I hadn’t heard of diagnosing which cylinder has a worn connecting rod bearing by disconnecting the spark plug, as the OP did. Anybody else hear of that?
Yeah, I’m with you. This isn’t my area of expertise, but disconnecting the plug isn’t going to make the piston stop moving, so I don’t know why it would make a noisy bearing suddenly not noisy.
Most shop oscillascopes have a cylinder shorting function to isolate missfire and other problems including piston and rod noises. And eliminating the spark will greatly alter the sound from a failing rod big end.
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OK, not the kind of thing I’ve dealt with. Good to know!
I still wonder what all else is worn out, if the rod bearing is shot…
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That was a clever way to narrow it down Ant_Schultz!
No fire in that cylinder makes the piston not slam so hard against the crank during rotations?
I’ve not heard of it either, and am guessing that may make a noticeable sound difference.
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A rod knock occurs when the rod reaches BDC with several hundred psi pressure forcing it down and suddenly must reverse direction. Doing so requires the rod big end to move from pressing downward on the journal to being pressed upwards by the journal and the loose fit of a worn bearing allows the journal to hammer the rod.
And BTW, years ago shady used car dealers had mechanics drop the pan and insert a piece of thin leather between the lower rod cap and insert to cushion the rod’s reversal and eliminate the knock.
Of course, you can also pull the fuel injector plug, as shown in the first few minutes of this:
That trick about killing a cylinder to determine which one has a knock has been around forever.
The same thing applies to a center main bearing thump on a Subaru. Disconnect the No. 3 plug wire and if the thump (usually very, very subtle) goes away then you know the center main is worn.
@GeorgeinSanJose, in regard to your question about wrist pin lubrication there is generally a hole on the small end of the connecting rods. The wrist pins are lubed strictly by splash from the oil being thrown by the crankshaft journals.
The cylinder walls are lubed by the same oil.
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