Use a short piece of hose from that nipple and plug it, then. You don't need to be bypassing that much coolant back to the water pump inlet. It's interfering with the operation of the thermostat.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!