In the process of moving my suburban into the shop, I started the engine and heard a knock. It was a consistent sound.
I figured it sounded like a lifter. So I thought, idle is ~800rpm, cam is ~400rpm, so 400/60 =~6.67 hertz.
I plugged it into a hertz/sound generator
https://onlinetonegenerator.com/
In square wave it sounds just like the knock rhythm I was hearing.
I pulled the intake, valley shield and spider and saw a bad spot.
So I pulled the cam for a better look.
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I've never had a failure this small, usually it's catastrophic. It looks too small to make the amount noise it was making. Maybe I am just over thinking it?
Greetings,
I like your diagnostic thought process of using the tone generator to figure out if the knock repetition frequency
is at camshaft or crankshaft speed. I actually followed your link, plugged in your values, and listened carefully.
What I found is that to me the sawtooth function sounded like an individual lifter at 800rpm, whereas the square wave
function sounded twice as fast, like a single piston/rod assembly making a noise? (The sawtooth has a single step
function per cycle, but the square wave has 2 step functions per cycle?)
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Which leads me to the damaged cam lobe. You definitely have found 'a' problem, but I don't know
if it is the source of the knock sound you are trying to track down? In English, the cam is bad, but
when you replace it with a new one I am unsure if it will cure the noise that you are hearing?
You didn't mention using a mechanics stethoscope prior to teardown, but I have found these do give
me positive identification of the noise source. I've used a stethoscope first on the valve covers, and
subsequently on the rocker arms of a slowly idling engine to figure out exactly which of the 16 don't
sound like the others? (
Mechanic's Stethoscope link)
My last observation is that over the years I've noticed valve train noise is always more of a midrange/treble
sound. On the other hand a piston/rod knock can be sharp, but is more of a bass sound? And main
bearings are a deeper rumble?
****
Obviously you have located a bad cam which requires replacement. After installation, it will interesting
to see if the knock sound clears up. (Fingers crossed that it does.) If it doesn't, then the next step
will be to pull the spark plug wires one at a time & see if you can find which piston/rod assembly is
knocking when the mixture fires?
Please keep us in the loop as to how the repair is progressing. And thanks for the sharp photos, it
helps.
Best of luck --
