My toyota van was overheating. Replaced thermostat and worked fine for about 2 weeks then stated over heating again, any ideas??

don't make assumptions , each problem has to be diagnosed ,first do the fans come on when it gets hot ? if it over heats right away it may be the head gasket , is the exhaust white ? Is there goop under the oil fill cap ? If the coolant is full and it over heats occasionally or the temp. gauge varies it may be the thermostat . I recently replaced one that was only 5 months old , you can put it in a pan of water and bring it to a boil to see , but then you might as well replace it if you take it out but you'll know what the problem is . If you've checked all that it may be the radiator or the water pump , things should be tested before you replace them .

Thanks for the input. Yes the cooling fans come on when it heats up, it takes a while before it over heats and goes up slowly, the exhaust looks good, I have not checked the oil fill cap but will do. It over heats all the time know.

The oil fill cap looks good, there is some black residue on the engine under the cap, but not a lot. The coolant was low so added some but still started to overheat. Water pump bad maybe?