Motor continuously overheats, otherwise it runs and performs fine (no hard start, rough idle, etc.). the heatercore, oil cooler, and radiator have all been flushed. The thermostat has been changed twice and the cooling system appears to be functioning as it should...but it still overheats. What else could it be?

 

Are you sure the car is actually overheating and maybe the gauge isn't just reading high? Or do you have other signs that the car is indeed overheating?

Have you checked all of your hoses to make sure one isn't kinked? Sometimes the hoses can look fine when they are cold, but when you heat them up they become soft and suck themselves closed. This can easily cause the coolant to just stop flowing....

 

it is definitely overheating...you can hear it and smell it. my husband tried it without the thermostat and it didnt overheat. so yet another thermostat and a water pump were replaced and it is still overheating. my husband has worked on cars for 20 years and this has him totally stumped. nothing makes sense!

 

I turn back to my second question then, When it is overheating has he checked the hoses to make sure none of them has collapsed under the vacuum pressure? This can happen if the hoses need replacement or if the radiator cap is no longer making a seal. This could also explain why the problem goes away with the t-stat removed, as the water pump won't be able to make any vacuum pressure since the system is always open and flowing. No vacuum = no collapsed hoses.

I would replace the radiator cap either way, since it is an easy thing to do. If he hasn't already.