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Trying to Diagnose Knocking Sound


Joe Flanagan

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The knock that you hear, is it a sharp high pitched knock, or a deep low pitched knock, cam shaft speed or crankshaft speed?  If it is the fuel pump then it will be camshaft speed.  If it is a rod it will be crankshaft speed and sharp.  I had two piston pin bushings in the rods extrude from excessive load from running too much advance.  This causes an unusual sounding knock that removing plug wires did not effect the sound.  I pulled the pan and rocked each cylinder over at top dead center to find the slack.  Blue Skies who used to post here all the time suggested I do so and I found my problem.

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I can't say that it's particularly high pitched. What it sounds like is someone hitting the inside of the block with a ball peen hammer. I don't know how to tell whether it's crankshaft or camshaft speed but at idle, the sound occurs faster than once every second (that's a guess, but I think it's accurate). It's definitely not tinny or metallic. It sounds like something of substance hitting something of substance, if you know what I mean. Is there anything in the valve train that could make this noise? I can't see how it would, but I'm obviously no expert.

Remember that this is a rebuilt engine with only 950 miles on it. I'd be surprised if carbon is an issue, though Pete Anderson said he found a lot of carbon in his engine after not a lot of miles. 

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I don't know if this is possible on these engines but could the pick up for the oil pump be partially blocked ?

When I was in high school a friend and I pulled an all nighter and rebuilt a SBC. Ran fine for a while but a few days later developed a pretty bad knock. When we dropped the oil pan we discovered that we had the pick up screen upside down. Fixed that and the knock never returned.

 

Jeff

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You don't want to hear my thoughts and it's probably not your problem.  It may be a broken crankshaft.  They don't always break cleanly and engines can continue to run with them broken and it may partially explain your wobbly pulley.

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This topic got busy. I'd be interested to find out what it is so keep us posted.

Usually a rod knocking will appear and disappear at different RPM, mostly when decelerating you'll hear it. When accelerating and you hear it most times its a piston or wrist pin. A main usually knocks all the time. I know you pulled your plug wires and the knock was still there, if its a rod most times it will stop knocking - it takes away the power stroke. That usually works on a worn or even spun bearing but don't count on it for a damaged rod or badly worn bearing. What comes to mind is the loose pulley bolt, if the pulley was wobbling it could have been throwing a vibration onto the crank shaft which would take it to the lower timing gear and the front main bearing.

Either way it sounds like you will need to remove the timing cover, if that does not show anything I would look at dropping the oil pan next.

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And finally, I keep going back to the weird noise I heard and that started this knocking yesterday morning. It was the first time moving the vehicle for the day. I let up on the clutch, went in reverse, and it sounded like something grinding or scraping. It was a really weird sound. Ever since then, the engine has knocked. It kind of sounded like I was backing over a lawn chair or something. Did the sudden torque on the engine cause something to let go?  

I just re-read this, a rod or bearing will not make a grinding or scraping sound. Never heard of it anyway.

I know you still hear it when you depress the clutch, so that would leave the flywheel and pressure plate.

 

Also, one way to tell a knock between the valve train and crankcase is with a timing light. It will light at about TDC on the compression stroke on one piston. So that can give you an indication of how often the piston is traveling. If the knock is slower than the light is blinking then it should be valve train related.

 

Also a rod knocking will usually get louder when you rev the engine and especially as the engine decelerates you will hear a rod increase in noise.

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You don't want to hear my thoughts and it's probably not your problem.  It may be a broken crankshaft.  They don't always break cleanly and engines can continue to run with them broken and it may partially explain your wobbly pulley.

This pulley has wobbled since the day I put it on, so unless the crankshaft was broken since the day I installed it, I don't think that's the problem. Last night I re-read Pete Anderson's account of the wrist pin bushing that went bad in his 50 Plymouth. He said that multiple attempts at isolating the problem by pulling the spark plug wires didn't tell him anything UNTIL the knock worsened. At that point, pulling the wires pinpointed the cylinder where the problem was. I would say this knock is getting worse, so in time I may be able to isolate it by pulling the wires one by one. 

