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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old March 25th, 2015, 04:26 AM
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Default Yikes!!! 305E V6 knocking like crazy!

And the funny thing is that it started happening right after I put a new set of plugs in! Here's a video...what ya'all think? https://www.youtube.com/watch?v=HyhgUvL-2wU
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Old March 25th, 2015, 04:34 AM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

If you swear on a GMC manual that you did nothing else !, then play with timing. If no cure, recheck that the plugs are the same and put old plugs back in to see a difference in sound. Obviously it is something you did and not an all of a sudden, coincidence. So backtrack and think. Hotter plug? Colder plug? wrong length plugs? moved timing? .... loosened manifold or crossover pipe?, high octane gas. Watch the temp gauge so as not to damage motor during trouble shooting. Need to replace items, tighten manifold bolts and re-time to get any kind of a change, this will lead to the remedy. Do one thing at a time and test or you will get confused/misled.
Be aware that a leaky exhaust manifold can make big knock sound. Did you check the heat riser flapper for noise?
Bottom line: Get a broom stick and hold it all around motor while up to your ear to isolate the noise if you can. Rich guys use stethoscopes

Last edited by AZKen; March 25th, 2015 at 04:56 AM.
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Old March 25th, 2015, 04:51 AM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Quote:
Originally Posted by AZKen View Post
If you swear on a GMC manual that you did nothing else !, then play with timing. If no cure, recheck that the plugs are the same and put old plugs back in to see a difference in sound. Obviously it is something you did and not an all of a sudden, coincidence. So backtrack and think. Hotter plug? Colder plug? wrong length plugs? moved timing? .... loosened manifold or crossover pipe?
Be aware that a leaky exhaust manifold can make big knock sound. Did you check the heat riser flapper for noise?
Bottom line: Get a broom stick and hold it all around motor while up to your ear to isolate the noise if you can. Rich guys use stethoscopes
Here's what I've done...New distributor cap, coil, plug wires, plugs, oil change, oil filter, and engine restorer, also a fuel filter. Thanks for the response. I'll pull the new plugs tomorrow and see if things clear up. I did have the autolights crossed over to NGK's and cross references have been known to be a couple heat ranges off. Timing has not been touched. Manifolds have not been touched. Haven't check the heat flapper for noise, the sounds seems to be loudest at the rear of the engine. I think if the manifolds were leaking it would have knocked when I first got it unless somehow it blew the manifold gasket while idling. lol I'll do some inspecting tomorrow. Thanks.
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Old March 25th, 2015, 05:01 AM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Changed oil and forget to put new in? Just kidding. Check plug cross reference online till you find three sources that all agree. FLAPS men can be very unreliable.
FLAPS= Friendly Local Auto Parts Store. You are on the right track, don't take anything for granted. Could be detonation or pre-ignition for sure if you say no exhaust leak. If you have the old plugs, put them in for sure, that's a quick check. Did you compare the old to new plugs as far as physically?
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Old March 25th, 2015, 05:22 AM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Yep I put new oil in. lol 6 quarts. I'll have to double check tomorrow. I'll be darned if I put the old plugs back in and it runs fine! Guess that'll mean another run back to Napa for Autolight plugs if that's the case.
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Old March 25th, 2015, 06:46 AM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Good job. Great result. "Plug" on.
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Old March 25th, 2015, 07:52 PM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Well I have no idea what I did but the engine decided to quit knocking all by itself. LITERALLY. I pull all the new plugs and put the old ones back in...still knocks...pull both valve covers to make sure everything is moving and nothing is bent. Has a little more sludge in there than I'd like to see.....Ran it till she got hot and pulled the radiator cap to see if I had a blown head gasket...no bubbles....now it only knocks on deacceleration.....so you rev it and while it comes back down to idle it knocks just a little now....with the old or new plugs. So the plugs aren't the issue. The knocking from deacceleration sounds like a very loose rocker arm but its not. Hard to explain what it sounds like and the camera doesn't help either. I'm starting to think one of the lifters is hanging up a little. Thoughts? Knocks when the engine comes back down to idle..sometimes.
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Old March 25th, 2015, 08:24 PM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Good information to help anybody who is doing basic troubleshooting. Thanks

Foley
Greenwood, Ca
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Old March 25th, 2015, 08:28 PM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Quote:
Originally Posted by TJ's GMC View Post
I'll be darned if I put the old plugs back in and it runs fine! Guess that'll mean another run back to Napa for Autolight plugs if that's the case.
Well you fooled me with that one, thought you were done. I guess my next guess is to guess..............................Rod Knock. There is an easy test for that. You may want to take off one plug boot at a time while motor running to see if it quits. That may not be possible if it's not knocking all the time. Anyway the test for rod knock is on YouTube. I think valve train noise is a little more "ticky tacky" than your noise. Yours is a hard knock. But it is hard to tell from my house.

Last edited by AZKen; March 25th, 2015 at 08:34 PM.
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Old March 25th, 2015, 08:32 PM
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Default Re: Yikes!!! 305E V6 knocking like crazy!

Quote:
Originally Posted by AZKen View Post
Well you fooled me with that one, I guess my next guess is to guess..............................Rod Knock. There is an easy test for that. You may want to take off one plug boot at a time while motor running to see if it quits. That may not be possible if it's not knocking all the time. Anyway the test for rod knock is on YouTube. I think valve noise is a little more "ticky tacky" than your noise. Yours is a hard knock. But it is hard to tell from my house.
I strongly second this.
I had a knock, it wasn't bad so I ignored it. At 55 mph the knock stopped, so did the truck. I had dropped a rod cap, the knocking was the piston slapping the head (had a nice imprint of the valves on it). BIG mistake to ignore any noise, or "get used to it" as I did.
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