Coolant leak

On my 95 aurora, I replaced the water pump last march, since than there's been a small coolant leak somewhere in the engine. I just put more water in ever 4 to 5 days and it would be good for another 4-5 days. Since the winter is coming up I wanted to fix this and put coolant in so the water wouldn't freeze. The mechanic did a pressure test and said there was no leak. When I picked up the car and was driving home the car over heated within a mile. I drove it back to the mechanic the same day. We put some water in it and it came right out. After looking around I found a hole about an inch big. It was almost as if a seal or a cap had come off. The hole is right behind the water pump if you are viewing it from the driver side. You can actually see the outline where the cap was. Has anyone seen anything like this?

Can the seal come off when the pressure test was done?
Why would GM put a cap on a hole?
Is there a fix for this? The mechanic says he can’t fix it.

PLEASE HELP.

Are you talking about a freeze plug?

Are you talking about a freeze plug?

I am not sure. Unforunately I never saw it before and now that it's missing I don't know what it looks like. I would assume it's a freeze plug?

Usually made of brass, looks like a large soda bottle cap(if you're old enough to remember). Several of them are pressed into the block at various locations and are designed to pop out at high pressure to save the block from cracking due to expansion from hard, prolonged freezing temperatures. They can be pressed back in with the right tools.

Usually made of brass, looks like a large soda bottle cap(if you're old enough to remember). Several of them are pressed into the block at various locations and are designed to pop out at high pressure to save the block from cracking due to expansion from hard, prolonged freezing temperatures. They can be pressed back in with the right tools.


Any idea on what king of tool I should be looking for?
Since the block is Aluminum, can it be welded?

This is something that I would pay to have done. If you try to drive one in yourself and it is not perfectly straight going in, it will come out again,...late at night in bad weather far from home.