Strange problem on an '84 GLT that my son is bringing back to life. Last year we replaced coolant hoses and I happened to notice that the radiator cap had a BIG crack. I ordered one and sent it to him, since he had returned to college. He called shortly after to say that the car was overheating, and there turned out to be a major water leak. I told him to refill it with 50/50 coolant and put the cracked cap back on, and it got him through the school year. Now that he's back, we looked at the cooling system, and saw nothing obviously wrong. I saw a little bit of seepage on one radiator hose, so tightened it and put the new cap back on. We went for a drive and after a few miles it began to overheat. We brought it home and popped the hood, and found a "hole" in the radiator. More specifically, there is a rubber grommet on the passenger side of the radiator, towards the top (above the fittings for an auto tranny, which the car does not have). Apparently something fit into that grommet, which popped out (I never noticed it, so have no idea what it was). Anyone know what it was?

Anyway, I replaced it with a 7/8" expansion/freeze plug and put the cracked cap back on. It seems to be running OK--only slightly above 9 o'clock position on a hot day with the A/C on, and going up a grade. But I don't like leaving it that way--anyone have an idea what's going on?