L98 accessory knocking sound
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Joined: Jul 2013
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From: Massachusetts
L98 accessory knocking sound
Light knocking sound about 5 mins after warming up. I can only hear it at idle. Remove belt, quiet, no knock. New belt, tensioner, air pump, alt. If I spin water pump, PS pump, A/C by hand, they seem smooth with no runout. The only other way I can stop the knock at idle is by slipping the clutch with e-brake on and pulling the engine down to ~6-700 rpm under load. Strange. Any ideas?
I checked all accessory mounts for tightness as well as pulleys and crank bolt.
I checked all accessory mounts for tightness as well as pulleys and crank bolt.
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From: Massachusetts
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From: Anaheim CA
You can tell if the outer ring of the harmonic balancer has moved by looking at the hole in the rear of the timing tab. The rear edge of the harmonic balancer should be visible through the hole. This isn't a C4, but it shows the relationship:
A mechanic's stethoscope will let you listen to the various accessories while the engine is running. You can find them for very little money with a little searching.
A mechanic's stethoscope will let you listen to the various accessories while the engine is running. You can find them for very little money with a little searching.
Last edited by Cliff Harris; Sep 18, 2013 at 01:48 AM.
Thread Starter
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Joined: Jul 2013
Posts: 67
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From: Massachusetts
You can tell if the outer ring of the harmonic balancer has moved by looking at the hole in the rear of the timing tab. The rear edge of the harmonic balancer should be visible through the hole. This isn't a C4, but it shows the relationship:
A mechanic's stethoscope will let you listen to the various accessories while the engine is running. You can find them for very little money with a little searching.
A mechanic's stethoscope will let you listen to the various accessories while the engine is running. You can find them for very little money with a little searching.
Race Director
Joined: Apr 2002
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From: Anaheim CA
The belt pulley is bolted to the center hub of the harmonic balancer. The outer ring can move without affecting the pulley alignment.
Last edited by Cliff Harris; Sep 19, 2013 at 02:48 AM.
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From: Massachusetts
Tried a few different things today. Made a mark between the two halves of the harmonic balancer. No movement, yet. Probably the most telling thing is this, If I put my breaker bar on the tensioner and push loose, sounds goes away 99%, if I pull and increase tension on the belt, sound goes away 100%. I think it must be the harmonic balancer but I want to be sure before I pull it. What do you guys think?
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From: Massachusetts
Checked static timing today and the marks line up, so I don't think the outer ring has migrated. I can't completely rule out the harmonic balancer yet but I'm thinking a cracked accessory mount or something loose I can't see is hitting when warm?
Melting Slicks
Joined: May 2010
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From: Conowingo Maryland
get a sepothscope 20 bucks at auto store. run car and place scope to every thing that could make a noise. The air pumps on these things are know for knocking then coming apart. they will turn by had ok but when you rev them up and put a load on them different story
Safety Car
Joined: Apr 2011
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You don't have to spend a dime to diagnose this !
get a section of hose, water hose, anything. Run the engine at idle and place the end of the hose on the various things and stick the other end to your ear...LISTEN. Listen and learn what you are hearing.
Suddenly small things like injector clicks are thunderous....and big things like bearings failing sound like cement blocks grinding against each other.
The idler WILL cause a squeal or chatter. The bushing inside is a plastic/poly material that wears out and lets the idler slip for/aft as it starts bouncing at idle. There is no fix, replacement only. Shop around...those can go for well over $100 while some
sell for <$75
The hose/ stethoscope works...you can use a screwdriver BUT, that can be very dangerous. You have to look away to get your ear on it and IF YOU SLIP....you can get tangled in the belt on a running engine and get a screwdriver stuffed thru your head.
No solid objects (including your face) around or near moving parts.
get a section of hose, water hose, anything. Run the engine at idle and place the end of the hose on the various things and stick the other end to your ear...LISTEN. Listen and learn what you are hearing.
