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Engine knock/loud tap


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  #1  
Old  August 15th, 2012, 12:23 PM
Engine knock/loud tap

I have a GMC Sierra (I know this is a Chevy forum but I'm desperate, as this is my only mode of transportation, and I'm afraid the engine is going to blow up if I don't fix this soon), 1/2 ton with 350 v8/5.7 liter that has a bad knocking/loud tapping sound but can't determine where it's coming from.

I thought it was probably lifter noise so I took the driver side valve cover off and attempted to adjust the rocker arms on the 1st cylinder. The front rocker adjustment made no difference in the noise, but when I loosened the rocker arm on the 2nd rocker, to the point of noticeable pushrod tapping, the loud tapping noise all but disappeared. I re-tightened the rocker and the loud tapping returned.

The tapping/knocking is there no matter what the engine temperature is, or RPM level. It is most noticeable at idle but the truck has Flowmasters and is quite loud under power so the tapping noise is drowned out a bit.

Does the fact that loosening the rocker decreases the tapping noise indicate that the lifter there is sticking or collapsed?

I've had 3 "mechanics" listen to the noise but none could tell where it's coming from, I've been told it was a lifter, wrist pin, bearing, timing chain (2 year old double timing chain so I doubt it's worn enough to cause this noise), exhaust leak, cracked flywheel, on and on and on.

I can't afford, nor do I trust any of the local mechanic shops around here, I've been told they're all crooks, so I'd like to know where to start tearing into this thing myself. Keep in mind, removing the valve cover and trying to adjust the rockers is about as knowledgeable as I am regarding engine internals, but I am mechanically inclined so could handle something relatively simple.

Thanks in advance!!

  #2  
Old  August 15th, 2012, 1:01 PM
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BluzDude9999 
I thought it was probably lifter noise so I took the driver side valve cover Does the fact that loosening the rocker decreases the tapping noise indicate that the lifter there is sticking or collapsed?
Be my guess.

  #3  
Old  August 15th, 2012, 9:02 PM
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    NC
your gmc sierra is pretty much the same exact truck as the 1500. Your situation sounds sucky, sorry to hear guy... You might be best off, cheapest and easiest route of approach, to get a head off another 350. Take it home and get your self some valve grinding compound. Get a torque wrench and the specs of how tight you need to put all your rockers/springs. you could also have the "new" head xray'd just to make sure its good, but thats on you. Anyways tear the "new" head down then put the valve grinding compound on each valve and hole (there are videos of HowTo on youtube). Clean everything up, put the head together.... then swap the heads. Then if you want to make your money back you can fix your old head and sell it on ebay. I think you probably got a peice of carbon, or peice of the piston head melted off, or even a piece of the sparkplug melted and got in the valve seat. It happens.
Keep in mind, I'm only going of what your saying about you making the adjustment to the valve and it stopped


Last edited by jesse06; August 16th, 2012 at 7:18 PM.
  #4  
Old  August 15th, 2012, 9:22 PM
try adding some ATF in your valve cover and then see if the noise goes away

  #5  
Old  August 16th, 2012, 3:41 AM
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replacing the head will not solve this issue, without hearing the engine I am going to say you have a bent push rod, bad lifter/cam lobe.

first thing you need to do is a compression test, then pull that push rod out (make sure you don't pull the lifter with it) and see if it is true.

if every thing checks out so far bring the engine up to TDC and open her up.

best thing for you do buy a book on that truck changing a camshaft is not hard but you need to follow rules and be gentle with the cam on bearing action.

good luck

Ross

  #6  
Old  August 16th, 2012, 3:42 PM
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You didn't mention the year, but some of the older ones had a piston slap.
For some reason the just keep running.
Our daughter has a older Tahoe and it does it on start up but then goes away.
It also sounds like a lifter..
BTW, GMC and Chevy are the same thing.
They just wear different clothing.

Just like the Big Olds, and a Cadillac.
of yesteryear,
Same car.

  #7  
Old  August 17th, 2012, 7:26 AM
staggerlee 
You didn't mention the year, but some of the older ones had a piston slap.
For some reason the just keep running.
Our daughter has a older Tahoe and it does it on start up but then goes away.
It also sounds like a lifter..
BTW, GMC and Chevy are the same thing.
They just wear different clothing.

Just like the Big Olds, and a Cadillac.
of yesteryear,
Same car.
The truck is a 1990 and has been gone through underneath completely about 2 years ago, new everything, shocks, steering, brakes, exhaust, complete engine rebuild with custom ground crank to work with the stock throttle body/computer. I've added custom wheels and Yokohama tires. The engine is pushing between 325 and 350 HP and gets 22 MPG on the highway, with 4 speed auto trans with overdrive and a shift kit.

The noise never stops, except when I loosened the 2nd rocker arm on the driver's side. There appears to be a dead miss in the engine, I assume due to whatever is causing the knock.


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