Need tips for isolating engine knocking noise
Thread Starter
Instructor
Joined: Jan 2004
Posts: 176
Likes: 3
From: Houston Tx
Need tips for isolating engine knocking noise
My 327/300 hp car has developed an engine knocking noise that I hear while idling. It is not a metallic tapping noise that you get from lifters, in fact I used to have that noise too and I pulled both heads, replaced all hydraulic lifters, had heads machined with new valves, guides, etc. Now that that noise is gone, I notice the oscillatory knocking noise. Sometimes it comes and goes, and is more noticeable the slower the idle speed and the more warmed up the engine is. Can anyone offer tips to isolate the noise?
Melting Slicks
Joined: Apr 1999
Posts: 2,675
Likes: 13
From: Sudbury MA
I had such a noise in my 327/350 and it turned out to be a excessive spacing in the distributor at the lower gear that is inside the motor. I thought it was bearings or lifters and it was only at idle. The machine shop found it by gently leaning on the distributor while the car was running. I removed and rebuilt the distributor with specific attention to the gap at the bottom of the distributor shaft (gear and shims) and it solved the problem. Cheap fix and quick also. It eliminted the end play whihc caused the distributor shaft to move slightly up and down causing a knocking sound.
Hope yours is something simple like that.
Kurt
Hope yours is something simple like that.
Kurt
Thread Starter
Instructor
Joined: Jan 2004
Posts: 176
Likes: 3
From: Houston Tx
I had hoped that was my problem, as I also had excessive play in the dist shaft. Unfortunitly, I know I recently rebuilt the dist. and have the spec amount of play in the shaft.
I do have a stethoscope and have tried to use it for this noise, but honestly it seems I can hear it all over the place.
Is there a way to tell from the frequency of the knock whether it is from the upper rotating parts (cam, etc.) versus the rotating crankshaft? 500rpm idle is about 8.5 rotations per second. I know the knock does not occur 8 times a second, more like maybe 2 or 3 times a second or even slower.
I do have a stethoscope and have tried to use it for this noise, but honestly it seems I can hear it all over the place.
Is there a way to tell from the frequency of the knock whether it is from the upper rotating parts (cam, etc.) versus the rotating crankshaft? 500rpm idle is about 8.5 rotations per second. I know the knock does not occur 8 times a second, more like maybe 2 or 3 times a second or even slower.
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Melting Slicks
Joined: Jun 2001
Posts: 2,429
Likes: 14
From: CA
Originally Posted by bondtimbond
Is there a way to tell from the frequency of the knock whether it is from the upper rotating parts (cam, etc.) versus the rotating crankshaft? 500rpm idle is about 8.5 rotations per second. I know the knock does not occur 8 times a second, more like maybe 2 or 3 times a second or even slower.
Last edited by 71coupe; Feb 3, 2005 at 01:06 PM.
Le Mans Master
Joined: Aug 2001
Posts: 7,804
Likes: 4
From: Standing still with all my might Texas
Cruise-In X Veteran
St. Jude Donor '09-'11
Try disconnecting one spark plug at a time.
Note:
If you do this while it is running make sure you wear really thick gloves!!!
If the knocking goes away with one spark plug disconnected you have narrowed the problem down to that particular cylinder.
I used this technique to find the knock in my old L-48 and found that the #1 cylinder was the problem. Since my oil pressure was good I assumed that it was a bad wrist pin causing the knock and continued to drive it for several years. Once I finally tore the engine down about a month ago I found that it was actually the rod bearing which was worn almost halfway through.
Hope your's turns out better than mine
Note:
If you do this while it is running make sure you wear really thick gloves!!!
If the knocking goes away with one spark plug disconnected you have narrowed the problem down to that particular cylinder.
I used this technique to find the knock in my old L-48 and found that the #1 cylinder was the problem. Since my oil pressure was good I assumed that it was a bad wrist pin causing the knock and continued to drive it for several years. Once I finally tore the engine down about a month ago I found that it was actually the rod bearing which was worn almost halfway through.
Hope your's turns out better than mine
Team Owner
Joined: Sep 2003
Posts: 88,393
Likes: 2
From: eville in
Originally Posted by ksbunting
I had such a noise in my 327/350 and it turned out to be a excessive spacing in the distributor at the lower gear that is inside the motor. I thought it was bearings or lifters and it was only at idle. The machine shop found it by gently leaning on the distributor while the car was running. I removed and rebuilt the distributor with specific attention to the gap at the bottom of the distributor shaft (gear and shims) and it solved the problem. Cheap fix and quick also. It eliminted the end play whihc caused the distributor shaft to move slightly up and down causing a knocking sound.
Hope yours is something simple like that.
Kurt
Hope yours is something simple like that.
Kurt
Burning Brakes
Joined: Apr 2003
Posts: 844
Likes: 0
From: Central KY
Take out the fuel pushrod (by first removing the fuel pump) and check it for straightness and for any burnish marks at all. If in doubt put in a new one, they're cheap! I had this issue with another car, turned out to be a bad fuel pump itself - was a new one and still bad!
Thread Starter
Instructor
Joined: Jan 2004
Posts: 176
Likes: 3
From: Houston Tx
Update on this thread: I tried several of the techniques mentioned, including retarding the timing and pulling plugs one at a time. None of that made a significant difference. Also, more detailed listening with a stethoscope did not show that the fuel pump is making noise. I replaced my right side exhaust manifold and associated gaskets, and I think an exhaust leak was the majority of the noise I was hearing. The gasket in between the heat riser and manifold was blown out on one side. Things are much quieter now! Thanks for the advice!
