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Old 04-09-2013, 09:25 PM   #1
jdl71
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5.3 knocks when warmed up

I recently purchased an '05 z71 5.3, 204,000. Had a noisy tensionser and water pump, replaced those. Also had a code on for knock sensor fault, so I pulled the intake, replaced both knock sensors and connectors. Now the engine has developed a knock, sounds like it's in the bottom of the engine and similar to a rod knock, but it won't do it until the engine is up to temperature. I also don't hear the usual "flutter" or whatever it's called when you rev and let off like a rod or bearing will do. I've been all over it with a stethoscope, and can't seem to get it pinpointed, other than it does seem louder on the oil pan than on the top of the engine. I can also hear it better on the right exhaust manifold than the left, can hear it right where the bellhousing meets the block, but not so much under the torque converter. It also will go a way if driven at highway speed for a short time, then returns after idling for a minute or two. I've pulled each plug one at a time to kill each cylinder, no change. I'm really at a loss here, maybe a wrist pin? It does increase with engine speed while revving the engine when parked, putting in gear, holding the brake and revving seems to make it go away although it's hard to tell for sure as it's not really that loud. Two other things of note, when I pulled the intake there was oil in it, however I've read this is a somewhat common problem with the PVC getting plugged or valve cover baffles getting plugged up. Occasionally when first started I will get "oil pressure low" on the display, but the gauge reads fine, and no abnormal noise. Other than that the truck runs wonderfully, and I've always thought the LS motors would run forever with basic maintenance. I don't know the truck's history for sure, but it was a one owner vehicle when I bought it so I would think whoever had it that long would have taken decent care of it as the body and interior and immaculate.
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Old 04-09-2013, 09:37 PM   #2
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Re: 5.3 knocks when warmed up

My money would be on a cracked or loose flex plate bolt.
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Old 04-09-2013, 09:40 PM   #3
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Re: 5.3 knocks when warmed up

I haven't ruled that out, but why would it not do it all the time? And do you mean flex to crank or torque to flex?
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Old 04-09-2013, 09:46 PM   #4
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Re: 5.3 knocks when warmed up

I would cut open oil filter and inspect element for glitter.
If glitter and you'd consider rebuild then I'd park it and continue inspection.
I would think that a loose rod bearing or wrist pin would sing a different note w/ deleted spark.
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Old 04-09-2013, 09:54 PM   #5
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Re: 5.3 knocks when warmed up

I would check both, flex to crank and flex to torque, You might be able to see the cracks, fractures. If not then run it until it knocks and get underneath and see if its hitting the bell-housing (wobbling) also look for rubbing. As far as it not doing it all the time my guess would be that when it heats the metal, it expands and somehow causes it to hit the bell housing. Almost like an exhaust leak, when cold no noise, when hot noise......Oh check for any exhaust leaks to near the collector
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Old 04-11-2013, 06:00 PM   #6
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Re: 5.3 knocks when warmed up

My sons 04 5.3 had a noise you would have swore was a rod. Raced the motor up several times in a row and the noise went away, let it idle for a while and the noise came back , sounded like a rod or broken flexplate to me. Turned out the catalytic convertor had come apart inside , cut it open , gutted it and welded it back shut .......NOISE GONE ....... Swore we were gonna have to put an engine in it. Something to think about ....!
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Old 04-11-2013, 07:10 PM   #7
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Re: 5.3 knocks when warmed up

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My sons 04 5.3 had a noise you would have swore was a rod. Raced the motor up several times in a row and the noise went away, let it idle for a while and the noise came back , sounded like a rod or broken flexplate to me. Turned out the catalytic convertor had come apart inside , cut it open , gutted it and welded it back shut .......NOISE GONE ....... Swore we were gonna have to put an engine in it. Something to think about ....!
I have considered that. I've beat on both cats with a hammer to see if the noise changed, nothing. I also put my stethoscope on them while I was under there and didn't really hear the knock as loudly as I do closer to the front. I do hear it pretty clearly on the exhaust manifold though, so I haven't ruled out the cats just yet. Did your noise increase with engine speed until it went away? Mine does, that's what's making me lean towards something loose in the rotating assy, but I'm sure hoping that's not the case.
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Old 04-11-2013, 09:45 PM   #8
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Re: 5.3 knocks when warmed up

Dead lifter? The oil passages are tiny and get plugged easily.
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Old 04-11-2013, 09:58 PM   #9
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Re: 5.3 knocks when warmed up

I had a 98 Blazer with a 4.3 vortec started doing this at around 100,000 miles, I tried all the above except the catalytic converter, kept on knocking. I put another 100,000 miles on it then sold it in 2007 & last I heard it was still running and still knocking. I would have loved to known what the knocking was but I couldn't justify tearing it down while it was running so good otherwise.
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Old 04-11-2013, 11:29 PM   #10
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Re: 5.3 knocks when warmed up

I have a 2006 Silverado with the 5.3 with 70000 miles that had similar symptoms. It would only knock once warmed up. I put 30000 miles on with it running fine just with a knock. I finally pulled the pan, looking up at it you could see that one of the lifters had gone out and it had wore the cam lobe bad.
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Old 04-12-2013, 01:18 AM   #11
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Re: 5.3 knocks when warmed up

