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Knocking coming from cat


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Phantom305
User

Apr 24, 2021, 9:32 AM

Post #1 of 34 (652 views)
Knocking coming from cat Sign In

I have a 97 chevy c1500 with the 305...shes got 200k on the clock and runs great but im hearing a very slight knocking from the passenger cat. Its not like a broken honeycomb that rattles around it sounds more like engine knock but im only hearing it from this cat. Oil pressure is good. I have noticed a slight decrease in power so im thinking it could be a clogged cat. The knocking doesnt increase with rpm either. Also, i can only hear it when the truck is at normal operating temperature.

anyone else have this happen or any tips? Im really hoping its not a rod or bearing


(This post was edited by Phantom305 on Apr 24, 2021, 9:35 AM)


Tom Greenleaf
Ultimate Carjunky / Moderator
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Apr 24, 2021, 9:42 AM

Post #2 of 34 (636 views)
Re: Knocking coming from cat Sign In

Noise are always tough to impossible. You hear something and notice loss of power? Sure, a clogging converter would do that not so familiar with them making noise?
Have you looked at connections there and I think a plate with bolts then heat shields on anything more on OE mufflers on these there is a good fix and cheap if so and it's all there just rusted spot welds.
Tap on exhaust parts with a rubber hammer I doubt this noise is from the cat/converter as much as some other hardware if an exhaust issue at all??


T



Phantom305
User

Apr 24, 2021, 9:47 AM

Post #3 of 34 (630 views)
Re: Knocking coming from cat Sign In

Yea the welds look good, the exhaust is pretty clean for 25yo... the shield is also not rattling. Ive used the screwdriver trick to isolate the noise. Not coming from oil pan, manifolds, or anything else in the immediate area. My flywheel ticks a little bit but the two noises are easy to distinguish. It’s definitely from the cat. Ive hit the cat and im not hearing a rattle. I would say that maybe i have back pressure from the muffler maybe? I have a 2.5” super 10 and the rest of the exhaust is 2.75 so its a little smaller but idk if it would cause that much back pressure


Hammer Time
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Apr 24, 2021, 9:53 AM

Post #4 of 34 (624 views)
Re: Knocking coming from cat Sign In

If your Cat is producing a noise, it can't be anything but a broken honeycomb. It's the only thing in there besides some partitions.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Phantom305
User

Apr 24, 2021, 9:59 AM

Post #5 of 34 (616 views)
Re: Knocking coming from cat Sign In

Yea thats what im thinkin but this noise is consistent with the rpm at idle and very low sounding knock...idk if broken internals of the cat would cause it


Hammer Time
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Apr 24, 2021, 10:08 AM

Post #6 of 34 (610 views)
Re: Knocking coming from cat Sign In

Then maybe you are wrong about the source of the noise.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
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Apr 24, 2021, 10:11 AM

Post #7 of 34 (605 views)
Re: Knocking coming from cat Sign In

I even happen to own this but a very HD 2500 4X4, no miles still 5.7. IDK how much is the same exhaust wise?
Noise from flywheel too? Have you run over something and bent anything? If you really had engine bearing knock it wouldn't necessarily lose power yet and those make distinct noises.


You see stuck on backpressure so there is a chance muffler is rust gutted on inside known of one sound like a shaker full of small gravel still had all power.


For power loss is maybe the real clue. Why? Get codes read see what they say for starters.
If you are still on back pressure you can just vacuum test with a "T" to actual manifold vacuum. At other than super high altitude it should be about 18is Hg at idle and the same with no load with RPMs held at about 2,000. Excessive back pressure would be lower at the 2,000 RPM reading.
Other things? Any supports for things even exhaust hangers + I forget how trans is supported but see if you can move it.
Calling it "knock" is a bit damning for a problem vs a rattle for things I've mentioned is just choice of words makes web and text hard for noises said up top.
Do get codes read if real engine issues it's going to have something to read pop up. 200K is either OK for these engines IMO with very on time care all along or plenty if times oil changes neglected especially.
So far nothing is really adding up. Worn engines usually worst when cold or upon start up then quieter later.
See if you really think backpressure is it try that vacuum test but try hard not to bust anything to access real vacuum it's old and miles enough just removing any hoses can bust things.
That's it for the moment for ideas and I still don't understand flywheel if that's screwed up I would think it would bust soon after heard?


