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Old 12-26-2003, 01:09 PM
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Engine Knocking Noise

I've got a 1988 Chevy pickup with a 350 engine that has a strange knocking sound when the engine warms up. The motor sounds great when the engine is cool but when it heats up to about the point when the thermostat opens up it begins knocking regardless if the truck is just idling, in gear, or going down the road. I have checked the valves thinking they may need adjustment but have determined that the knock is deeper than that. (it sounds like it is coming from a lower part of the motor). It has good oil pressure and is not buring oil or anything else. It still runs good and has a lot of power. From this description of the problem, what would you suggest that I do to try to determine/fix the problem? Do you think it is a rod knocking and if so, is it rebuild time?

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Old 12-26-2003, 02:40 PM
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Sure sounds like a bearing knock to me. It's quiet when it's cold cause the oil is thicker. No way to fix it but take it out and check it out. Usually you'll find a lot more will need attention once you tear it down and inspect it.
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Old 12-26-2003, 02:45 PM
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duxjak7,

I have to agree with M.S. Tom, it sounds like a bearing.
Pull one spark plug wire at a time until the noise goes away. That is the one that need the MOST attention.
Good Luck
scholman
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Old 12-26-2003, 02:53 PM
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Pull ALL the wires off and ALL the noises will go away!
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Old 12-26-2003, 03:07 PM
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Is it possible that the problem is an exhaust leak or would an exhaust leak occur regardless of the tempature of the motor?
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Old 12-26-2003, 03:11 PM
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That would sound more like a clicking or snapping noise than the sound you described.
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Old 12-26-2003, 03:46 PM
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Quote:
Originally posted by duxjak7
Is it possible that the problem is an exhaust leak or would an exhaust leak occur regardless of the tempature of the motor?
The second part of your statement is true. It sucks, I know, but it's time to yank the motor and throw in new bearings.

I had a friend that had the front bearing go out because he let it idle on his VERY steep driveway. At least that's what he said had happened. Anyone think there's any truth to that?
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Old 12-26-2003, 03:58 PM
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Quote:
but it's time to yank the motor and throw in new bearings.
Is it possible to check and change the rod bearings from underneath the vehicle with the engine still in the truck?
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Old 12-26-2003, 04:04 PM
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I wouldn't even attempt that...it may be possible, but would add a couple more days to the process....
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Old 12-29-2003, 10:07 AM
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Well I put the truck up on the lift yesterday and replaced all of the rod bearings (none of them looked very bad), checked that the main bearings were ok, and replaced the oil pump while I was at it. I put the oil pan back on and filled her full of oil, cranked her up..... no oil pressure. DOH! I only let it run for a couple of seconds hoping the oil pressure would rise all the while the engine was making a horible rattling / knocking sound. So now it's back up with the lift to remove the oil pan to see if the oil pump didn't engage properly or something.

Still I'm concerned that nothing I did yesterday was the solution for tthe knocking problem that I was after. About a week ago I changed the oil in the truck and when I went to crank it up for the first time it made a very similar knocking / rattling sound then. With all the bearings in good shape, even with new rod bearings, I have a feeling that after I get the oil pump issues solved (if that is even what is wrong with the oil pressure) that this will not solve anything.

This is becoming a project from hell.
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Old 12-29-2003, 02:52 PM
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Might be a pin or loose piston tolerances. But these sounds will come from higher in the engine and often represent themselves as a higher pitched sound.
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Old 12-30-2003, 03:49 AM
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If its an auto trans, check the flexplate. I've seen cracked flexplates that sound just as bad as bearing knocks. Take a flashlight and after removing the shield look closely for signs of rust-apppearing streaks. Those streaks are usually cracks. They originate at the mounting holes, and travel outward to the larger holes, Ect...
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