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Hello all,

I have a 2004 Envoy with about 150,000 miles and it is making this knocking noise from the engine. I have taken the belt off and none of the pulleys are bad. It has been making the noise for years now and it doesn't effect how the truck drives. below is a video of the noise.

 

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olds bravada
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I have a 2002 Bravada 180,000 miles with the same noise, just not as loud. Goes away when warmed up completely. I've done the fan clutch,water pump,tensioner,idle pulley and alternator in the last few years. I listen and it doesn't sound like the timing chain. Valve train ticks in what I'd call a normal way. I'm thinking piston slap like everybody says. Not rod bearings as they don't go away when warm and I've always changed oil every 3000 miles.
So today I bought a compression tester as I was going to change the spark plugs for wintertime. To my surprise I have 180 psi in all cylinders! I was shocked and amazed. I'm thrilled. 10-1 compression engines are 200 psi when new says the manual. So only a 20 psi loss over all these miles. So what is the noise? How can you have piston slap when the compression is this good? Somebody has mentioned Cold Carbon Knock? I can see down the spark plug holes with a pin light and see tops of pistons are black. So I'm running Techron in my gas each tank. Will carbon on the tops of the pistons make the knock only when cold in the mornings? It seems to be worse now that the weather is colder this fall?
Help somebody? This SUV has been great except for this noise? Drives me crazy! SUV is otherwise perfect shape. I've maintained it to death! Thanks !!!
 

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2004 Chevy Trailblazer LT 4.2
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I am getting the same type of low tap as well and i am starting to think it might be a bad chain tensioner / stretched timing chain / worn guide /` s ?
Putting a screwdriver to my ear at the valve cover bolt at the front where the noise seems to be strongest
To me it doesn`t sound like the valvetrain / rods / pistons making the noise

After watching this video someone uploaded i wonder if this is the issue causing the knocking with the chain slapping the upper guide at the valve cover location ?



 

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olds bravada
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I am getting the same type of low tap as well and i am starting to think it might be a bad chain tensioner / stretched timing chain / worn guide /` s ?
Putting a screwdriver to my ear at the valve cover bolt at the front where the noise seems to be strongest
To me it doesn`t sound like the valvetrain / rods / pistons making the noise

After watching this video someone uploaded i wonder if this is the issue causing the knocking with the chain slapping the upper guide at the valve cover location ?



Wow, that is a possible! I did the listen as you did and didn't seem to hear it however. But that would make sense wouldn't it? As the engine warms up the oil tightens up the tensioner? I've never changed the timing chain. Today it made no noise whatsoever. But it's not cold outside today. Also you got the valve cover off without removing the air intake? That would be good! I've not taken the cover off to date as I thought you had to remove the intake and that is a pain. Thanks!
 

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2004 Chevy Trailblazer LT 4.2
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Not my video as i said and yes the intake manifold Must be removed in order to remove the valve cover
Its a possible answer to the noise we experience when other types of noise do not sound like this as the video eluded to
 

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olds bravada
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Timing chain! Everybody in posts say rod knock, fan clutch and piston slap. Timing chain
guides and tensioner wears out! You get that knock at cold start and it goes almost away after warm up but if you listen close you hear the chain going tick tick tick. That is the slack in the chain. The tensioner was fully extended to the max. Guide had groves worn into them.
I change the oil on schedule and use Mobil One. So it's just wear from lots of miles!
 

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2004 Chevy Trailblazer LT 4.2
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Very Weird , When The Engine Is Noisy You Take Large Needle Nose Pliers And Squeeze The Hose Shut Across It As You Can Access It Somewhat Easy And Hold It Closed For About 30 Seconds Or So The Noise Stops
Release The Pliers And Noise Comes Back
You Will Create A Vacuum With In The Engine From The Hose Not Being Able To Circulate Gases / Blowby But How Does It Stop The Noise ?

CAM ( Valve ) COVER TO AIR CLEANER

Hose Location Viewed From Front of Engine ,Behind And Just To The Right of The Oil Fill Cap
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You Would Suspect The Check Engine Soon With Related Codes Would Be Triggered If A Timing Chain Has Slop In It
Crank Correlation Or Cam Timing Codes Would Be My Guess When It Gets Too Worn Out
A Shame They ( stupid GM engineers ) Didn`t Leave Any Service Inspection Hole To Check The Chain For Wear Or At Least Make It Easier To Remove The Timing Cover
I Know How To Check Older Distributor Style Engines by Turning The Crankshaft Backwards And Watch The Distributor Move Within Reasonable Travel
Mine Is Just Under 100,000 Miles On The Odometer So It`s Not Like There Is A Lot Of Miles On It
 

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'05 TB 4.2 EXT
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Very Weird , When The Engine Is Noisy You Take Large Needle Nose Pliers And Squeeze The Hose Shut Across It As You Can Access It Somewhat Easy And Hold It Closed For About 30 Seconds Or So The Noise Stops
Release The Pliers And Noise Comes Back
You Will Create A Vacuum With In The Engine From The Hose Not Being Able To Circulate Gases / Blowby But How Does It Stop The Noise ?

