I recently swapped a B18B into my 93 civic and since I've had it in the engine isn't warming up to normal operating temperature. I drove 2 hours on the interstate and the car never fully warmed up. I could turn on the heat and it was decently hot after a while of driving. If I just let the car idle without driving, the engine will eventually warm up to normal operating temperature (according to the temp gauge). My thoughts are that either the thermostat is stuck open or the ECT gauge sensor is possibly bad, but I'm leaning more towards the former since the gauge still works. I just wanted to get your inputs on possible causes or ways to check before buying any parts to try to fix it. Thanks.

 

tstat is stuck open

 

Have u checked to see if there is a thermostat?