Suddenly small things like injector clicks are thunderous....and big things like bearings failing sound like cement blocks grinding against each other.
The idler WILL cause a squeal or chatter. The bushing inside is a plastic/poly material that wears out and lets the idler slip for/aft as it starts bouncing at idle. There is no fix, replacement only. Shop around...those can go for well over $100 while some
sell for <$75
The hose/ stethoscope works...you can use a screwdriver BUT, that can be very dangerous. You have to look away to get your ear on it and IF YOU SLIP....you can get tangled in the belt on a running engine and get a screwdriver stuffed thru your head.
No solid objects (including your face) around or near moving parts.
Instructor
Joined: Jan 2013
Posts: 104
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From: Moriac Victoria
I had a similar noise on my 88 Vette, it sounded like it had a cam shaft gear drive installed, it had a really distinctive whine that I swear was coming from inside the engine. Even using a piece of hose as a stethoscope and checking all the accessory drives didn't narrow it down.
With the serpentine belt removed the noise went away so I knew it had to be either the ps pump, alternator, smog pump etc, they all seemed fine when spun by hand.
I just happened to have a spare new alternator so I swapped it in, that fixed it, noise was gone.
It weird sometimes how noise can travel/resonate and lead you in another direction.
Ken D
With the serpentine belt removed the noise went away so I knew it had to be either the ps pump, alternator, smog pump etc, they all seemed fine when spun by hand.
I just happened to have a spare new alternator so I swapped it in, that fixed it, noise was gone.
It weird sometimes how noise can travel/resonate and lead you in another direction.
Ken D
Last edited by Bandit1977; Apr 7, 2014 at 07:11 AM.
Instructor
Joined: Jan 2013
Posts: 104
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From: Moriac Victoria
......bit of a typo there by me!
Thread Starter
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Joined: Jul 2013
Posts: 67
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From: Massachusetts
I had a similar noise on my 88 Vette, it sounded like it had a cam shaft gear drive installed, it had a really distinctive whine that I swear was coming from inside the engine. Even using a piece of hose as a stethoscope and checking all the accessory drives didn't narrow it down.
With the serpentine belt removed the noise went away so I knew it had to be either the ps pump, alternator, smog pump etc, they all seemed fine when spun by hand.
I just happened to have a spare new alternator so I swapped it in, that fixed it, noise was gone.
It weird sometimes how noise can travel/resonate and lead you in another dire action.
Ken D
With the serpentine belt removed the noise went away so I knew it had to be either the ps pump, alternator, smog pump etc, they all seemed fine when spun by hand.
I just happened to have a spare new alternator so I swapped it in, that fixed it, noise was gone.
It weird sometimes how noise can travel/resonate and lead you in another dire action.
Ken D
Exactly. It's not easy to find the source of the noise. I've replaced the alt, smog pump and tensioner. The sound goes away when I take off the belt. Will be using a stethoscope today to try and narrow it down.
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Joined: Jul 2013
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From: Massachusetts
It's either the water pump, PS pump or A/C. Everything else has been ruled out. Could it be a cracked accessory mount that only knocks when it gets warm? I've used a prybar where I can fit it but can't find anything.
Safety Car
Joined: Apr 2011
Posts: 3,660
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Cool !
I've discovered that these tensioners can be responsible for a whole bunch of weird symptoms and sounds...
they get weak, bearing gets loose and they cause chatter, thumping rattling noises that just can't be explained.
if you take note of the nylon bushing inside the tensioner and how much you can see...the more that slips out and the more that can be seen is an indicator of how bad the tensioner is.
these belt tensioners ARE wear parts just like brakes and tires.
I've discovered that these tensioners can be responsible for a whole bunch of weird symptoms and sounds...
they get weak, bearing gets loose and they cause chatter, thumping rattling noises that just can't be explained.
if you take note of the nylon bushing inside the tensioner and how much you can see...the more that slips out and the more that can be seen is an indicator of how bad the tensioner is.
these belt tensioners ARE wear parts just like brakes and tires.