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alex44

240z l24 rod knock - advice needed

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 https://youtu.be/4kStjnFagcU

 

so as shown on video cylinder 2 seems to have a nice rod knock, I drained some oil and noticed copper flakes in the oil so I'm 100% it's the rod bearing. Car was sitting since 1988 and we got it running yesterday to find this noise, oil looked new and was at proper level. So now I'm wondering two things, how did this happen and what is the best plan of attack? Replace all 6 of the rod bearings and put it back together? Disassemble and send the crank / block to machine shop then reassemble with new main, rod bearings, and piston rings?

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31 years is a long time for the motor to sit. Hopefully you at least swapped the oil out before cranking it over several times before actually starting the motor. Oil can take a long time to circulate especially if the passages are clogged.

 

It doesn't sound like knock to me. Although microphones are really variable. I would suggest running a compression test. I wouldn't be surprised if the valve seals or something has dried up and making a super loud ticking noise and having the compression gas leak past the valve seal or something. 

 

The copper in the oil depending on the amount could be telling. It would be hard to tell until you take the motor apart. But if it was rod knock you definitely need to take the whole block apart and have it cleaned, those metal fragments could get stuck in passages and cause problems depending on how much the bearing deteriorated. Not doing the piston rings while you are in there would be silly. Having the head machined and a valve job with new valve seals and seats potentially replaced with non bronze seats would be a good idea as well. Depending on the rod knock you might find damage on the rod itself from overheating or the piston from wobbling in the bore. 

 

There is a method to doing all this as you can find out information each step of the way. I'd start with a compression test though.

 

It could also be something silly like a blown out exhaust gasket or a cracked header. It is odd that the car is misfiring when you rev, makes it seem like there is more than one thing wrong like timing or firing order as well.

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Yes so if it was a rod knock wouldn't it keep knocking if the plug wire was pulled ? The rod is still moving ? Also mine was much louder . my engine sat for 30 years ran great for a couple thousand miles before the sludge broke loose and clogged the pick up tube 

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hey so it seems i might have jumped to conclusions too soon on this (hopefully). first of all the drain container we used to remove some of the oil out was very old / dirty so it is impossible to tell what was from our oil and others cars oil we are going to remove all of the oil today into a new container and see what comes out. the misfiring i think was from not enough choke it doesnt do that when it warms a bit and choke is off. compression is 150-150-165-150-160-150. i sent the video to a mechanic of mine (im not a mechanic , obviously lol) and he was sure it was a exhaust leak so we're in the process of replacing the exhaust gasket (if the oil checks out into a new container). first things first though i want to check the oil quality properly and replace it with new oil if i cant see anything weird in the old stuff. from videos ive been watching recently it just doesnt sound like a rod knock (that gross heavy metal on metal knock) sounds very very similar to a ram truck which has a leaking exhaust manifold. fingers crossed! also thanks for the replies i do appreciate it.

Edited by alex44

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Keep us updated. 

 

Like I had never heard rod knock in person, but when it happened, there was no question what it was. It physically hurt to listen to, but that may be my mechanical empathy.

 

Check the gasket, it will show carbon if it is still intact where it was possibly leaking. Look for cracks. Also look for warping on the manifold.

 

Last bit which is a bit rare is to measure the height on the flanges for the intake and the exhaust. If someone shaved the runner down at some point it might just be loose, you would need a stepped washer or to plane down the intake flange. 

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8 minutes ago, grannyknot said:

Those don't look too bad at all, as long as there is not a corresponding scratch on the journal, what does the other side of the bearing look like? Quite often it is the upper half of the bearing that takes most of the wear.

 

 

The upper half of the rod bearings had a bit more wear but nothing crazy, no shiny spots or copper colored patches they all looked like that. Lol so we've decided to cross our fingers and bolt it back up, just got the bearings torqued and put the oil pan back on after cleaning it until spotless, filled it up with oil and new filter next I got to extract 2 exhaust studs then we will put the intake/exhaust on and start it up.. wish me luck I'm praying to the z gods!

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oh wow sometimes i can be such a neanderthal i swear. Found the source of my supposed 'rod knock' ... a nice 1/8" hole under the #2 air smog pipe that connects all the exhaust runners on the manifold together, like i cant believe i didn't even stick my hand around that area to quickly check for a leak. Heh though i don't regret checking the rod bearings and i certainly don't regret cleaning the filthy oil pan, all's well that ends well :)
 

thanks everyone!

Edited by alex44
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