rod knock quiet down when engine warms up?-video added - Chevelle Tech
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post #1 of 73 (permalink) Old Mar 22nd, 11, 10:52 AM Thread Starter
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rod knock quiet down when engine warms up?-video added

Hi Guys.

Just curious....Id think not, but will a rod knock quiet down when the engine warms up?

Earlier this winter I started the car and had some noise that resembled either lifter noise, or a rod knock. When I blip the throttle and the revs settle back down its more prononced for a brief second or two.

I was trying to find the location of the knock and I found(makeshift stethascope) that its the loudest on the trans bell on the passenger side. I though it was a converter bolt, but found nothing wrong there.

I started thinking rod knock, and I tried to investigate this past weekend. Once the water temp hits around 140 or so...the noise is entirely gone. I kind of ruled out lifter/rocker noise because I can barely hear it through the valve cover, so Im back to trans, piston noise, or rod noise(but based on the location of the noise...trans or rod "seemed" mostly likely). Would you expect rod noise to go away with heat?

I added a vid. Its only in the 40's right now, so prime time for noise. Number one..its waaay louder in the vid that real life. Number two, notice when I bring the rpm's up around 2 grand, it completely goes away. And after it warms up...its silent as well. Sorry there isnt much of a visual. This was taken down under the car by the passenger side collector(thats where its loudest).


Last edited by greg_moreira; Mar 22nd, 11 at 8:44 PM.
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post #2 of 73 (permalink) Old Mar 22nd, 11, 10:56 AM
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Re: rod knock quiet down when engine warms up?

No, I'd think it would get worse as the oil warms up and thins out.
Sounds like piston slap, but I'm curious about what the experts say...

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post #3 of 73 (permalink) Old Mar 22nd, 11, 11:03 AM
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Re: rod knock quiet down when engine warms up?

Don't overlook your harmonic damper. Could very well be loos and once heat gets into the crank it tightens up a bit. This would also transmit the sound to the area you were hearing it. Just some else to look into before getting to serious.

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post #4 of 73 (permalink) Old Mar 22nd, 11, 11:07 AM
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Re: rod knock quiet down when engine warms up?

Flexplates are known to crack around the bolt holes, could be hearing that.

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post #5 of 73 (permalink) Old Mar 22nd, 11, 11:08 AM
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Re: rod knock quiet down when engine warms up?

Could it be a wrist pin?

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post #6 of 73 (permalink) Old Mar 22nd, 11, 11:25 AM
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Re: rod knock quiet down when engine warms up?

I was thinking wrist pin I had one that made noise when cold but quit when it was warm. I also had a skirt broke off a piston once that done the same thing. Is the noise slow or fast usually valve train noise is faster paced.

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post #7 of 73 (permalink) Old Mar 22nd, 11, 11:32 AM
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Re: rod knock quiet down when engine warms up?

what kind of pistons do you have Greg? A forged piston with clearances on the fat side will typically slap (knock) when cold....if the noise goes away when it heats up expansion is "fixing" the noise so look at things that expand...Dave

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post #8 of 73 (permalink) Old Mar 22nd, 11, 12:02 PM Thread Starter
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Re: rod knock quiet down when engine warms up?

It is a pretty fast paced noise. reminded me of valvetrain noise...but the origin is whats throwing me(not being able to hear thru the valve covers).

I thought piston slap, and they are forged. If thats the case though...its only one piston, and it never happened once in warm summer weather. Just in the 50ish degree weather of now.

Maybe the 30 degree outside air temp difference literally is enough to keep these pistons "tight" in the summer, but let them loose enough to be noisy this time of year?

As you can tell....Im tryin to talk this issue out of being a problem with the bottom end, and I hope its listening! First things first is I suppose just gotta look closer at the flexplate, and if I have to...yank the trans and get the whole trans/converter out of the mix and see what the noise does.

best case scenario would actually be flexplate, or valvetrain. Its still got the flat tappet in it, but Ive got a whole new roller valvetrain going in. Rockers are on backorder, so I cant start yet...but yeah, everything must go. Problem solved. I guess a little piston slap when cold isnt the end of the world either as long as clearance isnt ridiculous.
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post #9 of 73 (permalink) Old Mar 22nd, 11, 12:06 PM
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Re: rod knock quiet down when engine warms up?

