Inlet air temperature is a BIG factor in the detonation equation. The higher the inlet air temp, the higher the propensity to detonate. In the winter I can run 87 PON in my M103, but not in the summer!

Most FI engines have a cold air induction system with a duct to the area in front of the radiator to pick up cool, ambient temperature air. This both reduces detonation tendency and increases power relative to aspirating heated, under hood air. Make sure your cold air duct is intact.

The recommendation for my erstwhile '84 2.3 was 87 PON but I believe your '87 2.3 is 91 PON, same as the M103 six-cylinders.

Duke