Team Chevelle banner

rod knock quiet down when engine warms up?-video added

52K views 72 replies 39 participants last post by  bad66427 
#1 · (Edited)
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).

 
See less See more
#2 ·
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...
 
#3 ·
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.
 
#6 ·
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.
 
#7 ·
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
 
#8 ·
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.
 
#9 ·
Re: rod knock quiet down when engine warms up?

Don't know your setup but I have heard fuel pump rods make noise to.
 
#12 ·
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.
 
#14 ·
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.
 
#15 ·
Re: rod knock quiet down when engine warms up?

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. ;)
 
#17 ·
Re: rod knock quiet down when engine warms up?

or atleast regular pliers with rubber coated handles. . ;)
I vote for NOT using these.... I got knocked on my ass by a Briggs by trying to pull the plug wire off with a pair of rubber insulated Channellocks... :eek:

They make tools specifically for pulling off wires..... :yes:
 
#16 ·
Re: rod knock quiet down when engine warms up?

find someone that has an early '00s Chevy Silverado with a 5.3 or 6.0 and have them start it up cold.. if it makes the same noise as what you are getting from your car, then it's piston slap. GM says this is normal..
 
#19 ·
Greg, do you have exhaust manifolds? Could be the heat riser.
 
#21 ·
Bad plug wire maybe? I had one tick in the boot so loud it sounded like a lifter.
 
#22 ·
Sounds like an exhaust leak to me, but audio over the net can be deceiving. Do you have headers? And what gaskets did you use? Did you try tightening the header bolts? I still say to try that diagnostic test I discribed in post #15. At the very least, it will rule out the possibility of a rod bearing knock, and at best, it will uncover it. I've used this test before, and it works great, and it's simple enough to do by anyone.
 
#25 ·
Sounds like an exhaust leak to me, but audio over th net can be deceiving. Do you have headers? And what gaskets did you use? Did you try tightening the header bolts? I still say to try that diagnostic test I discribed in post #15. It at the very least, it will rule out the possibility of a rod beatring knock, and at best, it will uncover it. I've used this test before, and it works great, and it's simple enough to do by anyone.
x2. And you can kill two birds by swapping a different wire on the cylinder that quiets down with this test too just to rule out my bad wire theory. Hey, I'm reaching I know, but a noise that audible you would think would be easy to find. Sometimes the search for an errant noise will make you lose your hair.
 
#23 ·
Thats one thing I can rule out for sure. The wires that are on it now are brand new taylor's. Bout a week old. Prior to that I had straight boot plug wires on it. I was not aware of the header clearance issues I was going to have when I grabbed the straight boots, and I burnt a couple new wires last fall(noticed the noise first time last fall with the set of wires I initially put on and subsequently burnt).

I replaced the burnt plug wires and used 4 new straight ones, and 4 old 45 degree wires that I had in the garage. Noise still there.

Then...I replaced all 8 with the taylors this past weekend....but of course, still no change in sound.
 
#26 ·
Thats one thing I can rule out for sure. The wires that are on it now are brand new taylor's. Bout a week old. Prior to that I had straight boot plug wires on it. I was not aware of the header clearance issues I was going to have when I grabbed the straight boots, and I burnt a couple new wires last fall(noticed the noise first time last fall with the set of wires I initially put on and subsequently burnt).

I replaced the burnt plug wires and used 4 new straight ones, and 4 old 45 degree wires that I had in the garage. Noise still there.

Then...I replaced all 8 with the taylors this past weekend....but of course, still no change in sound.
I gotta learn to type faster!
 
#24 ·
Greg,

I am going to second the exhaust leak. I have been fooled by them before and they do get better as the headers heat up, expand, and seal better.

Only thing else would be piston slap, but that usually comes on slowly over many thousands of miles, not all at once. My 99 Tahoe sounds like it is coming apart if I have the window down when I start it in the garage in the morning :)
 
#30 ·
good advice from Billy :yes: :thumbsup:
..since there is no spark with the wire off,there is no "firing" load on that cylinder,hence you will not really hear the knock.I have used this method a few times over the years to diagnose lower end noises.Does kinda sound like an exhaust leak in your case.
 
#31 ·
This may not be the best idea, but an old school mechanic of 35 years told me he could find a hard to find exhaust leak by pouring a TINY amount of tranny fluid in the carb while the engine is running. It will smoke like crazy out the tailpipes, but the smoke will also blow out where the leak is. But, if your headers aren't a pain in the keyster it's probably better to just unbolt em'.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top