This morning upon starting my 67 L36 427/390 I heard a distinct knock that sounded like a hydraulic lifter problem. The knock is very clear at 700 RPM but disappears as the RPMs increase. Definitely sounds like something is trying to escape. When I pulled the valve covers everything including the lifters looked fine. The engine was rebuilt without any problems over 4,000 miles ago. When I had it on the road last weekend I did have a couple of major misses where the engine stumbled but I did not think anything was particularly unusual about that. Are there any suggestions about what I might check out before tearing the engine down any further?
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Re: Big Block Knock
i recently had a similar sounding problem with an old chevy 216 ci in our 39 woody. the **** they sell as gasoline today is just that--sewerage treatment plant effluent.If one allows a gasoline engine to sit for a month or longer without starting it and with today's gasoline that is at least a couple of months old, then valves can stick in their guides. In my case , about 3 valves stuck and i got 3 bent pushrods. I pulled the valve cover and removed the rocker arms and took a brass-faced hammer and tapped on all the valves. the three with the bent pushrods would depress on meeting mr. hammer but wouldn't pop back up till i sprayed a ton of Kroil on the exposed valve stems and tapped on the valves till they freed up.I started adding gasoline stabilizer to all our cars that aren't run regularly. haven't tried to restart and run any of the dormant vehicles as its too damn cold to go outside.Will know by spring if gas stabilizer works. mike -
Re: Big Block Knock
Thanks for the help, Mike. Before I pulled the valve covers, I was sure it was a valve related issue. Everything looks good and appears to function as it should on the top end, yet the knock is definitely there.Comment
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Re: Big Block Knock
You might have a cam going flat turn the motor over with the valve covers off and look for a valve that does not open as far as the others and then inspect the rocker arm and pushrod on that one if they are ok GEt a dial indicator and measure the pushrod travel I hope I'm wrong but it could be.
BillComment
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Re: Big Block Knock
John,
When I hear a knocking sound, I tend to think rod bearings. Valve related problems are usually more of a clicking sound.
If you were to replicate the sound you hear by tapping on something with a ball pein hammer, what would you tap on?
Do you have hydraulic or solid lifters?
tcComment
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Re: Big Block Knock
Tracy,
The sound is definitely a knock, not a click. Actually, it sounds like "clack, clack, clack." The sound is like a ball peen hammer striking a piece of sheet metal sitting on top of a quarter inch sheet of plywood. The sound disappears when the engine is revved but returns at idle. There are some momemnts when it disappears at idle. One thought was to get it up on a lift and pull the oil pan to see if anything looked amiss from below.
I'm really hopeful that I can find the problem without having to tear down the engine.Comment
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Re: Big Block Knock
A rod knock is pretty distinctive in sound. If it actually sounds like sheet metal, check the sheet metal around the clutch area to be sure nothing loose is hitting the flywheel.Comment
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Re: Big Block Knock
I would at idle pull and replace a plug wire off the Dist cap one at a time and see if the knock gets quieter or goes away. That will tell you if it's a rod bearing or cracked piston.
If you have a Auto. trans, put it in R gear or 4 speed with the emergency brake on and in first gear slowly let the clutch out and see if the knock goes away. That will tell you if the rear crank bearing is worn. This would be a good starting point and set your mind at ease.Also see if your balancer is not loose.Comment
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Re: Big Block Knock
I replaced the balancer last year. For some reason, the balancer is the last thing I think to check. Sounds like a possible candidate. Thanks for the other tips.Comment
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Re: Big Block Knock
John, is it possible that you may be getting some piston slap. If the cylinders were overbored .060 when the engine was rebuilt and piston fit is not tight,it could be that when cold you may hear a clacking sound.Comment
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Clack Clack Clack sounds like a flat lifter to me
With the engine running at idle, take a very long shank screw driver and place the tip up and down along each valve cover as you hold your ear to the handle.
When you find the loudest area you have a good idea which cylinder is affected.
I wouldn't waste my time pulling the pan unless you are going to pull the rod caps off and look at each bearing. you can't see didily otherwise.
If one of your hydraulic lifters is failing to pump up then you have the valve train rattling around at that location.Comment
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Re: Big Block Knock
Check the harmonic balancer.
JoeComment
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Re: Clack Clack Clack sounds like a flat lifter to
Thanks, Tracy. Gives me something else to explore. Everything looked pretty good when I pulled the covers and looked at the lifters and valve train.Comment
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Re: Big Block Knock
The fuel pump can also cause a knock/clicking noise. A long screw driver with the tip placed on the engine at different places and your ear on the handle or a stethoscope should pin point where the noise is most pronounced.
RobComment
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RARE alternative....
I've only seen this once or twice, but it was SERIOUS.... Engine's crank/cam timing gears had apparently escaped heat treat/hardening. After a few K miles, the timing chain ate into the gaps between gear teeth causing the gears to 'shrink' and slack/slop to develop in the timing chain.
Noise ONLY exhibited itself at/near idle + under decel and was the timing chain slopping against the inside edges of the timing chain cover. All the king's horsies and all the king's men who went to find/kill the 'knock' failed. All resolved itself when sufficient 'slop' developed to allow the timing chain to jump gear teeth!
Of course, this was NOT good for the engine....valves kissed pistons and push rods turned to pretzels. If attempts to localize the knock with a stethoscope defy identification, for grins, place the stethoscope on the timing chain cover...Comment
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