Thread: Help Identify Knock Sound Chevy 350

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Hi,

I am new to the forum. I have a Chevy 350 engine with turbo 400. I am getting a knocking sound in what seems to be the passenger rear of engine between about 700 to 1200 rpm (no tach so guessing), then it goes away.

I put the stethoscope on it but could not really determine it's location. However, if I was to do my best guess it seems upper rear passenger.

I adjusted the rockers running to 3/4 turn.

The rockers are adjusting fine and do not seem to clack when adjusted. By theory, if a lifter is bad, wouldn't the rocker not adjust or keep clacking if it is bad enough to cause a knock sound?

Also pulled the trans bell pan to see if the torque converter nuts were rubbing or if there was any wear on the flexplate but did not see anything.

Running 5-30w oil. Already put in Lucus wear protector honey and marvel mystery oil. Just ran it mostly for hours no change.

In conclusion, the engine runs awesome past the 700-1200 rpm no knock sound. Lower the idle down and it goes away too.

Your help is appreciated.
Charles

Exhaust leak, manifold to head.
http://www.ebay.com/itm/SBC-Square-P...-/182509641451
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I will check it today and make sure they are all tight. I just put on brand new ones before starting it.

I am used to exhaust having more of a pinging sound and not a knock. Can they have an internal knock sound?

Try some seafoam in the oil.

Oops, I reread and see you already have marvel added.

I'm not a great fan of multiple additives..

You might try some fresh oil?

Thank you for your help guys.

Yesterday I had put it in gear with some load and hear the knock still. It was same level not worst and no less.

I went back tonight and tightened down all the exhaust manifold bolts and no change. These are 1 week old gaskets also.

I did more stethoscope and it is so hard to determine the exact whereabouts. However, my feeling is that it is the rear 8 or 6 cylinder area to engine. For good measure, I put the scope against the fuel pump as well as the block in that area and did not hear it strong there.

I lowered the idle again and at very low idle it is non existent.

I am really curious though. If a lifter is failing, won't it effect the rocker lash? In my case the rockers effectively adjust without clack when adjusted.

I have also done the revving but for a little longer. I will do it again. Once thing I can say is that the oil is showing 60psi and the oil shoots out of these rockers super well on every one of them.

I will record tomorrow.

I have the trans cover off as I was seeing if there was an issue there so tomorrow will check the play.

My concern is piston slap. Crossing my fingers.

I checked the play on crank by prying the torque converter back and forth. Did not find any play at all. I would get a slight bang sound when the flexplate flexed toward the trans and torque converter moved back a tad.

Find included the video of the engine. Did in drive and out.

http://cruisinstreet.com/videos/SmallVidTruckKnock.mp4

Ya should look for a broken valve spring before you run it any more

I rule out the valve springs because I have been in the valve covers 4 times adjusting valves, inspecting rockers and oil flow and appeared fine. However, I will look one more time.

It also runs fairly smooth, whereas I would expect a valve issue would cause it to run rough wouldn't it?

I have driven the truck around a few blocks and truck drives well.

Here is a larger pixel video of the engine and sound:

http://cruisinstreet.com/videos/SmallVidTruckKnock.mp4 <- small video size

http://cruisinstreet.com/videos/largeVidTruckKnock.mp4 <- Large video size

Sounds suspiciously like an exhaust flange gasket leak to me. Try to shroud the rear exhaust flange where it meets the head with a wet rag wrapped around it to see if the sound changes or goes away; the rag will absorb the noise from a leaky gasket. If the knock is still there, its probably somewhere inside. I had a stuck lifter that knocked one time, drove me nuts, but when the bolts on the headers got loose, the leak sounded just like that.

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I had to turn the volume way down to hear it, but oh yeah, sounds like a lifter.

Oil additives didn't do it, change lifters.

I'm tempted to say oil flow issue, but you say the lifters all spurt real good.

It might be worth playing with valve lash.

ID the culprit, and dink with the stud nut, see what happens.

After working with the rockers 4x in the near past few days, the rockers can be adjusted to clacking, then zero and 3/4 turn with no clacking on the rocker. However, the knocking sound still persist.

It has a high volume oil pump and each rocker pumps out a ton of oil. I examined each rocker and they all seem to pump equally and raise up and down fairly equal.

So one question still remains. Can a lifter make this noise if the rocker is properly adjusted and does not make any clacking?

I will do the test tomorrow. So to confirm, take the flat head screwdriver and gently push it between the under side of the rocker tip where it meets the valve and push it between the rocker and the valve, correct?

Also in person and the valve covers off, it does not sound like the rockers but actually inside the engine.

".......I would get a slight bang sound when the flexplate flexed toward the trans and torque converter moved back a tad....."

I suspect you found your problem, which is likely a cracked flex plate.

On the bright side the transmission does not necessarily have to come all the way out. Most times you can do the following and save some time and aggravation.

Support the transmission and remove the drive shaft and cross member. Lower the transmission slightly and support the rear of the engine (make sure you don't squash the distributor cap into the fire wall). Use a floor jack with a piece of plywood or metal plate and support the transmission.

Take off the convertor nuts/bolts.

Remove the lower bolt on each side of the transmission and replace with long bolts (with them screwed in you should have 3-5 inches of length sticking out) if you can't find bolts long enough, use pieces of threaded stock with nuts on the ends. Remove the rest of the transmission bolts.

With the jack supporting the transmission roll it back and slide the transmission back far enough that you can get a long slim wrench in to loosen/remove the flex plate bolts. Make sure the convertor remains seated in the transmission.

There should be enough slack in the speedometer cable, cooler lines and vacuum modulator lines they won't need to be disconnected, but if necessary remove them.

When you install the new flexplate it will have to be properly indexed for all the bolts to start, so make sure you start them all before you start tightening them.

The long bolts you have installed will have kept the transmission properly aligned, so when the new flex plate is installed the transmission will just slide forward and you can reinstall everything (if the convertor uses studs make sure they are lined up with the flex plate holes as you slide it forward).



Hope this helps.




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