sounds like the water pump has a hole in the housing somewhere and is recirculating water through itself at idle, but cant flow enough under load or higher rpms.
might be a bhg, but im leaning towards a wp. had the same problem on my buddies dodge dakota 5.2. would idle all day long, but put it on the freeway, itd over heat in 2 min. 1/2 inch hole in the pump housing...gaudamnit..
but like said, do a compression test. check for any coolant hose leaks (small hole can suck air in instead of leaking).
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87 toyota 4x4 single cab short box pickup, 7MGZE.
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