<VV> Engine knock diagnosis

[cmot at ix.netcom.com]

	In the past I had seen strange difficult to diagnose lower end
knocks in vehicles that sometimes seemed to come and go but were very loud
and disturbing but ran fine. One time is was a cracked crank on a Ford V8
that had cracked in two on a casting line directly across the web between
the main and rod journals and was difficult to see even in direct view. More
common on Auto trans vehicles was to find the flex plate between the crank
and torque converter had developed tiny stress cracks around the crank bolt
circle or near the converter attach points that made a horrendous noise.
Again difficult to see initially unless it sometimes had a rust "smoke"
trail emanating from the crack. Both required careful examination to find.
The first time I heard a Corvair with a standard trans flywheel with loose
rivets it was a baffling sound with the noise telegraphing throughout the
lower end and transmission with no direct relation to speed or cylinder.
	A trick for isolating the area of noise is to short out cylinders
one at a time while the noise is present and listening for a change or
abatement of the sound. If you have access to a engine analyzer with this
feature it's easy. If not you can get vacuum tubing about the same diameter
as your plug wires, cut pieces 1 1/2" long or so to insert in the
distributor cap and fit into the end of the plug wire terminal. The rubber
in vacuum hose has carbon in it and is conductive. Use a jumper wire to
touch each piece of hose one at a time to test. This can help you determine
if the problem is isolated to one cylinder or not, whether it is a lower or
upper end knock. You may have to check in gear and out. If in gear make sure
you chock the wheels with E-brake on, preferably with someone behind the
wheel as well. You don't mention if the noise is different under load or
idle. Safety first in testing!
	If you suspect a valve problem a vacuum gauge will usually pulse
with the bad cylinder rather than read steady. If you have one hooked up
while shorting cylinders compare the effect on readings with each cylinder.
Hope this helps. Good luck. Chet Motyka

From: "Jedd Hebrink" <jhebrink at anvilcom.com>
>
>    There does not
> seem to be a direct relationship to the pace of the knocking and the 
> engine speed.
>
>> Jedd Hebrink
> 66 Monza 110 PG