Help me zero in on my Rod Knock?

[76zimmer]

95-350. I'm getting a knock when it sits for more than 30 minutes. It is very low in tone, and is easily noticeable, it goes away as soon as oil pressure is noticed on the guage, and doesn't voice itself again until it sits for the 30 minutes or more. 95% of the time this is the scenario....Once in a while it will be very faint, even after sitting all night.
I'm thinking a rod, but the 5% it doesn't make noise, makes me question that. Oil pressure is a minimum of 20# at hot idle, and about 35 or more, from 2000 and up.

 

[rcamacho]

Sounds a bit low off idle. You want 10PSI per 1000RPM generally. If you have a lot of miles on it, best to teardown and measure clearances. Rod and main clearance can be measured with the pan off if you're averse to R&R without cause.

 

[4X4HIGH]

I would tend to lean towards a piston knock. A slight bit too much piston clearance will have a slight knock sound when cold and either completely go away once warm or change in sound considerably when warm.

 

[79rustyk10]

I had a TBI 350 that would do that until it got what little oil pressure there was to be had. ran for a long time like that. turned out it was a rod bearing.

I cant believe that engine lasted as long as it did, between being used as a farm truck, then I got it when I was 16 and it spent alot of time at 5000 rpm (wouldnt usually rev much higher than that) 190K on it when I pulled it.

 

[buefchris]

I've heard many stories about the durablelity of the TBI 350.
They may not have a lot of power but, you can get some mileage out of them.

 

[76zimmer]

I would tend to lean towards a piston knock. A slight bit too much piston clearance will have a slight knock sound when cold and either completely go away once warm or change in sound considerably when warm.

Scott, I was ruling out a piston or other parts?, as the knock is only noticeable when it has been off for 30 min. or more, then it immediately goes away when oil pressure comes up on the guage....it will knock like 5 times, then nothing, not even a faint knock.
I'm still thinking rod.

 

[dudeiwin86]

sounds like piston slap to me.

chevy is known for this in their later model 350's
(like 97 and later corvettes... probably earlier too, but i havent researched much but the C5 vettes)

it means that the piston to bore clearance is slightly too much and when oil pressure is low, it lets that piston rock in the bore. thus a slight knock. ive heard of chevy engines go for 100k+ miles with the piston slap.
supposedly its pretty "normal".

now... for a early TBI model... i dont know if its that normal (the vettes run 8 injectors in the manifold.. LS1) but... its exactly what your describing.

--

newb question... (im used to late model stuff)... are the lifters in these trucks solid? or pumped up by oil?
maybe a failed lifter and its rattling until you get oil PSI up to "pump it up"?

 

[76zimmer]

definitely not a lifter....this is a bottom end knock, and not a ticking like a bleedout lifter. They are hydraulics since.....72? in Chevy V-8's.
And I didn't know a piston problem was common in the SBC's pre 97.
And wouldn't it carry on for more than just a few knocks if it were piston?

 

[dudeiwin86]

i cant say if it was "common" or not for sure pre-LS1. but i know they get it.

id drain the oil, let it settle some in the pan, put a light on it as your turn the pan on angles and see if you can see "sparklies". if theyre golden its likely bearing material.
put a magnet in there and see what it pics up...

put a light back on it and see if theres any more sparklies in the oil.
this will at least tell you if the metal is ferrous or non-ferrous.
could cut the filter open too and see how she looks

or if there is any at all. if there is no material to be seen, id venture more towards piston slap since it doesnt typically wear too badly compared to a rod bearing being out.

also possible is a wrist pin. but typically that doesnt go away when you get PSI up...

some reading (i searched "piston slap vs rod knock" on google):
http://www.chevytalk.org/fusionbb/showtopic.php?tid/235289/

 

[76zimmer]

now I'm totally confused....if its lifter I'll just keep running it, I'm worried about a bearing though. I'll dump the oil, and check as you suggested, when I get my 2500 running and legal, then I can work on it a bit....man it sure is eating up my K5 time working on DD's though.

 

[76zimmer]

I know you can check rod knock by taking a plug wire off, but it don't last long enough to do that...maybe I can do one each time I start it?
Is there a way to check for piston slap....
I'll put my stethescope on it and have a listen, I got remote start, so that will give me a few seconds anyway.

 

[4xcrazy]

when's the last time you changed the oil & filter? what weight are you running.

My previous older, higher mileage motors used to do this quick couple of knocks when the oil got old, would change it usually with a 20/50 weight, as i was in Arizona at the time and usually cleared up for the most part.

I have heard other engines in the past with this piston knock as some call it, someone else i knew called it piston slap, said it was a typical tiny bit of slop in the wrist pin until the oil pressure built up. Either way, i personally have never had any major malfunctions from the noise.

 

[dudeiwin86]

id spend the 20$ to drain the oil and look for bearing material too.

 

[76zimmer]

when's the last time you changed the oil & filter? what weight are you running.

My previous older, higher mileage motors used to do this quick couple of knocks when the oil got old, would change it usually with a 20/50 weight, as i was in Arizona at the time and usually cleared up for the most part.

I have heard other engines in the past with this piston knock as some call it, someone else i knew called it piston slap, said it was a typical tiny bit of slop in the wrist pin until the oil pressure built up. Either way, i personally have never had any major malfunctions from the noise.

