One for the engine guys!
Thread Starter
Pro
Joined: Jun 2009
Posts: 613
Likes: 3
From: Fleetwood PA
One for the engine guys!
Ok....I have a 454(1974 by the engine code) in my 68. I have a loud knock when the car is at idle(not a valve tap, a knock) It seems to go away when I give it some gas while driving(engine is under load) and only if I'm rolling, (the knock is there if I'm just revving in neutral). As soon as I get off the throttle, it comes back. I know what ever it is isn't good, just wondering where to start looking first.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
Ok....I have a 454(1974 by the engine code) in my 68. I have a loud knock when the car is at idle(not a valve tap, a knock) It seems to go away when I give it some gas while driving(engine is under load) and only if I'm rolling, (the knock is there if I'm just revving in neutral). As soon as I get off the throttle, it comes back. I know what ever it is isn't good, just wondering where to start looking first.
Heel & Toe
Joined: Oct 2005
Posts: 22
Likes: 0
From: Howell MI
Ok....I have a 454(1974 by the engine code) in my 68. I have a loud knock when the car is at idle(not a valve tap, a knock) It seems to go away when I give it some gas while driving(engine is under load) and only if I'm rolling, (the knock is there if I'm just revving in neutral). As soon as I get off the throttle, it comes back. I know what ever it is isn't good, just wondering where to start looking first.
Back in the 70's I had an old Ford pickup that did the same. I rebuilt the engine and found it was the timing chain slapping at idle. The chain was so stretched I can't believe it didn't jump teeth.
Drifting
Joined: Oct 2004
Posts: 1,255
Likes: 7
From: Coralville Iowa
Get an engine stethoscope. They are fairly cheap and indispensable if you have engine noises. You might be able to hear it and find the locale of the sound. Sounds a little like a rod bearing or wrist pin to me. Under load the chambers have a lot of positive pressure holding the rods tight to the crankshaft. On deceleration or idle they are a little "loose" on the shaft and you will typically hear a loud knock.
Here's a little info:
Listen with your stethoscope. A rod bearing makes more noise at the oil pan than elsewhere, and a wrist pin more racket up on the water jacket. Hold RPMs at 2500, jerk the throttle open and let it snap closed. This will accentuate rod knock, whereas pin noise won't change very much.
Start shorting out cylinders. A bad pin will quiet down, but a rod knock will double its cadence. (This is due to no power on compression stroke because the plug is is not firing)
Pull the pan for a visual inspection. (Check for metal in the oil. Bad bearings make the oil look kind of like metalflake paint ) If the bearings are good, you know you've got a pin problem.
Here's a little info:
Listen with your stethoscope. A rod bearing makes more noise at the oil pan than elsewhere, and a wrist pin more racket up on the water jacket. Hold RPMs at 2500, jerk the throttle open and let it snap closed. This will accentuate rod knock, whereas pin noise won't change very much.
Start shorting out cylinders. A bad pin will quiet down, but a rod knock will double its cadence. (This is due to no power on compression stroke because the plug is is not firing)
Pull the pan for a visual inspection. (Check for metal in the oil. Bad bearings make the oil look kind of like metalflake paint ) If the bearings are good, you know you've got a pin problem.
Last edited by highschool67; Sep 30, 2009 at 12:14 PM.
Burning Brakes
Joined: Jan 2007
Posts: 1,057
Likes: 4
From: Concord Calif
Ok....I have a 454(1974 by the engine code) in my 68. I have a loud knock when the car is at idle(not a valve tap, a knock) It seems to go away when I give it some gas while driving(engine is under load) and only if I'm rolling, (the knock is there if I'm just revving in neutral). As soon as I get off the throttle, it comes back. I know what ever it is isn't good, just wondering where to start looking first.
Thread Starter
Pro
Joined: Jun 2009
Posts: 613
Likes: 3
From: Fleetwood PA
Thanks guys for all your suggestions!!!!
I have a feeling it's a rod or wrist pin. It's a loud knock, not an octane or detonation kind of knock. I have a stethoscope, but haven't been able to really narrow it down. I will definitely use your suggestions Highschool...great info
. I'm hoping for wrist pin(could still drive it now and then till winter hits) I plan on a complete tear down over winter anyway. 
I have a feeling it's a rod or wrist pin. It's a loud knock, not an octane or detonation kind of knock. I have a stethoscope, but haven't been able to really narrow it down. I will definitely use your suggestions Highschool...great info
. I'm hoping for wrist pin(could still drive it now and then till winter hits) I plan on a complete tear down over winter anyway.
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I've seen converter bolts do that.
Melting Slicks
Joined: Dec 2006
Posts: 2,669
Likes: 3
From: Kingston Ontario
I had a 460 (I know 
) in a boat that had piston slap at idle for over five years without incident. Older marine mechanic told me it was "normal for that engine". Drove me crazy for the whole five years! Sounds exactly as described, only at idle; and ran super strong. 
) in a boat that had piston slap at idle for over five years without incident. Older marine mechanic told me it was "normal for that engine". Drove me crazy for the whole five years! Sounds exactly as described, only at idle; and ran super strong.