Engine Knock Help Needed
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,322
Likes: 141
From: Green Valley Arizona
Engine Knock Help Needed
Engine is a rebuilt L81 in my '81, rebuilt several years ago. Has a knock that is connected with RPM....knock frequency increases as engine RPM. Really noticeable at idle of around 700rpm....and I can no longer hear it much above 1000rpm.
Noise seems to be coming around #1 cylinder. I have used a stethoscope and probed everything I could, the knock never got louder or more defined or anything anywhere I put the metal rod to. I hear it best with my head down near #1.
I hope it is just an exhaust leak; I put the metal tip of the stethoscope on the exhaust manifold at #1 and once again the knock did not get any louder or more pronounced.
All exhaust manifold bolts are tight.
Ideas to try???????
Noise seems to be coming around #1 cylinder. I have used a stethoscope and probed everything I could, the knock never got louder or more defined or anything anywhere I put the metal rod to. I hear it best with my head down near #1.
I hope it is just an exhaust leak; I put the metal tip of the stethoscope on the exhaust manifold at #1 and once again the knock did not get any louder or more pronounced.
All exhaust manifold bolts are tight.
Ideas to try???????
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,753
From: Charlotte NC
Worth a try...you can have a cracked crankshaft pulley...and or cracked flex-plate (automatic transmission)
Remove all of your belts AFTER YOU have made sure the noise is there. Then crank the engine and see if the noise is still there. IF NOT...I have run into crankshaft pulleys having crack in them and making a noise that is wicked.
Then If the noise is still there...because noise can travel and throw you off...I would disconnect the torque converter and crank the engine
You do not have to ave the engine running long...which is why you want to make sure the noise is there first ...before you begin.
Is you oil pressure good????
DUB
Remove all of your belts AFTER YOU have made sure the noise is there. Then crank the engine and see if the noise is still there. IF NOT...I have run into crankshaft pulleys having crack in them and making a noise that is wicked.
Then If the noise is still there...because noise can travel and throw you off...I would disconnect the torque converter and crank the engine
You do not have to ave the engine running long...which is why you want to make sure the noise is there first ...before you begin.
Is you oil pressure good????
DUB
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,322
Likes: 141
From: Green Valley Arizona
Ran out of time today, but next I will remove the two belts and see if the noise is there.
And the noise is there upon start...no waiting for it....it is always there.
Oh and trans is a manual and the timing chain only has about 8k miles on it and is a double roller.
The situation is this: car came back from the paint shop on a truck. I was told the shop had the engine running when he picked it up. What!!!!!!! The engine had not been started for 5 months and I was not there when they loaded it.
So I start the engine within the next few days and there is the knock. It was not there when I had the car delivered for painting. I know they were just pushing the car around in the shop, so why they started it.....did they rev the engine, smoke the tires, etc before loading onto the truck...???!!!have no idea and of course I have no proof so I was not even going to start the argument. They still have parts to paint.
I do not think it is a spun rod bearing....I sure hope it is not. But I may be dropping the oil pan and taking a look-see before long.
And the noise is there upon start...no waiting for it....it is always there.
Oh and trans is a manual and the timing chain only has about 8k miles on it and is a double roller.
The situation is this: car came back from the paint shop on a truck. I was told the shop had the engine running when he picked it up. What!!!!!!! The engine had not been started for 5 months and I was not there when they loaded it.
So I start the engine within the next few days and there is the knock. It was not there when I had the car delivered for painting. I know they were just pushing the car around in the shop, so why they started it.....did they rev the engine, smoke the tires, etc before loading onto the truck...???!!!have no idea and of course I have no proof so I was not even going to start the argument. They still have parts to paint.
I do not think it is a spun rod bearing....I sure hope it is not. But I may be dropping the oil pan and taking a look-see before long.
Drifting
Joined: Jun 2010
Posts: 1,781
Likes: 15
From: Oxford Ohio
Could be a number of things. I've seen loose converter bolts make that noise also. The noise can travel like DUB has said. If you have an engine driven fan, cut some cardboard so to block the air flow to the one side of the engine. On the cabin side of the fan. While running, take a propane torch set as low as you can get it, move it around your exhaust/ header to the head seam and watch for a puff of exhaust gas to alter or move the propane flame. You can rule out the header leak.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
Try a heavier weight oil. I had a 305 we beat the crap out of for years and that knock just told us it was time to change the oil then it would go away.
We went of 50,000 miles with that knocking bearing before everything else fell apart.
We went of 50,000 miles with that knocking bearing before everything else fell apart.
Melting Slicks
Joined: Aug 2012
Posts: 3,487
Likes: 83
From: Titusville Florida
Mine developed a knock similar to what you're describing shortly after I bought it, I never could locate the cause so I pulled the numbers motor and put in a crate engine. I have yet to pull the motor apart to see if I could find anything. good luck on yours and I'll be interested to hear what your knock turns out to be 
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,753
From: Charlotte NC
STUPID QUESTION: Are you sure it is not a rocker arm "clacking".
Again...Is you oil pressure still good? Reason being...if it is a bad bearing your oil pressure should not be that good....or get worse when the oil gets hot....depending on the type of oil you are using.
