you are trusting a factory electronic guage i suppose. good old fashioned meat thermometer with the radiator cap removed. by the way...........remember not to open the system when it too hot. if you find much of a descripancy, go with a decent aftermarket mechanical gauge. oh yeah, the heater valve is on full hot. ya just might have a vapor lock problem. take a hard look at the radiator cap while it's off too. place in a pan of water and as the water heats up. check the openig tepm against the temp on the meat thermometer.
