Kiotzu

Just swapped in a new engine on my 2002 Civic because of blown head gasket, so it was okay the first day or so I drove it then it started to overheat, coolant still full but bottom hose was cold while the upper rad hose was hot. I thought thermostat so went on vacation car didn't run for 2 weeks, today I went to fix it.

I had to move the car so I went to start it up and before I even started it the car was at running temp on the gauge, so I changed the thermostat and now it starts at halfwalf mark on the guage and just goes up.

I want to test to make sure the thermostat opens but I also don't want to be driving it when I can't accurately tell the temp.

What could the causes be?

mcnoople

Did you plug the wrong wire into the sender? Sometimes engine connectors share the same connector shell and can reach compatible sensors. It can cause fun headaches when chasing the issues this can cause.

Do you still have the original engine laying around. I would swap your original engine gauge temp sender into the new engine and see if that fixes the problem.

ezone

I would have fixed the head gasket.

A used engine may need a head gasket too. Almost all need one sooner or later.
Got a scanner? Got codes?
What is the ECT reading in the data list?