And I really do not want to mention this....and NOT wanting to start somehting...BUT...it could be that "something" got inside the carb...and got into a cylinder and is now embedded on the top of the piston. If you have access to a bore scope...a good one. It might be worth the time looking.
Also...as I have been told to try and has helped me in some situations....I have removed the spark plug on the cylinder that I believed was the problem...and the noise changed due to the compression and such were taken out of the equation.
DUB
Again...Is you oil pressure still good? Reason being...if it is a bad bearing your oil pressure should not be that good....or get worse when the oil gets hot....depending on the type of oil you are using.
And I really do not want to mention this....and NOT wanting to start somehting...BUT...it could be that "something" got inside the carb...and got into a cylinder and is now embedded on the top of the piston. If you have access to a bore scope...a good one. It might be worth the time looking.
Also...as I have been told to try and has helped me in some situations....I have removed the spark plug on the cylinder that I believed was the problem...and the noise changed due to the compression and such were taken out of the equation.
DUB
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
2027 Corvette vs The World: Every Model vs Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
10 Things C8 Corvette Owners Hate (But Won't Tell You)
Joe Kucinski
10 Best Corvettes Coming to Barrett-Jackson Palm Beach 2026!
Brett Foote
Every Corvette Grand Sport Explained! (C2, C4, C6, C7, & C8)
Joe Kucinski
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,322
Likes: 141
From: Green Valley Arizona
I removed both valve covers and ran the engine (short time) all rockers were moving the same, all appeared normal.
I have no interior at the moment so I do have an oil pressure gauge hooked-up.
I sure hope nothing got into the carb. The paint shop would have had to remove the air filter to do so. And no bore scope.
I have the two V belts removed right now and am going to push car out into driveway and start engine and hopefully the knock is gone. If not, then I will be pulling a plug wire to see what that does to the knock.
Will report back my findings.
I have no interior at the moment so I do have an oil pressure gauge hooked-up.
I sure hope nothing got into the carb. The paint shop would have had to remove the air filter to do so. And no bore scope.
I have the two V belts removed right now and am going to push car out into driveway and start engine and hopefully the knock is gone. If not, then I will be pulling a plug wire to see what that does to the knock.
Will report back my findings.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,753
From: Charlotte NC
I removed both valve covers and ran the engine (short time) all rockers were moving the same, all appeared normal.
I have no interior at the moment so I do have an oil pressure gauge hooked-up.
I sure hope nothing got into the carb. The paint shop would have had to remove the air filter to do so. And no bore scope.
I have the two V belts removed right now and am going to push car out into driveway and start engine and hopefully the knock is gone. If not, then I will be pulling a plug wire to see what that does to the knock.
Will report back my findings.
I have no interior at the moment so I do have an oil pressure gauge hooked-up.
I sure hope nothing got into the carb. The paint shop would have had to remove the air filter to do so. And no bore scope.
I have the two V belts removed right now and am going to push car out into driveway and start engine and hopefully the knock is gone. If not, then I will be pulling a plug wire to see what that does to the knock.
Will report back my findings.
What was you oil pressure reading???
I know the air cleaner lid would have to be removed. BUT...you never know...some people are ---holes...an want to "rain" on other people's parade. JEALOUS BASTARDS. Hopefully that is not the case and I am not implying that is what happened...but I have know it to happen.
DUB
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,322
Likes: 141
From: Green Valley Arizona
GOOD NEWS:
Knock noise is from #1 cylinder exhaust manifold leaking.
I could feel the leak in two different angles only after taking the fan/water pump V belt off. Got all the exhaust manifold bolts out and got the gasket out and about half of #1 is gone.
I have the stock exhaust manifolds on right now as I had to take the sidepipe headers off for the repainting. So I will probably just get a new gasket for now as I need to be able to drive the car back to the paint shop for several spots they missed and the sidepipes would be in the way.
I am so relieved that it is not a spun rod bearing. I did not think it was as I could not hear the knock with the stethoscope probe on the engine.....but that did not keep me from thinking the worst.
Thanks for the replies once again.
Knock noise is from #1 cylinder exhaust manifold leaking.
I could feel the leak in two different angles only after taking the fan/water pump V belt off. Got all the exhaust manifold bolts out and got the gasket out and about half of #1 is gone.
I have the stock exhaust manifolds on right now as I had to take the sidepipe headers off for the repainting. So I will probably just get a new gasket for now as I need to be able to drive the car back to the paint shop for several spots they missed and the sidepipes would be in the way.
I am so relieved that it is not a spun rod bearing. I did not think it was as I could not hear the knock with the stethoscope probe on the engine.....but that did not keep me from thinking the worst.
Thanks for the replies once again.
Last edited by LannyL81; Apr 30, 2014 at 05:42 PM.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
What's your oil pressure after a good shake down at idle? If it's not below 25psi. I would be looking elsewhere for the source of the knock. Torque converter bolts loosen up, fuel pump and even an exhaust leak can sound like a knock.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,753
From: Charlotte NC
He stated it is a MANUAL transmission.
DUB