I usually run 5w30 Valvoline, changed every 2500, because the burb isn't an everyday driver....I started noticing the knock about 3500 miles ago, and I changed oil and filter (Napa Gold) w/ the 5-30 2300 miles ago.
The knock was a little better, but still quite noticeable, and 1900 miles ago I changed just the oil to Shell Rotella 15w-40. That was just before going to Charlotte (1900 mile trip). It used less than 1 qt. on the trip, but the knock was just as noticeable as before changing oil the last time to 5w-30.
How would the oil pressure affect the wrist pin, as they aren't pressurized?

 

[4xcrazy]

How would the oil pressure affect the wrist pin, as they aren't pressurized?

yeah, after reading your reply and mentioning this, i realized that as well. Honestly it's been so long since i was informed about this i don't remember the exact wording, but i do know the knock you are referring to.

really though, if the motor is a higher mileage unit, i would try a thicker oil and see if this helps, that's another reason i would run such a thick oil.

I just ran a brand new motor, with just the break-in mileage on it, from Arizona to north east Indiana, i changed the oil before the trip with 20/50, mainly because i figured i was pulling a heavily loaded trailer and the Burb was pretty heavy as well, was gonna do some searious pulling. I get here, just checked the oil now and it's gettin alittle darker already, almost 2000 miles still fine otherwise. But i do plan on switching to a slightly thinner oil, maybe synthetic for the cooler weather here.

I dunno, again, just my opinion, i would try a thicker oil and see how it likes that, i never was a fan of that 15-40 stuff, seemed to always break down to thin in too short of a time in the motors i tried it in previously.

 

[76zimmer]

I sold this Burb about a month ago....to a guy at works' kid. He just told me they pulled the engine out, took the pan off, and #3 rod bearing was in bad shape. They are polishing the crank, and putting a new bearing in that journal, along with a new oil pump. He said they removed the other rod caps, and the others look pretty good for 150K. Same with the mains. He said the pan was pretty clean inside, no sludge, or buildup at all. I've used Valvoline in it for the 50K I've owned it.
So what makes one rod bearing start to sh!t at 145K.
If this lives, he got a good deal.

 

[toomany]

I sold this Burb about a month ago....to a guy at works' kid. He just told me they pulled the engine out, took the pan off, and #3 rod bearing was in bad shape. They are polishing the crank, and putting a new bearing in that journal, along with a new oil pump. He said they removed the other rod caps, and the others look pretty good for 150K. Same with the mains. He said the pan was pretty clean inside, no sludge, or buildup at all. I've used Valvoline in it for the 50K I've owned it.
So what makes one rod bearing start to sh!t at 145K.
If this lives, he got a good deal.

Am I reading this right? They are only going to polish that journal and replace that bearing? Why go through all the work of pulling the motor only to do the one journal? I'd atleast grind all the rods/mains. Bearing are cheap enough to try to skimp and save money here.

 

[76zimmer]

They pulled the motor on my suggestion

I figured it would be easier to pull the engine and take the pan off, then if it was more serious than a simple fix, the engine would already be out of the vehicle. And it would be easier than removing the drivetrain components (front diff stuff) to get the pan off. And I'm not really into laying on my back and working on engine internals anymore.

I didn't look at anything, but they said they pulled the caps, and mains to check them, and they all looked good. I suggested they use some plastigage on em, but I don't know if they did. Maybe they felt it best to leave a sleeping dog lie.

 

[Fordum]

I had 180K on an engine when I sunk it. Damn culvert collapsed when I was crossing it and the front end went under water. I winched out, but some mud got in my starter contacts and it would not crank after I got out.
The engine had been running all the time I was getting out, so I knew it did not ingest any water. I got a friend of mine to pull me home and just parked the truck.

Week later, I drained the water and oil out of the front end, and decided to check the oil before I cranked it.
There was rusty sludge halfway up the dipstick.

Water had gotten in the lower part of the engine.

I changed the oil and filter with some cheap oil, cranked it. Ran it bout 5 minutes, changed the oil and filter again. And repeated that after 10 minutes.
Then, put in my good oil.
Oil pressure looked good, no noises, so I drove it.

About a month later, I started getting worried. The engine was running fine, but it just nagged me.

So a friend of mine and I eased the engine up and pulled the pan. Pulled off one bearing cap.
The crank journal looked fine, but the bearing was toast.
Pitted, discolored, not good looking.

We wound up replacing all the main and rod bearings to keep things together until I could get time for a rebuild.

We didn't touch the journals, just put in new bearings.
I know we mic'd the crank and rods, but for the life of me I can't remember if we went stock or .010 oversized.

As usual, life happened, and when I looked around again, I had put about 50K on those bearings.
I got a deal on a blueprinted engine that a guy had had built and totaled his truck on the way to pick it up.
I gave the old engine to another friend of mine, with the story of the bearings.
He put it in his barn, poured the cylinders full of oil, poured the crankcase way overfull so that the crank was covered, and left it sealed up.
About a year later, before he had gotten a chance to rebuild it, the motor on his truck seized.
It was harvest time, and he could not have that truck down. So, he drained all the oil out of my engine, refilled it, put it in his truck, and put at least another 80K on it before he sold the truck.

As far as I know it never used any oil between changes as long as I owned it, and never did up to the day he sold it.

We both wish out loud sometimes that we had that engine back. .....


J.

 

[76zimmer]

Its amazing how much abuse they will take, but at the same time can be fragile.