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Replaced radiator cap and it blew up my radiator

92 Toyota Pickup (22RE auto). New block, rebuilt head, new water pump at 300k about a year ago
Sounds strange I know but that’s what seems to have happened. Ive never been able to get heat from the truck since Ive owned it so Ive been trying to fix that. I flushed the coolant out and replaced the tstat. I brought the car to a mechanic and he put a pressure tester on the cooling system and he said it wasn’t holding any pressure. He also pulled off the heater core inlet hose with the truck running and no coolant was coming out. He thought there was a blockage somewhere but didn’t have an idea of how to fix it. I figured I might as well try replacing the old radiator cap with a new one so I did. Once I did this, the first time the truck went out, it overheated and the radiator crimped upper seam (on the plastic) burst outward.
So all in all…something is wrong. Thoughts?

Just a thought: the cap was preventing the system from pressurizing (much). The rad was weak, but not having to contain much pressure, you’d never know.

The cap was replaced, allowing the system to operate under “normal” pressure…which was too much for the weakened radiator.

I’ve purchased defective new radiator caps where they opened below the specified pressure, or opened above the specified pressure.

Before installing the new radiator cap onto the new radiator, have it pressure tested to see if it opens at the specified pressure. You don’t want to blow up a new radiator because of a defective radiator cap.