I’ve looked through the threads and can’t find the source of my issue here. I have recently rebuilt my N12 motor and everything seems good so far with 110 miles on the new motor. However, once the vehicle reaches operating temperature the expansion tank fills up to the cap. It doesn’t look like it’s boiling it just rapidly fills the tank. Once I let it cool down it goes right back down to the max line. I doesn’t seem to be leaking anywhere, and I replaced most of the cooling system anyway. I’m monitoring the temperature while I drive using my OBDLink MX. The coolant sits right around 223F to 226F. The fan kicks on when it’s supposed to. I’ve tried bleeding the system several times following the steps posted on threads here. Anyone know what’s going on?

 

You sure the T-stat is installed correctly and works opens when it is supposed to? Before I install a new T-stat I always drop the old one and the new one in a pot of water on the stove and heat the water and monitor its temperature with a digital thermometer and verify the new T-stat opens at the right temperature then after the heat is turned off and the water allowed to cool that it closes.

You sure a towel/shop rag didn't end up on one of the spigots the hoses attach? Do you have the hoses routed correctly? Did you install the wrong head gasket or install the right one upside down or the wrong way?

 

The thermostat housing is a new OEM one. Made the mistake of buying aftermarket once. The hoses are installed correctly. Verified with some diagrams. Head gasket was correct and installed right side up. I’m gonna run a compression test tomorrow. I’m starting to wonder if it’s related to another issue I’ve been having. The motor has been idling rough. I just pulled my valve cover and found two stripped bolts and one half threaded in. Looks like I was being a little negligent there. Car was running very lean. I’m gonna to fix the idling issue and then I’ll update on any changes.