[RICK LAKE]

Undy with this spike problem I would first go out and get a infrared temp gun. Start the car and watch the temp readings. It could be the gauge, sendor or a grounding problem to the block or if you have teflon tape rapped around the threads to seal the coolant it could cause a poor ground until thing heatup. At 250 there should be steam and water boiling coming from your tank outlet. If not than something is giving bad readings. Are your gauges electric or manual with a solid tube coming from the sendor to the gauge. How much pressure is on the coolant system before the temp drops? I know I am reaching but, who put the motor together and is it possible they installed a head gasket on wrong. This has happend more than once. The headgasket should look both the same with the ends of the gasket sticking out, if not, possible head gasket in upside down. Motor will run but get hot over a long drive. 1 hour or more. For the $15.00 dollars go to the pepboys and look for the themostat old style for a dodge 383-440. look at the outlet, it's a 1/2" larger than the FE one. Trim the out side edge to fit in the opening of your intake manifold and twick the 2 top supports to fit in your tank. Drill the holes. This should work. Is the motor new and tight? how many miles on it since assembly. Rick L. I only use tap water and 2 bottles of water wetter in my car. My coolant recover kit sucks down the whole system so I get no air pockets. Good luck. Rick L.