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Old 01-06-2011, 05:40 PM
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Diagnosing engine noise SBC 350

1978 k5 Blazer. 80-84 sbc 350 2bolt.

Drove round the corner, engine bay started to make loud rythmic scraping sound. Much lighter than a knock and with a high pitched tone to it. Seems to be coming from the front. Like something rotating and hitting on one point. Follows the revs, at about 2k is less loud. Immediately shut engine down checked oil. Needed a quart, pressure was good. Was 1.5 miles from home so drove home VERY carefully. No difference in performance etc.

Hoping it could be a waterpump bearing or fan bearing? (realistic/plausible?)

I'm hoping it's not a front cap and if it is that I can just pull the pan and replace it...

It's definetely not a knock, it's too scrapey and doesn't sound quite as deep. It's a real metal on metal kind of sound. It's not a flexplate either as it's very rythmic and follows the engine's revolutions.

Any ideas?

Might get a video up after the weekend when I've had time but would love to hear input.

cheers,

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Last edited by DQ81; 01-06-2011 at 05:45 PM.
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Old 01-06-2011, 05:56 PM
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Quote:
Originally Posted by DQ81
1978 k5 Blazer. 80-84 sbc 350 2bolt.

Drove round the corner, engine bay started to make loud rythmic scraping sound. Much lighter than a knock and with a high pitched tone to it. Seems to be coming from the front. Like something rotating and hitting on one point. Follows the revs, at about 2k is less loud. Immediately shut engine down checked oil. Needed a quart, pressure was good. Was 1.5 miles from home so drove home VERY carefully. No difference in performance etc.

Hoping it could be a waterpump bearing or fan bearing? (realistic/plausible?)

I'm hoping it's not a front cap and if it is that I can just pull the pan and replace it...

It's definetely not a knock, it's too scrapey and doesn't sound quite as deep. It's a real metal on metal kind of sound. It's not a flexplate either as it's very rythmic and follows the engine's revolutions.

Any ideas?

Might get a video up after the weekend when I've had time but would love to hear input.

cheers,
First place I'd suspect, given the age of the vehicle, is the timing chain is stretched out and hitting the timing case cover.

Bogie
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Old 01-06-2011, 06:14 PM
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Any of the accessories- alternator, P/S pump (check the fluid level first), A/C compressor, water pump could be making the sound.

Either use a mechanic stethoscope or a piece of fuel hose held up to your ear to listen to each component- including the timing cover, as Bogie suggested.

Be careful of the fan blade! You could also remove one accessory belt at a time, seeing if one stops the noise.

Obviously, run the engine w/o the water pump LAST, and just long enough to see if the noise stops, so the engine doesn't run hot.
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Old 01-06-2011, 06:29 PM
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How about a broken motor mount on the drivers side letting the fan hit the shroud when it torques up? Have someone brake it, put it in gear and some gas, see if it lifts.

I had a '76 Blazer that made a noise like that from the front hubs, a ratcheting in and out of gear sound while turning even a little, going down the road, I replaced them with manual hubs it went away.
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Old 01-06-2011, 06:51 PM
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Originally Posted by 68NovaSS
How about a broken motor mount on the drivers side letting the fan hit the shroud when it torques up? Have someone brake it, put it in gear and some gas, see if it lifts.
Thats what I was thinking too. Could be the clutch fan if you have one.
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Old 01-06-2011, 07:23 PM
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Take the belts of and run it. Has the balancer shifted?
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Old 01-06-2011, 07:24 PM
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Sorry Cobalt.
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Old 01-06-2011, 10:20 PM
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Check the timing tab, make sure it`s not touching the balancer.
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Old 01-07-2011, 12:15 AM
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My old 350 4 Bolt did that for the longest time. I took it to a mechanic and he told me it was just a sticky lifter and would only annoy me. Here I am six months later with a brand new 350 4 Bolt Engine. If it doesn't go away without the belts on, It might be worth pulling the oil pan and checking things out before that scraping turns into a knock, and then to a shot engine like mine. I pulled a lot of metal out of my oil pan when I did my swap. You might be able to stop it before it's too late.
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Old 01-15-2011, 06:02 PM
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thanks for all the responses.

I'm going to make a vid of it Mon/Tues and post it.

I was planning on removing the belts one by one to rule out accessories/waterpump and if that doesn't work pulling the pan. Good to know I was on the right track, never know what you might overlook.
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Old 01-15-2011, 06:38 PM
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Quote:
Originally Posted by DoubleVision
Check the timing tab, make sure it`s not touching the balancer.
I chansed on of those for a while
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Old 01-15-2011, 06:44 PM
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Originally Posted by T-bucket23
I chansed on of those for a while
What does that mean?
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Old 01-15-2011, 07:23 PM
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I'm assuming he meant "chased"?

My gut says it's a bad fan clutch, or that's what I'm hoping, though it was low on oil when the noise started

I'm really hoping the damage is limited/fixable. It's at the shop right now getting an estimate as some idiot drove straight into the side of my truck coming out of a parking spot. Got a downpayment on a sweet, solid '52 Chevy 3100 shortbed and the Blazer needs to go to make way for it's forefather and replacement

I've got a good spare 350 but I really don't feel like swapping them but who knows, if this one's chewing up a bearing it's a good excuse to tear it down and stroke it... either way fingers crossed!

I don't see how the tab could spontaniously bend and start rubbing btw? It's more of a scraping/grinding sound anyway, like if you were to pass one rock over the other...
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Old 01-16-2011, 07:22 AM
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Quote:
Originally Posted by DQ81
I'm assuming he meant "chased"?

My gut says it's a bad fan clutch, or that's what I'm hoping, though it was low on oil when the noise started

I'm really hoping the damage is limited/fixable. It's at the shop right now getting an estimate as some idiot drove straight into the side of my truck coming out of a parking spot. Got a downpayment on a sweet, solid '52 Chevy 3100 shortbed and the Blazer needs to go to make way for it's forefather and replacement

I've got a good spare 350 but I really don't feel like swapping them but who knows, if this one's chewing up a bearing it's a good excuse to tear it down and stroke it... either way fingers crossed!

I don't see how the tab could spontaniously bend and start rubbing btw? It's more of a scraping/grinding sound anyway, like if you were to pass one rock over the other...
You know, if you ever ran around a sharp turn and lost oil pressure due to low oil level you'd never forget that sound. It's like a high pitched wirrring sound. It will make you sick to your stomach and feel like hurling on the spot!
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Old 01-20-2011, 04:25 PM
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Allright. I took a quick video of the noise my engine is making tonight. Sound quality is pretty good but I'm a bit too close in and the fan is getting in the way slightly and other moving parts. But between 10 and 18 seconds you get a pretty good taste and towards the end as well.

I had the truck at my local shop who know these vehicles well to get an estimate on the damage done by some idiot, while it was there their mechanic had a listen and reckoned it was something like a pulley or even a cambolt loose and rubbing on the timing chain housing?

At the very least he concurred that it wasn't bottom end and it def. is coming from way up front in the motor.

Anway, would like to hear your guys ideas:

http://www.youtube.com/watch?v=NQWAARai1rM
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