Don't know your setup but I have heard fuel pump rods make noise to.

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post #10 of 73 (permalink) Old Mar 22nd, 11, 12:22 PM
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Re: rod knock quiet down when engine warms up?

from your description and forged pistons I agree with piston slap. mark
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post #11 of 73 (permalink) Old Mar 22nd, 11, 1:23 PM
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Re: rod knock quiet down when engine warms up?

my guess is main bearings.
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post #12 of 73 (permalink) Old Mar 22nd, 11, 2:10 PM
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Re: rod knock quiet down when engine warms up?

My "guess" is Piston Slap. I just took apart my 350 that was REALLY loud when cold and noisy when warm. Could never tell exactly where the noise was from. But when I took it apart, I could move the pistons side to side in the bores. These were Hypereutectic pistons as well so they should have been tight. The short was reassembled with forged pistons and it visibly has less clearance than it did then.

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post #13 of 73 (permalink) Old Mar 22nd, 11, 5:45 PM
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Re: rod knock quiet down when engine warms up?

I say piston slap. I have an older engine that has severe piston slap in cold weather that lasts for about a minute. Once it warms up, it's completely quiet.

Ray
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post #14 of 73 (permalink) Old Mar 22nd, 11, 6:19 PM
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Re: rod knock quiet down when engine warms up?

An old timer once told me that if it gets louder as the engine warms up, it's bad. If it gets quieter as it warms up, no worries. I'm going with piston slap or wrist pin/s. My tired ol' 396 has an ever so slight cold knock when the ambient temp is below 45 degrees and only for a min. During the summer it's quiet as a church mouse. But, if you're gonna tear into it anyway for upgrades, might as well rule out the balancer and flexplate just for peace of mind.

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post #15 of 73 (permalink) Old Mar 22nd, 11, 7:44 PM
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Re: rod knock quiet down when engine warms up?

Quote:
Originally Posted by greg_moreira View Post
Hi Guys.

Just curious....Id think not, but will a rod knock quiet down when the engine warms up?

Earlier this winter I started the car and had some noise that resembled either lifter noise, or a rod knock. When I blip the throttle and the revs settle back down its more prononced for a brief second or two.

I was trying to find the location of the knock and I found(makeshift stethascope) that its the loudest on the trans bell on the passenger side. I though it was a converter bolt, but found nothing wrong there.

I started thinking rod knock, and I tried to investigate this past weekend. Once the water temp hits around 140 or so...the noise is entirely gone. I kind of ruled out lifter/rocker noise because I can barely hear it through the valve cover, so Im back to trans, piston noise, or rod noise(but based on the location of the noise...trans or rod "seemed" mostly likely). Would you expect rod noise to go away with heat?
Greg, I've had a rod knock in one of my previous cars, so I've been through that before. There's a very simply and sure way to find out for certain whether it's a rod knock or not:

Get a pair of rubber gloves, and/or a pair of sparkplug boot pliers or atleast regular pliers with rubber coated handles. Start the engine up, and while it's running and making the knocking noise,(in your case, when it's stone cold) pull one of the sparkplug boots off to see if the knocking noise stops. If it does, then push the boot back onto the plug while the engine is still running, and the noise will immediately return as soon as you do (if it's a rod knock in that corresponding cylinder). If that doesn't eliminate the knock, then go to the next boot, and pull that one off and put it back on again with the engine running. Do this with all 8 of the boots one at a time. If it's a rod knock, you will immediately hear the difference by ear beyond any shaddow of doubt just as soon as you tug off the boot, and the noise will immediately return the second you push the boot back onto it's spark plug.

If you go through all 8 boots, and it doesn't change the knocking noise, then it isn't a rod bearing that's knocking, and you've ruled out that possibility. this is a simple and effective diagnosis procedure. If it's anything in the valvetrain, the noice will not stop regardless of which sparkplug boot(s) you remove.

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