As at the time, Superman was split in two by Lord Satanis, he could not accompany the Titans further.
Is that what it was? I just remember that he didn't have his super powers.
We are both right. He HAD lost his powers, and he had been split in two by magic. More specifically, Lord Satanis, while searching for the runestone in the 20th Century, split Superman into two, each one only having HALF of Superman's powers. It was this "half split" Superman that saved the Titans from the JLA watchtower.
Also, it wasn't #14, because the Doom Patrol story with the death of Madame Rouge and General Immortus's De-Evolution Pit started in this story (I always liked that in the trap, all of Titans became gorillalike creatures, whereas Starfire became something more feline). The story started sometime in the teens, though, and as I recall, ended in the first New Titans Annual.