Here is a Quick Video I took today at the shop demonstrating what is known as “dropped” exhaust valve guides. The vehicle is a 2000 Subaru Outback with the SOHC Subaru Engine. Once I have more time I will take some better pictures.
This was a very common thing to happen to the Legacy 2.2l from 1996 to 1998 with Single port exhaust but we have seen it more and more on the 2.5l engines, but I will add, typically only on the ones that are not taken very well care of.
What typically occurs is carbon and sludge developing on the stem of the valve and the inside of the guide. This causes first the valve to want to stick or hang up in the guide and then under the right conditions as the aluminum head has expanded from heat away from the steel guide the guide will shift a bit, at first and once it has moved will continue to do so until it actually causes a cylinder misfire.
This can be avoided with good maintenance normal oil changes, and no every 7000 miles isn’t good maintenance unless you like expensive repairs later. I also suggest a fuel induction service every 60k as good preventative maintenance as well. The carbon buildup that I try to point out is after we had already cleaned the heads in a parts washer. Using good fuel, not ignoring a check engine light are all forms of good maintenance.