Questions about potentially bad rod bearings
09-11-2008 | 08:29 PM
#1
Thread Starter
Joined: Jan 2005
Posts: 2,928
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From: Mooresville, NC
Questions about potentially bad rod bearings I just picked up a '97 manual corvette, 63k miles with a light knock.
It sounds like rod knock to me. But before I go digging, I'd like a little help. I think it is a rod bearing because of the sound - and also because the noise is more distinct when the car warms up than when it is cold. It's never really loud, but more noticeable. The worst is when you rev it up in neutral - 1800rpm it knocks like crazy. Does not seem to come from one side more than the other. Did not sound awful from underneath either.
1) Does a bad bearing always leave trash in the oil and filter? I changed the oil in it tonight. It was black and thick, but not metallic at all. I cut apart the oil filter and it was super clean. Not a speck of metal anywhere. No shards, shavings or needle bearings/etc came out of the oil (I strained it through a small screen...nada).
2) What effect on oil pressure does a bad bearing have? It came to me with a bad oil pressure sending unit (gauge is maxed out). I'll replace it this weekend. What kind of changes in oil pressure would hint towards a bad bearing?
I was wishfully thinking it may be a bad flywheel or something in the clutch/pressure plate. How common is that? The noise seems to disappear altogether if you get into it. It does not run poorly. The scanner didn't show any misfire codes or record any misfires (so I think that eliminates valvetrain...)
Any other suggestions are appreciated. I was sure that the oil/filter would tell me more about the problem, but now it's not so clear.
It sounds like rod knock to me. But before I go digging, I'd like a little help. I think it is a rod bearing because of the sound - and also because the noise is more distinct when the car warms up than when it is cold. It's never really loud, but more noticeable. The worst is when you rev it up in neutral - 1800rpm it knocks like crazy. Does not seem to come from one side more than the other. Did not sound awful from underneath either.
1) Does a bad bearing always leave trash in the oil and filter? I changed the oil in it tonight. It was black and thick, but not metallic at all. I cut apart the oil filter and it was super clean. Not a speck of metal anywhere. No shards, shavings or needle bearings/etc came out of the oil (I strained it through a small screen...nada).
2) What effect on oil pressure does a bad bearing have? It came to me with a bad oil pressure sending unit (gauge is maxed out). I'll replace it this weekend. What kind of changes in oil pressure would hint towards a bad bearing?
I was wishfully thinking it may be a bad flywheel or something in the clutch/pressure plate. How common is that? The noise seems to disappear altogether if you get into it. It does not run poorly. The scanner didn't show any misfire codes or record any misfires (so I think that eliminates valvetrain...)
Any other suggestions are appreciated. I was sure that the oil/filter would tell me more about the problem, but now it's not so clear.
09-12-2008 | 07:47 PM
#3
doesnt sound like rod knock from what your describing, i had it on my first motor in my 99 subaru and ive changed a few motors due to it (cousins supra had it really bad, never heard a motor knock so much without losing the rod) , shavings were always present and it would get louder as RPM's went up ( til the rod went through the block 
)
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09-12-2008 | 08:15 PM
#4
This just happened to me recently. It totally sounded like a rod knock at first. Turned out to be a broken valve spring. Pull your valve covers and take a look. I was lucky enough not to hurt anything and a new set of Patriot Golds fixed everything.
