1996 Chevy 1500, What is this rubber cap on my radiator?
 

1996 Silverado, 5.7 liter, I think heavy-duty radiator. This truck has a small (3/4") rubber cap clamped over a nipple on the radiator a couple of inches below the radiator cap. It split yesterday and soaked me. I've looked on all sorts of parts diagrams, and its now shown. Anyone have a clue of what its called or a part number? Thanks!

Check with a good radiator shop they might be able to help.

Red neck temporary fix
 

Thanks. Since the nearest shop is 25 miles away I came up with a temporary fix this morning . . . a short piece of 3/4" PEX plumbing pipe and a plug. I noticed the factory clamps were Cinch clamps like used on PEX, had a clamp tool and nothing to lose.

Sounds to me like it is for the overflow. Is there a plastic tank near the radiator that isn't hooked to anything?

This is a guess, but that rad might be a universal that has this fitting for vehicles that the heater dumps into the rad not the motor.

The size (3/4") and location if it is 3/4" would certainly make sense. I was actually questioning the diameter.

More facts
 

It's not the overflow, but is directly below the overflow fitting. The actual size is slightly smaller than .67", since that's the ID of 3/4" PEX pipe. The Clinch Clamps were identical ones with what I use to plumb PEX piping. I suspect Old Wrench is correct - it was put there for some special application used by another vehicle. Next trip back to civilization I will take the rubber hose cap to a Chevy dealer and let them scratch their head. I'll still take my truck over my sister's CTS Cadillac any day. It cost her over $200 to have a front turn signal light replaced. Yips!

Plug it back off

hi, got the same issue. I've been dealing with it for 20 years. That nipple is fir the heat return. Mine never had one so I've been buying the rubber cap like you mentioned and also made a similar fix. It sucks. Let me know if you find a permanent solution.