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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old June 11th, 2014, 02:56 PM
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FetchMeAPepsi FetchMeAPepsi is offline
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Default New Oil, Old look...new sound?

I just swapped out Cecilia's oil and the old stuff was dark as night, which I expected. But after running her for 100 yards the new oil is just as black as the old oil was. I assume that's just sludge and "yuck" in the motor coming free?

But now I have a sound kinda like a hammer on a board in the engine when she's running. It sounds like a quick bang, not crazy loud but a little faster than a heartbeat while she's running. Is it possible I broke something with an oil change?

It sounds like it's coming from the bottom of the motor, but that could just be because she doesn't have a muffler so most of her noise is from down there.

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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old June 12th, 2014, 12:25 PM
BarryGMC BarryGMC is offline
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Default Re: New Oil, Old look...new sound?

Crap. You sure dont want to hear anything like that. Its hard to diagnose with out hearing it.
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Old June 12th, 2014, 06:10 PM
George Bongert George Bongert is offline
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Default Re: New Oil, Old look...new sound?

Hey Fetch!

As Barry said, it's not a sound you want to hear, and hard to diagnose without hearing it firsthand. How's your oil pressure? It is possible to have a loose rod bearing, but that makes a "clattering" sound which increases in intensity when the engine is revved up quickly, and decreases in intensity when the throttle is released. A main bearing noise is noticed as more of a "thud" type of noise that would be more consistent with a steadily beating heart--think of it as a steady thump, thump, thump which will remain constant, and go up, or down depending on engine speed. You didn't break anything when you changed the oil. Get a good mechanic friend of yours to have a listen. He will be able to give you an idea what may be wrong with your engine.
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Old June 12th, 2014, 06:22 PM
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Default Re: New Oil, Old look...new sound?

Thanks Barry and George. All my good mechanic friends are here on this board

I'll record it this weekend but the second one you describe is what I'm hearing. It's not like a "bad gas" ethanol clatter but more like a deep down war drum kind of noise from the old westerns.
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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old June 13th, 2014, 03:37 AM
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Default Re: New Oil, Old look...new sound?

Yo Fetch. I was just thinkin' a couple of things. Since we can't be there to see it or hear it or relive what you did, I'd ask a few questions:

Was this the first time you had changed the oil in Cecilia since you've owned that truck? You said the oil was real black, which is fine, but when it drained out, was it oily and free moving or was it thick and gloppy?

You also said "running for 100 yards". Does it make the same exact noise when the truck was under power and moving during the 100 yards, as it makes when it just sitting still and idleing?

Can you tell if it is a thump that seems to happen at a regular paced interval and increases as the rpms of the engine increase or not?

We'll have to wait for the audio as that would tell alot I hope.

Foley.
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Old June 13th, 2014, 03:44 AM
Crosshairs6 Crosshairs6 is offline
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Default Re: New Oil, Old look...new sound?

I am not sure how you plan to record the noise but if I can make a suggestion, film the truck and put it on Youtube. Then attach the link here so we can click on it the see the video. I think that would be the best way for us to see/hear the problem. Take care Friend and good luck.
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Old June 17th, 2014, 04:11 AM
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Default Re: New Oil, Old look...new sound?

Quote:
Originally Posted by Foley View Post
Yo Fetch. I was just thinkin' a couple of things. Since we can't be there to see it or hear it or relive what you did, I'd ask a few questions:

Was this the first time you had changed the oil in Cecilia since you've owned that truck? You said the oil was real black, which is fine, but when it drained out, was it oily and free moving or was it thick and gloppy?

You also said "running for 100 yards". Does it make the same exact noise when the truck was under power and moving during the 100 yards, as it makes when it just sitting still and idleing?

Can you tell if it is a thump that seems to happen at a regular paced interval and increases as the rpms of the engine increase or not?

We'll have to wait for the audio as that would tell alot I hope.

Foley.
Hey Foley!
The oil was thick and black as sin, but oily and moving around.
The I can't tell if it makes it when it's moving. It's hard to hear inside the cab.
The thump is at a regular interval. I can't tell if it increases with RPM due to the loud sound Cecilia makes without a muffler. :/

Quote:
Originally Posted by Crosshairs6 View Post
I am not sure how you plan to record the noise but if I can make a suggestion, film the truck and put it on Youtube. Then attach the link here so we can click on it the see the video. I think that would be the best way for us to see/hear the problem. Take care Friend and good luck.
Roger that!
It is less loud now that I put the zinc additive in there. I went ahead and ordered the Archoil stuff too just to see if it changes things.


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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old June 20th, 2014, 04:16 AM
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Default Re: New Oil, Old look...new sound?

Can you guys hear it? I just got my archoil in and I'll be putting it in there tomorrow.
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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old June 20th, 2014, 04:25 AM
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Default Re: New Oil, Old look...new sound?

Pep I can hear it but its hard to say. I can't tell because its the same rpm. Things change under load and float and deceleration. Crap I wish I was close.
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Old July 18th, 2014, 03:16 AM
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Default Re: New Oil, Old look...new sound?

My 66 305E had a knock before we overhauled it. There were two pistons that had the skirts scored. I have a 70yr old friend that was a GMC mechanic back in the day of V6s. He said they were notorious for that, and had his own cure by lightly knurling the skirts for better oil retention.We knurled my new pistons and so far so good. I have a spare parts truck that did not knock until I changed the oil and it did the same thing as yours as soon as I did. Don't know why, but it sounds the same as the one with the scored pistons. Maybe a coincidence??
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