Originally Posted by p1mark
worn valve seals wont cause a compression loss. normally when these go you get smoke for a minute or so after start up (talking bikes and cars here with verticalish cylinders, not sure if it would be the same with horizontal scoob pots) but then it goes.
we run some race bikes without any seals deliberately and you still only smoke on start up, so i dont think your smoke is down to this.
worn valve guides wont cause it directly either, but this will give your valve seat and valve 45deg sealing angle a harder time, making them recede/wear quicker. then obviously you may get compression losses and/or a bit of smoke.
if your valve to guide clearance is like a **** in a bucket, you may get a bit of smoke but yours does sound a bit more than that.
i think you are gonna have to take the engine out and whip the heads off to confirm though unfortunately
Usually, valve stem seal failure is recognised by the engine momentarily smoking after a period at idle. The high idle manifold depression pulls oil down the valve stems into the tract above the valves and thence into the combustion chamber, which, at idle is not hot enough to burn the oil. When you press the loud pedal and put the engine under load, the temperature rises and the oil burns and blows out as a brief puff of smoke.
Very often, it is possible to change the stem seals (and springs) without removing the head, but I think accessibility would rule this out on a Subaru.