I have a 95 4.9 straight 6. I have replaced the radiator and water pump (less than a year ago). Just replaced the thermostat and that seemed to have solved the problem. I ran it to work (17 miles) and it started running hot again (gauge running on A usually rock solid between O and R). I added more fluid because I thought I have had an air pocket. That again seemed to fix the problem until the next morning. Did the same thing! I have had to add fluid every morning and afternoon for the past 6 days. Not alot of fluid but (less than a quart) still having to add it. There is no sign of leaks from hoses, water pump, thermostat.... Nothing. So if any one has a suggestion feel free.

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could be ur heater core. try puttin alumiseal in it that stuff works great i use it and it will stop a pretty good leak u can get it at autozone

 

get some radiator flush and clean everything out before you start dumping in gimk junk that never works(for long atleast).

check your oil for a milky discoloration

check the hoeses and clamps ( put a turn or 2 on the hose clamps)