(SOLVED) Dead cylinder at idle? Low temp on header collector

Any mechanical work done to it recently?
You try moving in closer with the temp gauge.
Swapped the intake for a fresh blue painted one. Wonder if I need to plug the hole in the floor? People say no need to plug, but I left the upper plane threaded hole unplugged and those 4 cylinders were running like dog **** until I plugged it. Lower plane hole is still wide open.