Swapped in a d16y8 running obd1 p28

runs fine. was going down the highway today at 4k (Si transmission), temp started to rise to half way.

checked fitv (i think, it's a 2 wire big sensor connected to back of intake)
top bolt was loose, tightened it. one hose was a little wet.

stopped for a few mins, took the same trip a little slower (3-3.5k), no issue.

rad filler neck still bubbles after 10-15mins running. coolant level lowers. overflow bottle is filled past full (wasn't when i refilled my rad last)

bad head gasket? bad thermostat? no white smoke, so i'm not really sure. can smell coolant in the engine bay tho, can't find a visible leak!!!

any insight is appreciated.

(also the footwell is always warm, not sure if that has to do with this or not)

 

Do a leak-down test and that will help pinpoint your problem. May as well compression test it too while you are at it to see the shape of the combustion area.