V6 262 'W' motor 190k miles
The truck will heat to 195 upon first starting but when thermostat opens, it drops immediately to 160. Currently has a brand new genuine Delco stat installed which opened and closed correctly in a pan of hot water before installation. Before that I installed a succession of aftermarket stats, trying to fix this problem. I have also replaced the two temperature sensors on the manifold, and had the temp guage checked for accuracy. Finally, I am confirming the water temp independently of the temp gauge by placing a thermometer in the radiator. I've never seen anything like this in 40 years of working on cars. Somebody PLEASE show me what simple thing I am missing.
thanks, Phil