To start with if its a hot climate and you run without a thermostat it is going to over heat. Why? Because the coolant is just continously circulating, it doen't get a chance to slow down and get cooled off.
If its a cold climate then it will over cool, as it passes through a constantly frozen radiator and never gets a chance to warm up. May not make sense, just try it and you'll see.
If its clean and has good flow, the pump is working etc. then like others mentioned timing, and lean fuel mixture will do it too.
Did you trouble shoot it? Check outlet temps and inlet temp, check temp across the radiator etc. Is there good airflow through the radiator?
I would put in a "checked" 180 thermostat, don't run with out it.
Last edited by Truckrolet; 06/09/2013 4:34 PM.
