Help Identify Knock Sound Chevy 350

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
.

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.

.

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.




.

Denny W. A few days ago we did pull each wire to see if it would isolate the noise but it stayed the same on all cylinders.

Regarding transmission, I will get up under there and flex it more on the converter and see if it sounds like it can be an issue or just a normal movement. When I put the engine in I did not notice any apparent cracks and driving was limited to our long driveway. It is possible it got cracked somehow. If it turns out to be an issue I do have one sitting on another engine.

Is the flexplate likely to crack if the starter gear does not engage properly? I know this has happened a few times where the starter hits the plate not engaging gears as I think we need a shim in it.

Denny W

So you think lifters? My confusion still is though that the rockers appear not to clack and I would assume if it is a lifter that it would also clack the rockers and never adjust.

Regarding oil, I live in a cold state, IDAHO and it gets very cold in the winter. I have always been suggested 30w is the best all around. You feel 10/40 is best?

Also I have added Lucus oil stabilizer which is very thick so would imagine it is much thicker than 30w now. If a person is going to use additives such as Lucus, is it best to use thinner weight being you will be thickening it?

Denny,

I appreciate your assistance. A real hair pulling ordeal. Other than the time it takes to pull the intake, replacing the lifters and gaskets is fairly inexpensive.

I did order the clips as mentioned. One more clarification to make sure I am correct. The screw driver test is to put the screw driver pressed down on the rock at the push rod side, correct? I assume the theory is to keep pressure on the lifter to take up any clearance.

Thanks again all.

Have you checked the rockers to make sure none of them are cracked below the center? I've had a couple crack and cause a clacking noise that was a bear to locate. I'd definitely do Denny's test though because it sure sounds like a bad lifter from what I can hear too. Hopefully you don't pull the lifters to find crowns starting. That's usually the luck I've had.

I am going to pull the cover off again today and check.

I'm going to be the negative Nelly here and say it sounds like a rod to me. Seems to be too deep for a lifter. I had a 3800 with a spun rod that did the same thing.

Hey Seth, my sons name is Seth.

How did you end up fixing your rod issue?

Thats pretty cool that he has the same name.. I replaced the motor because the crank and rod were ruined because of the spun bearing. I need to back up on it though, the more I think of it, it was quiet at an idle and knocked when I reved it up. I listened to your video again and it's hard to tell, but if it's a fast knock it can be something like a spun rod or wrist pin loose. That's because it will knock once going up and knock again when it changes direction going down. My brother in law had a motor that was knocking like a spun rod once and it ended up being the distributor cap was cracked causing pre detonation on one cyl. I had a knock on a motor that ended up being carbon build up on too of a piston smacking the head, just a few more ideas for you. I think if I were you I would drain the oil in a clean pan and let it sit overnight and check if any glitter settled to the bottom, also cut your filter apart and loo for any metal in it.

Charles,
I went back and re-read all of your posts. It seems to me that you've ruled out lifters, since you stated that as you adjust them you bring them to individual clatter, then silence them by adjustment, but the knock remains consistent as you adjust lifters.
One thing that you have not shared is the history of this engine. You mention "...when I put it in (the truck)..." that initial driving was limited to your long driveway, but you haven't said anything about the engine being new, old or in between. Can you share some details about when it was last apart, who put it together and if a machine shop or rebuilder was involved?

I hope not rod knock. However, I can say it does sound more prominent up higher than towards pan.

Rspear, the engine was slightly used. The only history I have on it is that it ran strong and was rebuilt. When I got it, it looked pretty fresh.

When I listen to the the engine run I believe the noise sounds like it's about half crank speed. If I am right that means it's in the valve train cam, lifter, push rod, rocker arm, valve spring, or valve, of course I could be wrong just my 2¢ worth

No I have flat hydraulic tappet.

I also tried the screw driver approach for noise but did not seem to make a difference. However, if I was to lean towards any speculation I would say it is first a lifter or two, second thought would be piston rattle.

Quote:

---

Originally Posted by fan4chevys

No I have flat hydraulic tappet.

---

So you have flat tappet lifters, and from post #19 you said:

Quote:

---

Originally Posted by fan4chevys

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.

---

If by saying "...the knocking persists" you mean that it continued to knock while you were adjusting the rockers, how can it be a lifter? You've also told us:

Quote:

---

Originally Posted by fan4chevys

....the engine was slightly used. The only history I have on it is that it ran strong and was rebuilt. When I got it, it looked pretty fresh.

