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symptoms for bad rod bearing

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Old 12-18-2010, 02:57 PM   #1
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symptoms for bad rod bearing

i bought a 92 with the 4.3 motor (90k miles original) in it for $900 bucks. the guy said he thinks it has a spun rod bearing. everything else on the truck is in great shape so i got it with the intention of 350 swap around the
summer.

what are symptoms of bad rod bearings. it gets about 40 psi oil pressure when driving. and it runs kinda rough, just thought it could use a tune up. what should i be looking for before pulling the motor. id like to use it as a work truck for awhile if the motors okay
Old 12-18-2010, 03:16 PM   #2
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Re: symptoms for bad rod bearing

Well the most obvious symptom is a knock.

If it's knocking, while running pull the plug wires one at a time. When you get to the cylinder with the bad bearing the knocking will be much less.
Old 12-18-2010, 08:17 PM   #3
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Re: symptoms for bad rod bearing

ok. planning on changing the oil tomorrow to check it and cut the filter open to see how material is in it.

as far as the knocking. you cant hear it unless you rev it up from the throttle body. it is not noticeble from the cab when driving. ill try the plug wire thing too. thanks
Old 12-18-2010, 08:21 PM   #4
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Re: symptoms for bad rod bearing

What's the oil pressure like? You sure it just isn't an exhaust leak or loose exhaust manifold bolts? Just throwing it out there.
Old 12-18-2010, 09:33 PM   #5
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Re: symptoms for bad rod bearing

I bought a truck that had a bad rod bearing and it had no oil pressure at idle but if you revd it up it would build pressure. I think as long as you have oil pressure through all rpms your bearings are probably good. It could just be a noisy top end like lifters or something. just my 2 cents.
Old 12-18-2010, 09:45 PM   #6
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Re: symptoms for bad rod bearing

Could be carbon build up also, might seafoam the top end before you change the plugs and such.......
Old 12-19-2010, 10:11 AM   #7
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Re: symptoms for bad rod bearing

Quote: Originally Posted by joyride
What's the oil pressure like? You sure it just isn't an exhaust leak?

The way you describe it, that is my bet
Old 12-23-2010, 10:56 AM   #8
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Re: symptoms for bad rod bearing

I had perfect oil pressure all rpm range. had 3 rod bearings and 2 main bearings spun + 2 rod bearings very bad.

I had bad noise from my engine.
Old 12-24-2010, 01:16 PM   #9
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Re: symptoms for bad rod bearing

Quote: Originally Posted by BBs-10
ok. planning on changing the oil tomorrow to check it and cut the filter open to see how material is in it.

as far as the knocking. you cant hear it unless you rev it up from the throttle body. it is not noticeble from the cab when driving. ill try the plug wire thing too. thanks
I didn't see anything in the posting about what transmission you have. If it's an automatic then i would check the flexplate around where the bolts hold it to the back of the crankshaft.

My daughter's '91 w/4.3 had what I would call "an intermittent knock". I thought "great, rods and/or mains".

Was talking to a friend of mine one day and he suggested checking the flexplate. Dropped the inspection cover unbolted the torque converter and slid it back, sure enough, cracked dang near all the way around between the bolts. Took around 2 hours to change and no more knock.

This is only my 2¢. YMMV.
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