Weird engine noise in crate 5.7L 350 tbi

[RileyCyras]

I have a 1993 gmc Sierra with a swapped crate 350 TBI with 91 octane core. It has less than 15k miles on it. It started making a weird engine noise when I was heading up to the snow to help recover a friend. I’ll attach a video but it’s a popping/loud click coming from what seems to be the driver side valve train. The noise has a pattern but isn’t super consistent. Doesn’t seem to be linked with valve movement. I’ve inspected the push rods, rockers, and the tops of the lifters and they all seem brand new. I’ve gone and looked at the pulley system and flex plate as well all seem to be in perfect working order. Replacing the distributor rn but I doubt that that is the issue. Been going at this for a while now thought I’d post here to see if anyone can point me in the right direction.

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[Hipster]

Kind sounds like a valve sticking in the guide, it's has a bit of a valve snapping shut sound to it. Can't really assess for bent pushrods until you get them out and roll them on a piece of glass. Broken valve spring maybe? There's been no other issues prior? Absolutely positive it's not exhaust/failing header gasket? They can mimic other sounds. Any attempts before or after this started with valve train adjustments? Engine seems rather mechanically noisy otherwise as well.

 

[Scooterwrench]

I saw valves not moving and the rockers look really dry. I think the cam is wiped out. That should be a flat tappet motor and if you didn't run high zinc oil that will wipe out a FT cam.

 

[Road Trip]

I have a 1993 gmc Sierra with a swapped crate 350 TBI with 91 octane core. It has less than 15k miles on it. It started making a weird engine noise when I was heading up to the snow to help recover a friend. I’ll attach a video but it’s a popping/loud click coming from what seems to be the driver side valve train. The noise has a pattern but isn’t super consistent. Doesn’t seem to be linked with valve movement. I’ve inspected the push rods, rockers, and the tops of the lifters and they all seem brand new. I’ve gone and looked at the pulley system and flex plate as well all seem to be in perfect working order. Replacing the distributor rn but I doubt that that is the issue. Been going at this for a while now thought I’d post here to see if anyone can point me in the right direction.

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Greetings RileyCyras, and Welcome to the GMT400 forum!

That is an interesting sound, intermittent but not exactly cyclical in time with the rotation of the engine?
(I concur with you, at first listen it doesn't seem to be linked to valvetrain activity?)

And in order to troubleshoot this correctly, I need to be able to explain how a sound like that
can occur once every so many engine revolutions?

You know, sometimes a spark that travels a fair distance can give you a 'tapping' sound like this.

Take a moment and listen to this short video for something (more cyclical) yet similar:

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NOTE: This actually may be easier to find if you were to run the engine at night in the dark.

And if you can't trace this sound to any intermittent arcing, then if it was me I would disconnect
any electro-mechanical widgets one at a time (ie: A/C compressor clutch, EGR valve, IAC, etc) ...and
see if you can't make that sound disappear. (On the theory that your computer is attempting to
force a periodic reset for whatever reason?)

By the way, do you have a SES/CEL illuminated? Any codes being stored, especially the ones
that are set but don't cause the check engine light to be illuminated?

I keep wondering, what if the spark delivery is OK, and all the electromechanical items underhood
have been tried and nothing found in the process of elimination? Then Hipster's comment about
how it sounds like a single valve sticking in the guide once every so many cycles is a possibility.
Q: Does it do this right from a cold start? Or does it have to run for awhile before it starts to
malfunction? This might give us a valuable clue.

One last thing. IF you discover that the sound is coming from an arc, shut the engine off before
probing around. Today's ignition systems have a bite that's worse than their bark. (!)

For what it's worth. Happy Hunting. And when you discover the root cause of this
strange intermittent percussive sound, please come back here and share the solution!

Cheers --

 

[RileyCyras]

Hey guys wanted to give an update. I want to preface this with saying I’m a very young and new home mechanic. I know a bit about engines from rebuilding dirt bike motors, but I have no real shop or diagnosing experience. I’ve always been scared to post on this forms because of this but everyone has been super kind and helpful.

When the noise first started I assumed it was driver side valve train. I had the truck towed to my friends dad’s garage cause it was the only weather proof working space I knew of, but I was not fully welcome there and was urged to rush. The noise was distinctively on driver side and I was in a rush to figure out what the problem was and how to fix it. So I only took off the driver side valve cover inspected the parts then moved on to the rest of the engine. Yesterday I was about to throw in the towel and take it to a mechanic but I decided to take off the passenger side valve cover just in case. The first thing I saw was cylinder number 2’s exhaust rocker that had snapped off the stud. Felt real stupid after that. At least I learned a lot more about my engine and learned a very valuable lesson to not rush when trouble shooting.

You must be registered for see images attach

I am now going to order all new rockers, studs, and pushrods. There was a small chunk of metal missing from the pivot ball that’s no where to be found. After I finish installing the new rockers and push rods I’m going to get a oil flush done before I start her again. I assume the noise now is my exhaust from cylinder #2 building up with no way to get out then forcing its way out the intake valve. I assume there is carbon build up all throughout my engine so I was going to run some sea foam through the system. I used my friends endoscope to look through all the spark plug holes when first inspecting and didn’t see and valve hang then but I will check again when I get the chance. Is there anything else I should do before trying to run her again? I’m also wanting some recommendations for rockers and how to find what ratio the original ones are. I have the 4 overhead bolts on the valve cover I’m pretty sure there’s some restrictions with what rockers I can get because of that.

