Need help identifying noise. Rod knock ?

[RamonRdzS]

I have this 98 C1500 that knocks, I can hear it more at the botom near the flexplate and the oilpan.
It more audible when I start the truck but once it's warmed up you can bearly hear it. Rod knock maybe ?
Is it worth fixing this or just find a new engine ? It's got 190k miles.

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[pressureangle]

Oil pressure tells the tale. If your OP is good, it's possibly the flex plate cracked. My experience is that rods squeak before they knock.

 

[RamonRdzS]

Forgot about that op will stay between 40 and 60 as far as I remember, it will dip a little below 40 when warm.

 

[pressureangle]

Forgot about that op will stay between 40 and 60 as far as I remember, it will dip a little below 40 when warm.

I did have a 350 that wore out the thrust bearing on the crank due to a ballooned converter, so much that a crank weight tapped the block at idle. It disappeared with any rpm increase. OP was normal.

 

[Scooterwrench]

With oil pressure like that makes me also think flexplate issue,maybe cracked or loose bolts. A good test would be to put the truck in gear while it's still cold and knocking and apply a little throttle while holding your foot on the brake. If it shuts up that pretty much confirms it. If it still knocks put it back in park and raise the rpm's to about 2000. If the knock gets worse then probably rod.

 

[RamonRdzS]

Did the test. Drive, break and accelerate and it's still knocking. Parked and accelerate to 1500rpm still ticks. Forgot to mention, tick is faster with rpms.
Oil pressure on start:

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[Scooterwrench]

Well,fudge!
Before you condemn the motor pull the tranny or slide it back and check the flexplate. You can use two long 3/8"-16 bolts screwed into the lower two belhousing holes to slide it back on. Hopefully you won't need to go any farther.

 

[Road Trip]

When folks ask me to listen to an engine, I always tell them
that Bass is lower end, while Treble is valvetrain.

I hear Bass. And if you want to narrow it down to which
cylinder has the noisy reciprocating assy, then pull the
spark plug wires one at a time and listen for the one
where the knocking sound lessens/disappears with
the combustion forces temporarily removed.

CAUTION: Don't dilly-dally, for you don't want to
wound your cats in the interest of science. Try
one, listen for a few seconds, plug wire back on,
run the engine for a couple of minutes to clear
the cats, and then repeat as needed until you
find the perpetrator of the noise.

EDIT: And if the problem isn't a rod bearing inside
the engine, then the noise will, if anything, become
more pronounced as you remove spark plugs one at
a time and add that 7-cylinder jerkiness downstream.

In other words, this may well help you figure out if the
problem is internal to the motor, or immediately downstream.
(cracked flexplate, loose fasteners between flexplate/converter, etc)

...and let us know what you find. Good luck!

 

[Road Trip]

I did have a 350 that wore out the thrust bearing on the crank due to a ballooned converter, so much that a crank weight tapped the block at idle. It disappeared with any rpm increase. OP was normal.

I've also troubleshot down low noises despite good oil pressure. In both
cases that I can recollect, the root cause of the knock was a bent/twisted
connecting rod.

In the first case a recently purchase truck had a sound
similar to the OP's video. But at the same time oil pressure was within limits?

After removal & subsequent teardown, it looked like the PO had suffered
a hydraulic lock (due to a head gasket coolant leak) into 1 cylinder.
The head gasket had been replaced, but the reciprocating assembly was
left as-is? And while the engine more or less ran on all 8 cylinders after
the head gasket job, but this one cylinder was noisy similar to the OP's engine?


In the 2nd case, the rod was twisted, and because of new stresses induced
because the wrist pin was working cocked, the piston slap had become
authoritative, and was present cold or hot?

Weird things can be hiding inside an engine, especially when it comes from
the used car manufacturing plant. The previous owner's 'good enuf' is now
your new baby's WTF?

Q: Was this engine ever quiet for you? If so, did it come on slowly over time
or did it suddenly appear? Or did you buy it for the right price with this
noise evident? This might help us narrow down the possibilities...

 

[Schurkey]

if you want to narrow it down to which
cylinder has the noisy reciprocating assy, then pull the
spark plug wires one at a time and listen

Mostly good advice, however don't "pull" the plug wires. This leads to an open-circuit for the secondary voltage, which then goes sky-high and is very hard on the coil, coil wire, distributor cap, rotor, and plug wire.

Instead, kill the spark by sliding small nails BETWEEN the plug wire and the plug wire boot at the distributor cap. Then use a 12V test light (incandescent bulb) or a jumper wire to ground the spark to each cylinder. I blunt the sharp point of the nails, and use a bit of Silicone dielectric grease on each one. DO NOT pierce the insulation of the plug wire/boot. The goal is to slide the nail between the wire insulation and the boot until you feel contact with the metal end of the plug wire.

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Grounding the spark isn't hard on the insulation of the ignition system, the way that an open-circuit is. Remove the nails when you're done, or they short on the air cleaner. If you're doing this with a grounded incandescent bulb test-light, the light bulb is not going to flash.

Kinda guessing that a test light using an LED or fluorescent tube may not work for this, and might damage the test-light circuitry.