LS swap temp sending unit not working

[EightiesOnly]

Alright, so I recently finished my 5.3 swap in my '86. I'm using a custom temp sender adapter and the temp sender doesn't work properly. It just reads about 100* unless I put it in park and let it idle, then it rises to about 195-200. Once I start driving, it gradually goes all the way back down.

I removed the plug on the passenger side and put the adapter there.



I thought maybe the coolant flow is just below the sender while driving, but it's full of coolant and others on here have used adapters in this spot just fine.

Any suggestions?