing) causing it to hit the spark plug and top of the combustion chamber. It sounded like the piston was banging against the cylinder head, which it turned out it indeed was.

https://youtu.be/UXSmWmSGa3E

We tore apart the engine, and #3 rod bearing was gone.

Edited by rundwark, 18 December 2016 - 12:54 AM.


#8 rundwark

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Posted 18 December 2016 - 12:56 AM

Also, for rod knock, don't pull your plugs in pairs. Pull them one at a time as each piston has its own rod. So pulling one at a time changes the load on the bearings one at a time. Pulling in pairs is used to determine if you spun a main bearing.




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