I have a 2001 suburban with a 5.3 motor. I bought a new thermostat from dealer and installed it. Problem is i can not stop it from seeping antifreeze. I have cleaned everything 3 times, have tried installing it dry and with sealant.(sealant was a non harding silicone) Any other ideas. The thermostat and housing came as one unit.

 

Is the gasket surface nice and flat?

 

Try this. Get some 400 grit sandpaper and put it on a flat strong surface and and lap the cover by just going around in circles. Similar to "wax on, wax off" This will give you a flatter sealing surface which will normally help with sealing problems.