---

No disrespect, but you bought a pig in a poke. You have no idea what "rebuilt" meant to the guy who sold this engine to you. To some, a set of plugs, some gaskets to stop the oil leaks and some rattle can paint means "rebuilt" (rattle can rebuild). To others, it's throwing in a set of new rings on old pistons without measuring anything, or having any machine work done. I hope that's not what you've got.

I been on the road for the past week-----------few comments that haven t been mentioned----rebuilt????? wrong size bearings, loose pistons in old tapered bores----piston in backwards so pin offset is extreme and will be noisy and/or maybe fracture skirt on thrust side of cylinder

Do all the lifters/pushrods spin same as motor idles????????

Passenger side rear ----------thats where the distributor sets in its boss/hole-------IFFFFFFFFFFFFF------the dist is not exactly straight into hole( manifold at top not centered perfectly in line) lower housing undersized, oil rings ( if used) bulged out into oil galley, and/or a few other conditions-----------at low speeds the flow of oil to those rear tappet bosses can bleed off enough that there isn't flow/pressure enough to do the job of filling the tappets at a rate that beats the collapse rate of the lifter-at higher speeds the lifter isn't in that part of the cycle for it to collapse far enough to bottom out. but-------at some certain rpm rates the lifters spend more time ( mili,mili micro sec) that total collapse is possible where at high rpm ( compare 600 rpm to 6000 rpm---10x ???) and add in the oil flow rates etc etc---------

Also-being an used older block that has been rebuilt-which brings up countless areas where clearances have been disregarded ( mainly $$$$$$$$$) in the rebuild process-------if a flat top piston is put on the rod backwrds, it will double the amount of normal wrist pin offset-----some HP forged pistons won't have any offset and are noisy at some idle range rpm with lighter oils---I'll try to look up the specs on the wrist pin offset so I can maybe explain this line of thought----

I have those rocker clips on there way and once I get them I will open up both valve covers and run it longer to really look at the pushrods turning and see how each one is pumping oil. I will video this as well.

By my best analysis, I am leaning towards 8 or 6 cylinder area or both. To me it does not sound like rod knock. I feel it is either piston rattle or lifters.

Something to add on our first starting of the used engine which I could not imagine having importance is when we dropped the used rebuilt engine in, it was popping through the carb with flames. Happened a good dozen times before we could figure out the issue. The accel distributor was bad with either bad coil or module. Was not getting enough spark acting retarded.

Then we put in a GM Hei that we had and it started right up.

Be sure to remove the rockers on the suspect cylinders and check those rockers too. Eliminate those possibilities before performing the screw driver test. You just might find some broken ones.

40FordDeluxe.

Will do. I am really starting to suspect the lifters now. I drove it around a bit and although it ran pretty smooth with good power, it started running rough like it had a sticky lifter or something. Like 7 1/2 cylinders.

I then changed the spark plugs, new fuel filter, rotor and cap and it stayed rough.

Do you have pic of the spark plugs??????

And are you ignoring my posts or are they above your level of experience????

Hi Jerry,

No, not ignoring your post. :) I am waiting to get the rocker clips in so I can run the engine with valve covers off while avoiding too much oil spill mess. I will get a picture of the plugs. They were fairly white with some black down low on the insulator.

Regarding the engine rebuild, have no idea. Was just told it was a performance rebuild. Not sure the rest.

My level of car experience is pretty good. Been around big blocks and small block Chevys for about 40 years. Built a few engines from ground up. Changed some cams, heads, etc. however, troubleshooting like this one, not sure of.

Just watched/listened to your vid----------noticed that the starter ring gear appears to have some wobble----are there any shiny spots on the flywheel that you can see------like maybe the teeth runout maybe hitting starter????????

Also---some rockers hit baffles and vents in aftermarket valve covers-noticed the breather at rear right side about where sound is coming from??????

Jerry and Denny,

I did find earlier in the week a few shiny spots on the valve covers hoping it was the issue. However, after grinding the spots down and rechecking to see if there was any more abrasion, the was not more marking and the sound persisted.

On the flywheel, I looked as close as possible for wear but did not see any. The flywheel has a slight wobble, but not sure as to why. The flywheel bolts are tight.

Tomorrow I will put the clips on.

Hi,

It is not yet fixed. We drove it a bit and runs strong. I changed oil again to 10w30 brand name oil with some Lucus oil stabilizer hoping maybe it would help but stayed the same.

Sound does not get worst when rpms raise or when driving around. It remains prominent at high idle mostly.

At this point, I am thinking the next and cheapest step to rule out the lifters, is to put new ones in.

Look at each of them very carefully as you take them out----also keek them in order for later referance-----and if any come up a littlle bit and then get harder to pull it will probably be because of a wiped out bottom leaving bad edge------and look at each and every cam lobe for wear patterns