Engine knock
 

Stock '67 L35 396 (Camaro), over 100,000 miles, never rebuilt, leaky/seepy valve guides, fresh spark plugs just put in today, stock timing setting. Starts cold with no fuss, idles fine. (New Q-jet a year ago.) During idle warmup the engine has an intermittent knock. Not loud but it can be heard by a trained ear. I'm thinking it's from carbon buildup in the cylinders? Is there a fuel additive that will help get rid of any carbon buildup in the cylinders? The engine needs to come out and be rebuilt but I'm trying to put that off until I have the $$$.

Thanks in advance!!!

Re: Engine knock
 

To de-carbon an engine, you might try Seafoam. Worked well for me so far.

However, the knocking you're hearing may be due to worn rod/main bearings, or perhaps excessive piston to wall clearance. Not uncommon for older engines with 100K+ on the clock.

How's the oil pressure?

Re: Engine knock
 

Thanks Eric! Oil pressure is good: 40-ish at idle, as high as 60 at anything above. It's definately a combustion knock. I've heard bad bearings and piston slap before and I don't think that's it--even though the old Rat has a ton of miles on it. It's not a steady knock, it's more occasional and not with any real rhythm.

Re: Engine knock
 

A cam going away will make some funky noises deep down...been there, done that, and almost ruined a 427 because of it.

Re: Engine knock
 

Bill...sounds like time to do some compression testing maybe? Have you changed your oil and checked it? My engine had a weird knock, too...and you know what happened after that!

Check the basics stuff and rule them out first...?

Re: Engine knock
 

Yeah Bruce, I'll do a compression test but I have a feeling all it will show is that a bad case of old motor has set in... After doing some research I'm 99% certain it's a combustion knock from carbon buildup in the chamber. The old plugs have oil buildup from leaky valve guides and I think there's got to be carbon buildup in the chamber. I think I'll nurse it through the summer (the Pac-Northwest All-Chevy Show is coming up and I already paid the admission) and then pull the old dog out and get it rebuilt.

Thanks guys for the inputs! https://www.yenko.net/ubbthreads/imag...lins/beers.gif https://www.yenko.net/ubbthreads/imag...ouguysrock.gif

Re: Engine knock
 

Bill,

I had a 396 many years ago that had one piston with
excessive pin clearance and it knocked until it warmed up.
It drove me crazy for about 9 months. So, after taking the
engine apart, I found that the piston in question had a
crack right through the boss where wrist pin was positioned.

BTW, I never beat this engine and changed oil every 2,500
miles. It just happens some times. Also, once the engine
warmed up, the knock went away. I once took the car to
Maine in October ( cold as hell ) and when I started it up
early one morning, it sounded like it was going to throw
a rod or something. I paniced all the way home and did the
rebuild shortly thereafter.

Consider this possibility as well.

Steve

Re: Engine knock
 

Highly unlikely it is carbon build up if it disappears after warm up.

Lynn Bilodeau

Re: Engine knock
 

It does it both when cold or when warmed up. I tried some of that Seafoam engine treatment and that seemed to help. The idle improved right away and the knock became less frequent. I ran 1/3 pint through the carb, as per the instructions, and poured the rest into the tank. Runs better but still a knock every now and then but not as often. Again, I'm just nursing it through this summer before pulling the trigger on a rebuild so it's band aids until the the rebuild can commence.

Thanks again for all the comments!!!

https://www.yenko.net/ubbthreads/imag...iggthumpup.gif https://www.yenko.net/ubbthreads/imag...iggthumpup.gif https://www.yenko.net/ubbthreads/imag...iggthumpup.gif https://www.yenko.net/ubbthreads/imag...iggthumpup.gif https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

Re: Engine knock
 

Didn't we used to pour auto trans fluid down the carb while running to get rid of carbon ? Makes lots of smoke.

Re: Engine knock
 

I recall straight water down the hatch with some rpm used to do the trick.
https://www.yenko.net/ubbthreads/imag...lins/dunno.gif

Re: Engine knock
 

Hmm, tranny fluid or water? (I remember old USAF KC-135A refueling tankers used water injection to get off the ground when fully loaded and the black cloud from that was incredible! That smoke trail made them easy to follow.)

