Found the source. Took of the heater return hose. No water in it. Took off thermostat housing. Guess what. No opening for the return hose. There is a flaw with the casting🙈. Water is restricted from flowing back. I will try to drill open the passage, and replace the whole housing with an original one later. Will report back on success😀

Non original part needless ti say

 

I would replace the thermostat along with the housing.
Just get rid of the P O's thermostat, & housing mistake.
You don't want to go back here again.
Be sure to replace both of the housing parts.
Some people get lazy, the upper housing torx bolts can be an issue to remove.
Don't make this mistake or you will have to go back later.
Good Luck To You

 

Will replace, but currently minus 14 deg C here so atempting a quick fix….

 

Could the thermostat be installed upside down in the housing?

 

I had a T Stat housing fail. Car overheated when stopped, cruising along no overheat. The plastic T Stat has internal channels, one broke and clogged/restricted the flow.
From the outside the T Stat housing looked perfect, no leaks. The day it failed it was very cold and I did hear a small noise/bang when I started the engine.
Dealer fixed mine for $500 on a Saturday.

 

I am having the same overheat conditions. I’m thinking air. But now want to go and check my part for blocked return. Was yours a success?

 

Much easier to get air out of the system when I opened up the blocked heater return.

If you have heat in the cabin, then you do not have the same problem.

br Martin