Cap-Armor aka FIGHTING CHANCE !
Mark Gruenwald was a great Captain America writer and was on the book a long decade run. He introduced a number of cool rogues who would become mainstays in Captain America's rogues gallery. Crossbones , Mad Cap and Super Patriot was a couple to mention. A few arcs were really regarded like the 2+ year long Johnny Walker storyline where he became Captain America. Or the "Streets of Poison" arc.
But like a lot of writers who stay on too long a book ; Gruenwald likely stuck around too long on Captain America as his final near 2 year arc showed. In it Steve Rogers is starting to feel weak and learns his super soldier formula is now running out on him. Doctors advise him to retire due to the strain on his body being Captain America and that the formula once its gone he will likely die.
Steve decides to go down fighting (Fighting Chance is the arc) and continues on ignoring this. He eventually informs the Avengers as his body is starting to feel the effects of it. Eventually to help him combat the serum leaving his body near shut down ; Tony Stark designs a robotic suit for him to work in awhile. The storyline ends as Cap at the end seemingly dies but the next issue Steve returns to life with a blood transfusion I believe.