I have a '97 V6 with a cooling issue. Recently changed the thermostat from stock to a 180 degree. Since then I am losing cooling through the overfill tank. It is backing up and overflowing.
I am getting cooling coming through the outlet so it appears to be flowing through the system.
Any help would be great.
Thanks

I assume you changed to a lower thermostat rating. The only things I can think of are 1) radiator cap seals not sealing correctly, or the sealing surfaces are not clean or 2)some type of coolant pressure surges due to not fully venting air from coolant. Make sure your radiator cap is designed for coolant recovery, i.e. double seals and that your filler neck surfaces are clean and smooth......SSF

I checked all seals and caps. All look to be in order and secure.
My new thought is just that the thermostat that I purchased is defective. When I ran the engine my temp gage read 210 degrees. The thermostat is registered at 180. Still getting an over flow of coolant out of the reservoir tank.
Unless it is a bad pump. Any other thoughts on this?
Thanks