The thermostat is completely dependent on water temp, period. It has nothing to do with the height of the radiator. The pressure difference between the old and new radiator heights is would be very small compared to the normal operating range of the cooling system, so that isn't an issue.
If the thermostat is sticking, you'll have problems with cooling obviously. The thermostat sticking is a completely separate issue though, unless there is something mechanically blocking the thermostat, or you have a hose pinched, etc.

However, if you have air trapped in the system above the level of the radiator, that can cause all sorts of problems. That is where I would look first. Also, are you basing your overheating diagnosis on what the temp gauge is reading, or on something else?

 

Brand new therostats can fail. Had it happen once with an OEM honda thermostat. Not likely but just throwin it out there.

 

thats o good point..very true...ive been thru 3 brand new thermostats that were all trash from day one

 

yeah i am not sure what it is, the radiator sits low and i know placement would have little to do with the water temp and causing it to stick or not open, It was brought up too me from a respectable mechanic, Because we couldnt figure out what was up. When it started to over heat i would nail the gas pedel and the second the turbo kicked it you would see the temp guage go down but we couldnt figure out why. I thought i would give it a shot on here and see what other people would have to say.. I am going off the temp gauge inside the car,which when it says the car is getting hot it dont feel sluggish at all. but still i am scared to drive it after it gets hot. Its going to the shop and have it looked at but thanks for the help

 

take the t-stat out and put it in boiling water to see if it opens. i think when you hit the gas it turns the rpms up and the water pump turns faster to put more pressure on the t-stat. that might be somethings. i think if the car did overheat wouldnt that bleed the air out anyway?

 

No, if the air is trapped somewhere above the radiator, it still won't necessarily bleed itself out. Plus, the hotter it gets, the more the air pocket will expand.

 

thats a good point. thanks! I was trying to figure out why it would go from hot to normal when i punched on it. the t-stat was sticking but i cant figure out why. maybe like others have said its just a crap t stat... i took it out for now until i can take it to the shop and have them look at it. i dont drive it but every 3 or 4 days so it just sits there for now..