T



Phantom305
User

Apr 24, 2021, 10:25 AM

Post #8 of 34 (601 views)
Re: Knocking coming from cat Sign In

Yea i gotcha, i have a slightly smaller diameter on my aftermarket muffler but its a high flow flowmaster super 10, so i dont think it would cause back pressure-its a 2.5 diameter the rest of the exhaust is 2.75. The muffler and the tailpipe that goes over the rear axle is all brand new

Ive tried my best to isolate the sound and id rather not call it a rattle because it seems consistent with the idle rpm. I use 10w30 synthetic oil for my oil changes every 3000 miles so its very well taken care of and ive yet to see metal or anything odd about the oil coming out. Not lifter tap(i dont wanna jynx it lol) and the motor runs great. And its not as loud as an engine knock but definitely noticeable if you bend down by the passenger wheel well where the cat is. I really wanna say its a leak but i cant see any holes in the welds or cat and cant see any air escaping. But if i put my ear right up to the cat i can definitely hear it but not at the upstream or downstream pipes

Also the check engine light is not on


(This post was edited by Phantom305 on Apr 24, 2021, 10:29 AM)


Phantom305
User

Apr 24, 2021, 10:52 AM

Post #9 of 34 (579 views)
Re: Knocking coming from cat Sign In

 

I tried my best to record it, it might be hard to tell but if you can listen closely you can hear the “knock” it almost sounds like a ford IDI 7.3

The rapid ticking in the background is the flexplate https://youtu.be/t7aJhDl_1Lc


(This post was edited by Phantom305 on Apr 24, 2021, 10:56 AM)


Tom Greenleaf
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Apr 24, 2021, 12:44 PM

Post #10 of 34 (544 views)
Re: Knocking coming from cat Sign In

That was so quick hard to say still. Normal for 6 seconds IDK that fast.


Other again is if anyone tried to bore out the converter who knows what would go wrong? Looked clean rust wise to me EXTERNAL parts there. No conclusion - take it off if you want and not sure if you'll need it mig welded back together right there or not,


T



Phantom305
User

Apr 24, 2021, 2:13 PM

Post #11 of 34 (533 views)
Re: Knocking coming from cat Sign In

Yea i gotcha, i recorded a better video if you could take a look at it, you can def here the knocking coming from the cat...but not from anywhere else

https://youtu.be/tNiLQ1IWd14


Hammer Time
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Apr 24, 2021, 2:39 PM

Post #12 of 34 (523 views)
Re: Knocking coming from cat Sign In

You already know the flex plate bolts are hitting the tin shield. Fix that first and it may be both noises.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
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Apr 24, 2021, 2:46 PM

Post #13 of 34 (521 views)
Re: Knocking coming from cat Sign In

That is a lot better now shifting the ''GUESS'' that it's a real engine noise that happens to show up thru exhaust to you.

Said right away noises are a pest + now older + enough miles? Things looked good in general leaves a real engine problem as a guess only.

At some point I have to suggest a totally qualified tech hear this it also lost some power YOU said so it's not just NOTHING.

Why? Who knows I can't be conclusive would take me time if in front of me I think,

Tom



Phantom305
User

Apr 24, 2021, 3:50 PM

Post #14 of 34 (515 views)
Re: Knocking coming from cat Sign In

What tin shield?


Phantom305
User

Apr 24, 2021, 3:51 PM

Post #15 of 34 (513 views)
Re: Knocking coming from cat Sign In

Yea thats what im thinkin now as well. Im gunna start by checkin the plugs, see if theyre not firing right...then maybe run some seafoam bc i know the spider injectors are prettt Sh!tty so maybe its them


Hammer Time
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Apr 24, 2021, 4:01 PM

Post #16 of 34 (503 views)
Re: Knocking coming from cat Sign In

There is an tin inspection plate to access the converter bolts through the flex plate. The flex plate rotates behind it with less that 1/8" clearance so if the bent that shield even a little when they removed the transmission, the torque converter bolts will hit it as they rotate. That sounds like what you have. You can just remove the tin cover for now to see if the sound goes away.





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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Phantom305
User

Apr 24, 2021, 4:53 PM

Post #17 of 34 (498 views)
Re: Knocking coming from cat Sign In

Oh i dont think mine has that, all i see is the flexplate and the torque converter..no dust shield


Hammer Time
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Apr 24, 2021, 4:57 PM

Post #18 of 34 (494 views)
Re: Knocking coming from cat Sign In

Well then, either the flex plate is cracked or the bolts are hitting something when it goes around, possibly both.

If the flex plate is actually cracked, then you need to stop driving it right away because it's only a matter of time before it breaks off all the way and leaves you stranded.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Phantom305
User

Apr 24, 2021, 5:42 PM

Post #19 of 34 (486 views)
Re: Knocking coming from cat Sign In

Damn ok, i just had the flexplate replaced and the noise never stopped. So yea maybe the bolts are hitting otherwise idk. And that’s definitely gotta be the flexplate i dont think that anything else in that area would make a ticking like that


Hammer Time
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Apr 24, 2021, 5:47 PM

Post #20 of 34 (480 views)
Re: Knocking coming from cat Sign In

They could have messed up the job by using bolts that were too long or the heads were too big.

