Help me diagnose a knocking noise...

Mike Manzo

Registered User
Ok here is the scenario...

Car sat for three years, needless to say stuff will go weird...

Anyway, at idle there is no noise and even when reving the engine...Oil pressure when at op-temp is upper 20s - 30s even (something like that)

Under load is when the "knock" happens and there is no oil pressure drop...If anything it climbs.

Now, I am trying to figure out is this lifter-related and pushrod problems or rod/piston related...

I know jack about engine internals and I admit I am probably the last person who should have entered the realm of modding this car, but I hope someone can add to what my brother in law says might be the case for both my SCs suffering the same issues with completely different situations...

Both have identical knocks, one sat for three years, one has a zillion miles on it and suffered a plugged cat for a while...

The "knocking" noise is not at a rapid pace and from what I recall from an actual rod bearing issue, the knocking noise was like, for lack of a better description, like the double bass beat from Pantera as opposed to what seems to be half-time with the RPM of the engine which is what this knocking noise is.

In what has been described to me, rod/piston/crank bearing issues where a knock occurs would be at the RPM of the engine and a rapid metal-metal beating...

What IS happening here at idle under load is akin to that of an alarm-clock pace, but a knocking "noise"...Of course this gets quicker when you step on it, but at idle at the SC typical 600rpm range in gear, its about 1 "knock" per second. What is being told to me is that a problem with the bottom end or pistons would be more crazy rapid than that.

If this is the case, then it could merely be a need for adjustment and maybe replacement of the pushrods? Like I said...no oil pressure drop at no point idling or driving hot or cold.

I plan to stethiscope the engine and see where I can hear the noise, but I gave up on both of them as toasted and bound for complete death....

I know I look like a complete idiot in this description, but at this point I could care less. I have two SCs, both with a tremendeous amount of work done to them, a crapload of killer bolt ons and this damn knocking noise! One engine supposedly dynoed at more crank horse than my Cobra and I really want to see it live...

Ha...either that or 10k takes both of them with everything and all the crap that isnt installed yet!
 
Mike diagnosing something like you're experiancing is very hard and tricky over the internet, it very well could be the bottom end of the motor as I've seen rod bearings that were on the ragged edge do just what you're describing. Having torn down several of these engines I can tell you without any doubt that if your engine hasd close to 100 K miles the rod and main bearings are wiped down into the copper shell with the Babbit bearing material gone. When idleing a knocking rod does'nt really sound all that fast paced either, its more the tone of the sound thats the giveaway and thats what makes internet diagnostics very difficult with this problem. Does the sound get louder as the engine heats up?
 
Noise...

MIKE 38sc said:
Mike diagnosing something like you're experiancing is very hard and tricky over the internet, it very well could be the bottom end of the motor as I've seen rod bearings that were on the ragged edge do just what you're describing. Having torn down several of these engines I can tell you without any doubt that if your engine hasd close to 100 K miles the rod and main bearings are wiped down into the copper shell with the Babbit bearing material gone. When idleing a knocking rod does'nt really sound all that fast paced either, its more the tone of the sound thats the giveaway and thats what makes internet diagnostics very difficult with this problem. Does the sound get louder as the engine heats up?

It actually gets quieter on both cars. It has been suggested that it is a pushrod and not a crank bearing, but I know jack about engines...I just wondered if the "fast-paced" vs "metranome" knock theory held water. What I was told was this...If it was a piston, then the knock would be very rapid and if it was a pushrod, then it would be (at idle) more like the speed of a ticking of a clock. What I have is the speed of a clock ticking. The theory made sense if the cam turns at half the speed of the crank and I have heard an actual piston slap before, which if memory serves, was very rapid when standing still and there was virtually no oil pressure.

On both cars, oil pressure is constant and good, so it was mentioned that it should not be a crank bearing. My brother in law had a car that sat for a long while and suffered bent pushrods due to poor rocker installation and possibly the fact that it just sat for so long. He said it sounded metallic, like knocking on metal with a hammer, but at a slow pace. The crank bearing were intact and the pushrods were "pretzels". While bent pushrods suck, at least such a replacement would be easier than an engine rebuild at this point.
 
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