Thank you all for the suggestions and your time. Sorry the real problem was just my incompetence but I’m coming out with a good attitude and valuable lesson learned.

 

[Hipster]

Screw in studs? modded or aftermarket head? There's china imported cast iron look alike castings floating around. Not all aftermarket studs are created equally, but have also seen them break studs from coil bind or retainer to guide stack up. Mal-adjustment and other things can come into play. That pushrod (roll on a piece of glass)still straight? Might need to dig a little deeper.

 

[RileyCyras]

Screw in studs? modded or aftermarket head? Not all aftermarket studs are created equally, but have also seen them break studs from coil bind or retainer to guide stack up. Mal-adjustment and other things can come into play. That pushrod (roll on a piece of glass)still straight? Might need to dig a little deeper.

They are screw in studs. It’s a performance 91 octane engine that I was told is making 330hp. Not sure if the heads were mess with though. Been trying to get more info from the old owner but no luck. Pretty sure that the rocker was over torqued which led to the stud snapping. When I did the lash on the other side I noticed that some were super tight and others were way to loose. Push rod didn’t roll straight so I was just going to replace all 16 along with the rockers and studs with something a bit more durable so this doesn’t happen again.

 

[WayneO]

Kind sounds like a valve sticking in the guide, it's has a bit of a valve snapping shut sound to it. Can't really assess for bent pushrods until you get them out and roll them on a piece of glass. Broken valve spring maybe? There's been no other issues prior? Absolutely positive it's not exhaust/failing header gasket? They can mimic other sounds. Any attempts before or after this started with valve train adjustments? Engine seems rather mechanically noisy otherwise as well.

Have you tried running it with the belt off

 

[Road Trip]

Hey guys wanted to give an update. I want to preface this with saying I’m a very young and new home mechanic. I know a bit about engines from rebuilding dirt bike motors, but I have no real shop or diagnosing experience. I’ve always been scared to post on this forms because of this but everyone has been super kind and helpful.

When the noise first started I assumed it was driver side valve train. I had the truck towed to my friends dad’s garage cause it was the only weather proof working space I knew of, but I was not fully welcome there and was urged to rush. The noise was distinctively on driver side and I was in a rush to figure out what the problem was and how to fix it. So I only took off the driver side valve cover inspected the parts then moved on to the rest of the engine. Yesterday I was about to throw in the towel and take it to a mechanic but I decided to take off the passenger side valve cover just in case. The first thing I saw was cylinder number 2’s exhaust rocker that had snapped off the stud. Felt real stupid after that. At least I learned a lot more about my engine and learned a very valuable lesson to not rush when trouble shooting.

You must be registered for see images attach

I am now going to order all new rockers, studs, and pushrods. There was a small chunk of metal missing from the pivot ball that’s no where to be found. After I finish installing the new rockers and push rods I’m going to get a oil flush done before I start her again. I assume the noise now is my exhaust from cylinder #2 building up with no way to get out then forcing its way out the intake valve. I assume there is carbon build up all throughout my engine so I was going to run some sea foam through the system. I used my friends endoscope to look through all the spark plug holes when first inspecting and didn’t see and valve hang then but I will check again when I get the chance. Is there anything else I should do before trying to run her again? I’m also wanting some recommendations for rockers and how to find what ratio the original ones are. I have the 4 overhead bolts on the valve cover I’m pretty sure there’s some restrictions with what rockers I can get because of that.

Thank you all for the suggestions and your time. Sorry the real problem was just my incompetence but I’m coming out with a good attitude and valuable lesson learned.

Hello RileyCyras,

One of the things that attracted me to the GMT400 forum was that the members seemed
to remember what it was like to for them to start out as a fledgling mechanic. And good on
you for sharing this photo -- it is valuable for other people who are new to the old truck hobby
to see all the different failures that are possible under the valve covers.

* It's hard to determine exactly what caused that stud to fail? Are we looking at a stud that
had a metallurgical defect when it was made and just it finally failed in use? Is the failure due to
a higher lift cam forcing a 'stock slot length' rocker arm into the side of the stud? (Seems like
there were stock slot vs long slot rocker arms?)

Given the adjustable nature of the rocker arms on the SBC, since we never drive the adjusting nuts
all the way to the bottom, I have a hard time explaining how the stud was broken by overtightening
the nut?

And while we're at it, I also can't explain how this knocked off #2 cylinder rocker arm could make the
'once per second' loud tapping sound heard in your video? It will be interesting to see/hear if this
unusual tap sound is still present or gone once you replace the stud & readjust this rocker arm?

By the way, elsewhere we were all comparing notes having to do with how to get the best possible
valve preload adjustment on the SBC, and the general consensus is that a 'running lash' (aka hot lash)
adjustment seems to give the best results? If you are curious, go over here and check it out: (HOT LASH)

Definitely let us know happens after replacing the failed parts & how it runs afterwards.

And if there are still issues then no doubt one or more of the members here will have a solution
based upon suffering through a similar failure themselves.

Best of luck!

 

[Hipster]

Given the adjustable nature of the rocker arms on the SBC, since we never drive the adjusting nuts
all the way to the bottom, I have a hard time explaining how the stud was broken by overtightening
the nut?

.

You can overdo the nut adjustment and stack the coils tight on a valve spring, Instant out of room/breakage/likely cam/lifter damage. Moreso, If things were tight to begin with. It can be not much to get there depending on what all has or hasn't been done as far as the build was done and subsequent adjustments. Lots of variables. Stud failure hope so but maybe just a symptom at this point. Assess full extent of damage before putting together a repair plan needs to be in play.