1993 Ford Ranger motor wont come up to temp

What was the reason for changing the thermostat in the first place? Overheating, running too cool etc. Make sure that the thermostat is installed correctly in the thermostat housing and make sure to completely burp the system of air.

I changed it because it wouldnt come up to temp thought it was stuck open so I changed it and when I did I filled it up and when I started it it was circulating immediately slowly but coolent was moving I lett it run for 1 hour and trhen drove it abought 155 miles never did come up to temp thanks for youre reponse

Do you know if any codes have been set on the ecm? I don't know this for sure, but if you're sure the thermostat is working properly and you are sure the vehicle is not getting up to proper temperature, you might want to see about replacing the temp sensor. Once again I do not know this for sure.