Are you in N. America?
If so, there was no 1988 300D, only an 1987 300D. I suppose
there might be an 88 300D somewhere else in the world, they
get more diesels than we do

Either way, you _probably_ have an OM603 variant motor
which does not require valve adustments like the older OM617
motors did. They stopped bringing the OM617 to the states
at the end of the 85 model year.

Chris Blanchard
1984 300SD 150k