It needs to be looked at by a tech.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



(This post was edited by Hammer Time on Apr 24, 2021, 5:48 PM)


Phantom305
User

Apr 24, 2021, 5:50 PM

Post #21 of 34 (474 views)
Re: Knocking coming from cat Sign In

I gotcha, thank you!


Tom Greenleaf
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Apr 24, 2021, 6:15 PM

Post #22 of 34 (469 views)
Re: Knocking coming from cat Sign In

I'm asking - What happened that you needed a flex plate?
Is the starter motor tight and intact on this? Only heard of it that this block in Vortecs can snap off a corner of engine block never saw that but left starter motor with one bolt! Never saw it.


The lower end of this engine is older than dirt a cast iron small block Cheby if you will hard to kill. The rest being a truck now could be different a lot from others IDK how many choices of the things were available when new?


I own this shares nothing but the Vortec part is and has been fine and predictable acquired used @ 7 years old no miles still low but plows snow and favors as a pick up some in need.
Same dust shield Hammer showed (I don't and not equipped to deal with stuff too heavy) looked like that since the 1960s did rub or bend assorted GM vehicles just took those off see where it's rubbing (ages ago friend) and put it back on after bending those.


You say no? IDK it's late here to go look my trans is a 4L80E with coolers everywhere I don't need to know more till it breaks.


Yours wouldn't be that one + IDK what choices there were?
That if like Hammer's graphics was an issue still unsure why on 100% were cars use this same block for an engine.
Arggh - IDK, back to the question top of this post "Why did you need to touch the flex plate?"


Tom


Phantom305
User

Apr 24, 2021, 7:33 PM

Post #23 of 34 (460 views)
Re: Knocking coming from cat Sign In

Haha i gotcha, i really appreciate the help. I heard the ticking when I bought the truck a year ago, i just had my tranny rebuilt and they said the flexplate was cracked so i said oh cool i guess i was right...well i was wrong because its still tickin.

As for the motor, yea its almost identical to the 350 except it has smaller bore, 3.75 compare to the 4.0 of the 350. I highly doubt its got a bottom end issues these things usually go 300k without bearing or rod issues and i know the guy who owned her before me and he took good care to do services on time.

So im in my garage right now because i ran the truck for 1 hour with 2 bottles of seafoam and it seemed like it got a little bit quieter but idk. So i figured ill start taking the plugs out see if theyre worn or contaminated. Well.. i took my airbox out and it reeks of gas!! So apparently that means its got a leaky injector, which would cause it to run rich obviously(which i knew it was) but also itll cause a knock because its forcing the cylinder to fire on the upstroke.

Im gunna worry about the flexplate later bc its not worth taking apart if i cant fix whatevers knocking in the motor. So this truck has spider injectors which a few people have told me are Sh!tty so im gunna look at them and see if maybe...just maybe... its a relatively simple thing causing this problem... btw keeping your fingers crossed while working on a motor is hard!


Hammer Time
Ultimate Carjunky / Moderator
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Apr 25, 2021, 3:44 AM

Post #24 of 34 (442 views)
Re: Knocking coming from cat Sign In


Quote
i took my airbox out and it reeks of gas!! So apparently that means its got a leaky injector,


That's simply not true. No connection at all. But a plugged cat could do that.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
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Apr 25, 2021, 4:09 AM

Post #25 of 34 (440 views)
post icon Re: Knocking coming from cat Sign In

So, you've only known this for about a year, started off with a trans job and this flex plate that was cracked. It was making noise then and still is - OK.


We or I am moving into rambles on figuring out what to make of this. Basically you don't know how or what it's life was problems or care to get it to 200k do you? Anything can be wrecked in no time or a few years of neglect. Do you know if this is the original engine at all?
Seafoam! Easy on that stuff it will eat a hole thru it's own container TWO bottles is excessive IMO. It's great and strong I use it by "caps filled" of it never as aggressive as you just were. Expect plugs to be fouled now - speaking of which they may not want to come out, use all care not to bust them off. Some brands can be made that leave thread in heads, hex and porcelain comes out you are in for a real treat or someone is. The originals were a high content of stainless steel TMK no sold as of 10-15 years ago but still AC Delco's suggested crossover it should want at least those.


You found fuel in the air filter housing? A first so I'm going to go look at my own will be the same injector type in this should NOT do that I'll come back after a good look how or why that could happen as IDK right now, T







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