383 engine noise cylinder 5

[davidwv72]

I tried to search for info and only found incomplete guidance. I have a 383 with an engine noise/knock. When I pull the plug wire for cylinder 5 it goes away completely. I have tried using a stethoscope to pinpoint but my hearing could not really confirm much. The noise appeared in spring when getting the car started after winter (it is stored outside sadly). Noise was not there when it was parked.

I believe engine was moderately rebuilt few years ago before purchase. I broke motor mounts and had to replace, and I also had to re-install exhaust header since there was coolant leak from one of the bolts. I cannot claim to have done a great job, best effort probably.

I see everything from spun bearing, valve train noise, rockers, and also exhaust leak for causes of the sound similar to what I hear. I have seen nothing that narrows it when the sound completely goes away when plug wire is briefly pulled for the cylinder in question.

I would like to not pull engine and rebuild, which I know is the answer for some causes so looking for something to check, possible causes, etc. Thanks in advance for any ideas.

 

[RemCharger]

As you mentioned, rule out the exhaust leak first.

 

[davidwv72]

As you mentioned, rule out the exhaust leak first.

Just to be clear, the sound going away completely when plug wire is pulled for that cylinder means it could be an exhaust leak? This was something I was not sure about. It sort of made sense to me, but I was not completely sure on the logic of it.

 

[RemCharger]

Yes the spark plug causes the explosion , thus the noise. Remove spark, removes combustion.

 

[BeatersRus]

id guess a spun bearing.
have you checked the Valvetrain for that cylinder?
rockers all good?
make sure nothings worn out or bent/pushrod?
if the rest checks out,its probably a rod bearing.

you can also do the old broomstick test.
grab one of those cheap dollar store plungers rip the plunger off,use the handle like a Stethascope,stick one end on the motor while its Running and the other end in your ear.
find out how Low or high in the motor the noise is,and that will tell you a Lot.
you can use any old piece of broomstick too,i just like having clean tools ;)
please let us know what you find.

 

[Geoff 2]

Exh leaks do not knock, they tick. Could be the rod brg, cracked piston. A problem with the piston pin is usually a double tap noise. Either way, I would strip it because it ain't gonna fix itself...& much worse damage could ensue if left...

 

[Diewilde13]

Also check if the Plugs are all tight.
Guess why i tell you that