Michael:
At the risk of appearing foolish, allow me to wade into this one. If one
has a perfect seal between the crankcase and the combustion chamber, there
would be no gas exchange between the two volumes. Since the seal is not
perfect at the piston, there is a certain amount of intake charge and
exhaust blow-by. This is the reason for a crankcase vent, which in the old
days was just a cap on the valve cover which allowed these gasses to
escape. More recently these gasses are redirected into the intake manifold,
by way of the PCV valve (Positive Crankcase Ventilation)which, by virtue of
the manifold vacuum, positively vents the crankcase. Similar gas exchange
will occur, to a lesser extent, at the valve stem seals. So far so good?
Now then, if the crankcase is under positive pressure as measured by the
hand-over-the-hole method, that says that either the crankcase vent is
plugged or the seals between the crankcase and the combustion chamber
(rings) are bad. A negative pressure may indicate that the valve seals are
bad, since cylinder vacuum is getting past the seals on the intake stroke.
This is confirmed if there is evidence of oil-burning. All this should be
followed by the usual regimen of compression checks.
If this is wrong, I'm sure I'll hear about it.
Philip E. Barnes '71 TR6 CC61193L
Cornell University
Newman Laboratory of Nuclear Studies
Ithaca, NY 14853
607-255-4951
peb3@cornell.edu
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