Engine knock

[427TJ]

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!!!

[VintageMusclecar]

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?

[427TJ]

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.

[Chevy454]

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

[Xplantdad]

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...?

[427TJ]

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!

[nuch_ss396]

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

[Lynn]

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

Lynn Bilodeau

[427TJ]

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!!!

[Charley Lillard]

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