I claim the ornamental design for an decorative exhaust system cap, as shown and described.
FIG. 1 is a rear view of the decorative exhaust system cap according to Applicant's new design.
FIG. 2 is a rear perspective view of the decorative exhaust system cap of Applicant's new design.
FIG. 3 is a side view of the decorative exhaust system cap of Applicant's new design.
FIG. 4 is a top perspective view of the decorative exhaust system cap of Applicant's new design.
FIG. 5 is a top view of the decorative exhaust system cap of Applicant's new design; and,
FIG. 6 is a rear perspective view of the decorative exhaust system cap of Applicant's the new design, shown attached to a muffler depicted in broken lines.
The broken lines depicting the external environment in which the decorative exhaust system cap is mounted, (FIG. 6), represent unclaimed structure.