light knocking can't find it??!! - Pontiac GTO Forum
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post #1 of 16 (permalink) Old 07-06-2008, 10:51 PM Thread Starter
 
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light knocking can't find it??!!

I have a 1970 455HO all original w/ the motor rebuilt about 300 mls on it. Over a year ago I did a sprint on the hwy and took it up to about 100 mph. After I got it home I heard a slight tapping or knocking and thought it was in the valvetrain. Put a stethescope all over it and cant locate it. Felt it was a lifter and wanted to put in a Ram Air 4 cam anyway which I did. Lifters, cam, and timing set from Comp Cams. Still there. Never gets louder and I can't really hear it when I rev it. I have had rod knocks before and this isn't like that at all. And a rod gets louder when hot and worse over time. So I put STP honey and 20w/50 on an oil change which I figured would quiet it down if I was wrong and it was a rod and it is exactly the same cold and hot. No change. Well I had it on my new lift running and had the stethescope all over it again and the sound seemed to strongest at the fuel pump. So I figured I would change it out only $31.00. No luck. The essentric on the timing gear for the fuel pump seemed fine when I did the cam. Are there more than one size and mabey it's smacking the fuel pump arm when it rides around? This is now driving me insane. Any ideas or known issues? Help!!!!!!!
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post #2 of 16 (permalink) Old 07-07-2008, 07:35 AM
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You could try to run the car for a bit with the fuel pump removed to see if the sound is still there.
It may be a bad wrist pin too, did you use forged pistons when you rebuilt it?
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post #3 of 16 (permalink) Old 07-07-2008, 12:12 PM Thread Starter
 
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No forged pistons. I know they knock a bit especially cold. Wrist pin?? When I put the stethescope all over the motor in doesn't seem to be louder along either side high or low.
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post #4 of 16 (permalink) Old 07-07-2008, 12:55 PM
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A good test to see if it`s a wrist pin or a connecting rod knocking, rev and hold the motor to about 2,000rpm (you should be hearing the knocking) then stab the throttle hard and then let off quick and pay attention to the knock when it revs up. If the knocking gets worse when it`s rev`ing up, it`s a connecting rod, if the noise goes away as it`s rev`ing, then it most likely is a wrist pin. Theory being if it`s a wrist pin, when the motor revs it puts the wrist pin under a load and the noise goes away. I would also try to run it just a bit with the fuel pump off just to eliminate that as the culprit too.
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post #5 of 16 (permalink) Old 07-07-2008, 10:43 PM Thread Starter
 
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I'll giveit a try tomorrow, starting to sound like it may be a wrist pin but of course i'll be pulling th efuel pump first. Will it run w/o pressure in the fuel line just being fed fuel?
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post #6 of 16 (permalink) Old 07-07-2008, 10:48 PM Thread Starter
 
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Well it was blue printed and built by a pretty good shop, the wrist pins are pressed in do you really think it's slopping around that soon w/ so little mileage. Seems like that would be a high mileage condition> The rod I agree could pop up any time if it wasn't right.
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post #7 of 16 (permalink) Old 07-08-2008, 07:35 AM
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Yes it will run without the fuel pump, at least until the carb runs out of gas anyway. Should be long enough to tell if the pump is making the noise.
And it won`t be the actual wrist pin that fails, but the piston around the pin. The wrist pin hole in the piston gets ob-longed so the pin moves up and down within the piston hole. That problem is more with cast pistons then with forged ones and it can crop up at any time, usually after a high speed run. I`ve also herd motors with noisy wrist pins run for thousands of miles making the noise too.
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post #8 of 16 (permalink) Old 07-08-2008, 12:25 PM
 
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Originally Posted by seamus2154 View Post
I have a 1970 455HO all original w/ the motor rebuilt about 300 mls on it. Over a year ago I did a sprint on the hwy and took it up to about 100 mph. After I got it home I heard a slight tapping or knocking and thought it was in the valvetrain. Put a stethescope all over it and cant locate it. Felt it was a lifter and wanted to put in a Ram Air 4 cam anyway which I did. Lifters, cam, and timing set from Comp Cams. Still there. Never gets louder and I can't really hear it when I rev it. I have had rod knocks before and this isn't like that at all. And a rod gets louder when hot and worse over time. So I put STP honey and 20w/50 on an oil change which I figured would quiet it down if I was wrong and it was a rod and it is exactly the same cold and hot. No change. Well I had it on my new lift running and had the stethescope all over it again and the sound seemed to strongest at the fuel pump. So I figured I would change it out only $31.00. No luck. The essentric on the timing gear for the fuel pump seemed fine when I did the cam. Are there more than one size and mabey it's smacking the fuel pump arm when it rides around? This is now driving me insane. Any ideas or known issues? Help!!!!!!!
I have a 1967 GTO with a 400 and it developed a tapping sound after I had the oil changed shortly after I purchased the car in January 2008. In the most recent edition of "The Legend," there was an article authored by "Mr. Goatwrench" that discussed the additive ZDDP, which used to be part of motor oil manufactured prior to Jan. 1, 2008. I contacted a company in Chicago that sells ZDDP as an oil additive and added a bottle to my engine. Once the engine reached operating temperature (around 160 degrees), voila, no more tapping noise. You may want to give this a try. A caveat -- I'm not a mechanic and Rukee has much more engine knowledge that I ever hope to have. Anyway, that's my "tapping" story. Let me know if this works for you.

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post #9 of 16 (permalink) Old 07-08-2008, 02:50 PM
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A way to isolate the cylinder making noise is to pull the plug wire off of each plug, one at a time, with the engine running, and see if the noise goes away or changes pitch. Rukee may have something: it sounds like a wristpin to me. Also, it COULD BE a wristpin if you're not running ZDDP additive. ZDDP was removed from most oils in 2006 to protect worn out, oil burning cars with CAT converters from fouling the converters. ZDDP is needed to prevent galling and wear in all flat-tappet engines. It is a metal based zinc and lithium product that coats moving parts, like wristpins! I run Delo 400 or Shell Rotella Diesel spec oil with an additive.
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post #10 of 16 (permalink) Old 07-08-2008, 08:38 PM
 
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Garage
Could also be a bad cam bearing or a loose flywheel /flexplate bolt.



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