After pouring that Seafoam stuff down the carb it is running better. There was a gray cloud of smoke after I did the Seafoam treatment and revved'er up a few times. I checked the timing and then took the car out after dinner tonight and it's running great and the knock occurs much less often.

Re: Engine knock
 

What is Seafoam?

Re: Engine knock
 

http://i13.photobucket.com/albums/a2...s_whitecap.gif

Ha.......Beat Rick to it........

Here you go Marlin.......(see you at the Nova Nat's)

Ken https://www.yenko.net/ubbthreads/imag...mlins/flag.gif

Re: Engine knock
 

I "Trained" you well...

Re: Engine knock
 

Seafoam is a top engine and fuel system cleaner, and it's one of the best I've ever used. Just be prepared to smoke out every bug in your neighborhood for a 2 block radius. https://www.yenko.net/ubbthreads/imag...mlins/grin.gif

Re: Engine knock
 

I remember using that stuff w/ my Dad on his '71 Polara....best do it away from home or someone may call the fireguys? .
It does sound oil related....have you run a few oil changes through it? . Sometimes a few changes of different viscosity works and one will 'take'? .
I've found exhaust obstructions before that would touch the system at certain revs only even internals of mufflers loose and sounds traveling up the pipe to the front. .
Make sure the trans has no loose bolts also that the flexplate is Ok if an auto. . Same w/ trans mount and crossmember bolts. .
And have fun! .
https://www.yenko.net/ubbthreads/imag...ins/3gears.gif

~ Pete

Re: Engine knock
 

[ QUOTE ]
Didn't we used to pour auto trans fluid down the carb while running to get rid of carbon ? Makes lots of smoke.

[/ QUOTE ]

Yes we did Charley, but it doesn't do the trick nearly as well as water. Can really stick to the carbon and gum things up.

Straight water is the best method for breaking up carbon on top of a piston. Keep the revs up, and pour it straight down the front two barrels of the carb, alternating slightly from side to side. You can even trickle it with a garden hose. You will get a feel for how much it can take as you go, but start slowly. Not sure how to describe how much it can take. Obviously, if you over do it, you will cause a hydro lock, and have really serious problems.


Lynn

Re: Engine knock
 

Yes I remember water as well.

Re: Engine knock
 

[ QUOTE ]
I recall straight water down the hatch with some rpm used to do the trick.
https://www.yenko.net/ubbthreads/imag...lins/dunno.gif

[/ QUOTE ] Watched my old auto shop teacher do this to a friends 64 GTO back in high school,we both thought he was crazy but the car ran alot better... https://www.yenko.net/ubbthreads/imag...ins/3gears.gif

Re: Engine knock
 

next time it makes the noise stick some fuel line down by the fuel pump and the block and plug in your ear, them goofy BB motors have a habit of making a scary sound from the fuel pump getting week and the rod is hammering around... https://www.yenko.net/ubbthreads/imag...lins/beers.gif

SeaFoam is the Bomb!! I run it in all my stuff now and it flat out cleans and unsticks things like it says, dont be afraid to dump two full cans into about 10 gallons of gas and run it through a car that has sat, it will straighten it right out and purr like a kitten.. https://www.yenko.net/ubbthreads/imag...ns/scholar.gif

Re: Engine knock
 

It's definitely a combustion chamber knock and when it does it the idle will stumble very, very slightly. That Seafoam stuff is great and I'm going to get more of it and treat the engine again. I went to two auto parts stores on Sunday--the first one was out of it and I got the last can at the second store. I guess it's popular stuff.

By the way, https://www.yenko.net/ubbthreads/imag...ouguysrock.gif

Re: Engine knock
 

Guys!

Are there any possible bad side effects from the use of
the Seafoam product? I've got way too much money tied up
in some of my motors to ever risk internal damage to the
cylinder walls, etc.. https://www.yenko.net/ubbthreads/images/graemlins/no.gif

I know the same holds true to all of you as well.
Steve