So I changes the expansion tank to the bmp tank due to a crack in my stock tank.
So I go for a drive and it is fine for 30-35 miles. Then bam the temp climbs. Coolant is coming out from the cap now. Air bubble or bad t stat?

Background on my car. Only had 2 weeks and got with bad head. Motor was half torn apart. Did all the machine work and did new head gasket and it runs great. Just this cooling issue. That was the longest drive I have been on.

 

I would start with the possibility of an air bubble first. With the engine running and the heater full blast add coolant till full at the access point near battery...not the expansion tank. Then replace cap and bleed using all 3 bleed points (one at a time). Be careful not to over tighten the bleed screws as they are plastic....lol. Fill the expansion tank to the top mark and monitor the level after a test drive. My overheat issue ended up being a small radiator leak that caused air pockets at times. One telltale sign was the loss of heat when heater was on.

Hope this helps.