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I have considered that. I've beat on both cats with a hammer to see if the noise changed, nothing. I also put my stethoscope on them while I was under there and didn't really hear the knock as loudly as I do closer to the front. I do hear it pretty clearly on the exhaust manifold though, so I haven't ruled out the cats just yet. Did your noise increase with engine speed until it went away? Mine does, that's what's making me lean towards something loose in the rotating assy, but I'm sure hoping that's not the case.
we beat on the cats also, didnt change anything.Used stethescope also , couldnt pinpoint it. Ours really sounded bottom end of the engine though. Son drove it back and forth to Indiana several times after we fixed it and absolutely no noise whatsoever.
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Old 04-12-2013, 01:19 AM   #12
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Re: 5.3 knocks when warmed up

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Originally Posted by 67ChevyRedneck View Post
Dead lifter? The oil passages are tiny and get plugged easily.
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They are notorious for this also.
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Old 04-12-2013, 07:49 PM   #13
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Re: 5.3 knocks when warmed up

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Dead lifter? The oil passages are tiny and get plugged easily.
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Wouldn't it do this all the time if that was it?
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Old 04-12-2013, 08:04 PM   #14
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Re: 5.3 knocks when warmed up

Click on image to play video

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Old 04-12-2013, 10:57 PM   #15
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Re: 5.3 knocks when warmed up

I would not rule out the cats.............!
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Old 04-14-2013, 10:59 PM   #16
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Re: 5.3 knocks when warmed up

That sounds a lot louder than a lifter tick:

https://www.youtube.com/watch?v=4RecrxxLT4s
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Old 04-15-2013, 12:26 AM   #17
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Re: 5.3 knocks when warmed up

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That sounds a lot louder than a lifter tick:

https://www.youtube.com/watch?v=4RecrxxLT4s
Good video.

To remove lifters:
Many 60's cars remove intake.
66 GMC remove valve cover.
New chevy 5.3 remove intake, and heads.

Are we making progress?
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Old 04-15-2013, 10:39 PM   #18
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Re: 5.3 knocks when warmed up

That sounds about like my Blazer did. the one thing I wish I had checked was the oil pump, the oil pressure was low (less than 20 psi) at idle after warming up and the knock would go away with acceleration, the oil pressure would also go back up to normal.
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Old 08-17-2013, 10:18 PM   #19
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Re: 5.3 knocks when warmed up

Well it's been 4 months since I noticed this knock, and would like to provide an update. Here's what I've done. Nothing. Still knocking and still running like a top. I've pretty much decided to ignore it. I seriously doubt it would have ran this long if it was something serious in the bottom of the engine. If anything changes I'll let you know. Thanks for all the advice.
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Old 08-30-2013, 12:36 PM   #20
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Re: 5.3 knocks when warmed up

I had this same noise in my buddies tahoe. The noise was just as you described. He was about to replace the engine because it sounded like a lower end knock. But before I ordered the engine I looked in the cylinders with a scope and saw a serious build up of carbon on the pistons. One spot had come loose and sounded like above noise when it was up to driving temperature. I ran two systems of BG fuel system cleaner through it and drove it hard until the carbon came loose and blew out of the exhaust. The product is not something you buy at a store. It's ran through an air pressurized system into the fuel system. It works believe me. He is still driving it daily. Its been almost two years.
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Old 08-30-2013, 10:07 PM   #21
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Re: 5.3 knocks when warmed up

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I had this same noise in my buddies tahoe. The noise was just as you described. He was about to replace the engine because it sounded like a lower end knock. But before I ordered the engine I looked in the cylinders with a scope and saw a serious build up of carbon on the pistons. One spot had come loose and sounded like above noise when it was up to driving temperature. I ran two systems of BG fuel system cleaner through it and drove it hard until the carbon came loose and blew out of the exhaust. The product is not something you buy at a store. It's ran through an air pressurized system into the fuel system. It works believe me. He is still driving it daily. Its been almost two years.
I've considered carbon knock but wasn't sure that something floating around in a cylinder would be so rhythmic. I have heard of BG, but not sure if anyone around here does it. You seem fairly knowledgeable about this so I'd like to ask for your thoughts on SeaFoam.
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Old 09-11-2013, 08:55 PM   #22
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Re: 5.3 knocks when warmed up

Sorry, ive been busy lately. Seafoam is a good product but it will not clean the carbon like a BG type product will. The BG product is used pressurized directly into the fuel system and is a very powerful cleaner. But with that being said you could always try a can of seafoam. It's a small investment with nothing to loose. The only reason I used the BG product is because it is readily available to me. That's all I ever use because of that reason. I know it works. I used a carbon cleaner product made by motorcraft years ago that I pulled through the booster vacuum hose. I let it soak then ran the snot out of the engine in my 4.3 s10. It fixed the spark knock I had in it. Seafoam or similar product should do the same.
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Old 02-01-2014, 08:36 PM   #23
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Re: 5.3 knocks when warmed up

Yep I agree with silvereagle07. BG Does work better.
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Old 02-02-2014, 03:17 PM   #24
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Re: 5.3 knocks when warmed up

I've pretty much given up on it. Been doing it for almost a year now with no apparent ill effects. I just ignore it and drive.
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Old 04-05-2020, 05:23 PM   #25
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Re: 5.3 knocks when warmed up

Sorry to revive a dead thread, but any update on your knock situation? My 05 z71 with the 5.3 (l33 version) has been doing this for a few months and I'm stumped.
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