>Compression is normally either lost around the rings or past the valves
>(although in Vanagons you can sometimes loose it into the coolant system).  As
>the rings wear they could stick to the piston and allow combustion gasses to
>pass by them and allow oil into the combustion chamber (hence bluish smoke
>when rings are bad).  Valves could become caked with carbon deposits and this
>could prevent them from closing all the way and allow combustion gases to leak
>out past the valves (also could allow oil into combustion chamber and bluish
>smoke results).  How to test to see which is causing the compression loss is
>fairly simple.  Put a tea spoon of oil into the combustion chamber through the
>spark plug hole.  Now do your compression test again.  If the numbers come up
>then it is the rings (the oil temporarily makes a better seal around the
>rings), if the numbers stay the same then it is the valves.
>Ken Wilford
>Van-Again
>John 3:16

Another mode of failure for valves, particularly exhaust valves is burning.
A valve burns when it doesn't spend enough time on the seat, or just
because of long wear. What happens is a pie shaped piece of metal gets
eroded from the edge of the valve. Also, with high milage, the valve guides
can wear out and cause all sorts of valve wear problems because the valves
don't always land in the right spot.

Joshua Van Tol -- jjvantol@uswest.net