CAM ( Valve ) COVER TO AIR CLEANER

Hose Location Viewed From Front of Engine ,Behind And Just To The Right of The Oil Fill Cap
View attachment 58287
I'm gonna chime in here because you seem to be running in circles, hearing zebras where there are only hoofbeats.

That said --- I cannot fathom WHY pinching a CV hose causes any difference in your noise.

Come with me as we take a little journey inside your engine .......

That hose you're pinching should be under at least a slight vacuum at all times, drawing back into the valve cover - never flowing into the sound chamber.

The vacuum is ultimately controlled by the vacuum value in the plenum AFTER to throttle plate, by allowing more or less throttle opening as controlled by the computer.

The nipple end on the valve cover has a small metering (fixed) hole of 'X'-diameter that is supposed to draw clean air from the zone between the air filter and the throttle plate.

That clean air is supposed to be drawn into that nipple and air flows into the valve cover to wash the crankcase of fumes from cylinder blowby and the after-byproducts of internal combustion - including water also from the humidity in the air.

If it does NOT purge the crankcase --- being drawn into the vacuum side of the PCV system --- it will keep collecting condensate (water) in the pan.

Every time you shut the engine off, then that water partially falls out of suspension and gets to reconsolidate and create rust and corrosion to the parts inside the engine.

OK --- so what does this diatribe by me info do for you?

If you haven't rusted the mechanism of the VVT - there's something else to consider.

When water collects in the lowest part of the crankcase (the pan) and gets drawn up by the oil pump where it gets mixed with the lube oil, it gets to eat your noble parts like aluminum and iron.

One of the things that suffers the most from water contamination is the camshaft and followers which are in critical need of fully slippery oil for it to live.
The bearings, piston rings and chains can take a lot of water abuse but the CAM is under serious loads that require very clean, non-contaminated oil at all times.​

It is not impossible to grind the cam flat or weld a follower to the lobe for lack of decent oil - and that won't necessarily show up on a compression test; so a compression testy will usually only find things that are really wrong.

A tossed follower or flat cam lobe WILL make a difference in performance at higher than cranking or idling RPM because you have 2 intakes and 2 exhaust valves per cylinder and I've seen these Atlas engine toss a single follower from time to time.

So - now I've given you another place to check out.

PART 2 --- I had a partially collapsed piston from the joker who bored my block and pressed the piston pins and rods.
........... because he crushed one of the pistons - not enough to be measurable - but enough to make a slapping noise until the piston got warmed up and the noise went away.​

I detested that little red-headed twerp for lying to me like he did - even to this day - as he cost me a stroker 350-383 build that I got to do a second time on MY dime!

You MAY have a broken piston. It gets quiet as it warms up --- that's a big hint.

I'm done slaughtering zebras for you ..... I also brought some of my own.
 

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2004 Chevy Trailblazer LT 4.2
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I cannot fathom WHY pinching a CV hose causes any difference in your noise.

Ditto that was my thinking as well.... Thought it was B.S. and someone trolling a video on others but as a skeptic i tried to debunk it
After i saw this video i tried the same thing but pinched the hose instead and i got the same results
I am also in the Dark why clicking disappears


Not my YT video but what brought me to the same issue


 

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Before i forget i did remove the belt and fire it up thinking it was the water pump / fan clutch or pulleys and noise was still there
That was the First thing i eliminated in my process of narrowing it down
 

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'05 TB 4.2 EXT
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If you don't have some "Steelman Ears" ................

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............then either a mechanic's stethoscope
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............ or a long screwdriver to your ear
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(this is not me)


.......................would do pretty good in locating the sound.

Just remember that some sounds telegraph and may not be exactly where you hear them.



This preferably forever unidentified and vapid-looking guy is using the wrong end of the screwdriver and then he needs a hammer to insert it fully ...

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I am NOT a <pthrrup!> "Scotty Fanboi" ..... <barf> <puke>
 

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2004 Chevy Trailblazer LT 4.2
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long screwdriver to your ear

Thanks for your feedback Ravalli Surfer
Exactly , thats what i used to narrow the location
Seems to me the loudest area is right ( red arrow ) at the front drivers side camshaft / valve cover bolt checking bolts along the path towards passenger side it lessens
Checked other areas where you would think it would originate from
Areas i checked where lower passenger side of block ( null ) all along it exhaust manifold ( null ) intake manifold ( null ) front of timing cover where belt is flowing( just enough room ) at drivers side access plug
Other than removing spark plugs and checking for piston slap or connecting rod play i am partial to further investigating it by removing the 2 access plugs and maybe it will allow me some more evidence on the chain tension ? or if i had a borescope ( need to get one ) to send up towards the top of the chain to see if the upper chain guide is looking a bit worn from the chain slapping ?


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