Knock Knock..

[Chevrolado]

.. Who's there?

This sound.

http://youtu.be/lNfR0d-AIIg

Anyone have any ideas?

It is only audible from base idle up to about 1500RPM and then it sounds completely normal.. but as soon as RPMs drop back under 1500 it starts to shutter and make this sound again.

Was hoping it was something as easy as a fouled plug causing bad detonation or something.. no such luck. New plugs in and still rough. Really hoping its not as bad as spun bearing or something worse. Looking for any ideas/tips/opinions.

Oh and this is on my 95 Tahoe - 350 TBI.

Thanks everyone!

 

[SunlitComet]

rod knock or a loose flywheel/converter bolt.

try removing one plug wire at a time safely while running. and see if it goes away. if it does you likely have a rod bearing goof on that cylinder.

start with that.

 

[Donnie Yukonie]

Sounds like a flywheel to me.

 

[Chevrolado]

Thanks for the tips guys! I will have to look into it little further. Figured Id just post it up though and get some input on where to START. Agh.. no good.

 

[Chevrolado]

So.. compression tests show that cylinder 8 is producing less than 90..
My mechanic testing tells me everything looks good spring wise, but he doesn't want to go any further unless I'm willing to pay for the labor of pulling the heads and continuing down into checking valves. And with all that work, I'm starting to break into high price tags that I might as well jump ahead and just move forward with my initial plan to buy a new motor and do the swap.

So now I debate.. should I continue to dig into why compression is shot on this cylinder and it knocks... or should I just make a decision on which motor and go forward with this question? http://www.tahoeyukonforum.com/forum/showthread.php?t=50517

 

[Gzes]

If it has low compression then maybe the piston ring is shot

 

[Chevrolado]

Yep could be!
I have reasons to believe that during a recent throttle body issue it ran extremely rough (smokey exhaust and terribly rough) and I'm thinking something may have messed up an exhaust valve. I read that it's possible that a large amount of carbon buildup can lead to low compression and such.. So since I'm already in the thought of replacing the engine, this weekend I'm going to run a can of Seamfoam through it and see if it breaks anything up.. Just figure Ill try stuff while I'm in the process of finding a new motor.

 

[SunlitComet]

adjust your valves and do a cylinder leak down test. it will help you figure out where the leak is. and yes 90 psi is below the 100psi threshold for engine.

 

[Chevrolado]

Thanks for the help man! I'll look into doin that.

 

[BigWheelinBubba]

One thing that causes failure in the old TBI engines is the oil cooler. It has a small check valve inside that over time gets restricted and the engine doesn't get proper oil pressure upon cold start ups and you end up wiping the rod bearings along with other things.
When it happened in my OBS years ago I did away with the oil cooler completely. The second engine had well over 200k on it when I sold the truck with no problems at all.
Note: This was my 1995 Silverado 2500 tow truck that had hard miles on it.