Over heating
You should go buy a new radiator cap, bleed all the air out, top it off and go for a ride.
I think your cap is old and its slowly, quietly pissing coolant out while you drive, then when it cools its sucking air into the system.
Get a new its $3.00 for a Stant with a pressure relief valve on it, if that desn't fix it then start looking deeper.
Air is getting in, thats the problem. You might have a fan issue too, but bad fans don't allow air into the system.
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That is what I am asking is the water pump bad when is pouring water out the the back?
If you're positive the coolant is coming out of the weep hole....change the water pump.
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