One thing I didn't do yesterday is remove the fuel pump and examine it. Do you guys think that is worth doing? At least then I could eliminate that as the cause. Plus it's pretty easy to do. 

Once I get the radiator back in and the pulley installed, I'll run the engine and see if I can determine whether the noise is crank speed or cam speed. I'll also pull the wires one by one again and see if that tells me anything. In the meantime, I'm going to see what's up with this pulley.

Edited by Joe Flanagan
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OK, thanks. By the way, I have confirmed that the pulley is bent. Pretty badly, too. I checked it with a combination square and it's way off. But you can also see it easily with the naked eye. It looks like somebody might have tried to pry it off at some time. I have a friend with a press whose going to repair it. 

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Yes, same pulley. But when this first happened a few years ago, I was able to make the sound stop by pulling the spark plug wire on cylinder #6.  When I started looking into things, I found that all the rod and main bearings were badly scored. I'd rebuilt the engine and it sat on a stand in my garage for 7 years, covered, but without the spark plugs in their holes. There was sandblasting going on outside the garage, which may have been a factor. Also, when I first started it, I had zero oil pressure for a short while before it finally came up. I really had trouble getting the pressure to come up. So that might have accounted for the scoring, too. I replaced all the bearings and had the cylinders and crank honed to remove the scratches. The machinist could find no cause for the knocking and he really went through it. He miked the #6 cylinder and the piston, too. Even though there appeared to be nothing wrong with the wrist pin in #6, I replaced it. I reassembled the engine and it ran fine until this past weekend. However, much of that was idling just to keep the engine working as I worked on the rest of the car. I didn't actually get the car on the road until this August. I'm convinced that whatever that noise was when I first moved the car last Sunday morning is the answer to this mystery. I just don't know what that was. 

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This pulley has wobbled since the day I put it on, so unless the crankshaft was broken since the day I installed it, I don't think that's the problem. Last night I re-read Pete Anderson's account of the wrist pin bushing that went bad in his 50 Plymouth. He said that multiple attempts at isolating the problem by pulling the spark plug wires didn't tell him anything UNTIL the knock worsened. At that point, pulling the wires pinpointed the cylinder where the problem was. I would say this knock is getting worse, so in time I may be able to isolate it by pulling the wires one by one. 

One thing I didn't do yesterday is remove the fuel pump and examine it. Do you guys think that is worth doing? At least then I could eliminate that as the cause. Plus it's pretty easy to do. 

Once I get the radiator back in and the pulley installed, I'll run the engine and see if I can determine whether the noise is crank speed or cam speed. I'll also pull the wires one by one again and see if that tells me anything. In the meantime, I'm going to see what's up with this pulley.

 

Can you run it for a minute or so without the radiator & pulley installed.....same as running it without the pulley installed (as has been suggested by many), since the water pump won't be working anyway? That way you can eliminate or identify the pulley as the problem before you re-install it.

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Yes, I plan to do that this weekend. Do you think about one minute is a safe amount of time to run it like that? I guess I could just run it until it gets up to temperature and then shut it off. 

 

Yes a minute is ok but I wouldn't go too much longer. Remember the temp gauge won't work right not submerged in water and since you've got the radiator out it shouldnt be in water anymore. You can also stick a garden hose(running) in there to help.

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How will you know what the temperature is, if the coolant is drained out?  

 

I suppose you could plug  the hoses and have coolant in the block.  That would absorb some of the heat and make the temperature gauge work. 

 

Or you could connect the two hoses and rely on gravity to circulate the coolant.  But with no radiator, not for long. 

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Worth a shot for the belt. I swore my water pump bearing was going out because of the grinding and metallic sound. Took off the belt and water pump was fine. Tightened the belt and noise was gone. I would have expected a squeal from the belt - I had not heard one make the noise I had.

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