[RICK LAKE]

Undy He have a coolant flush and fluid transfer machine that removes the coolant after run to operating temps. Vacuums the whole system down and replaces with new coolant. The vacuum parts keeps air pockets and air from being caught inpockets in the heads. Have you tryed to purge the coolant system with the motor on the first startup cycle? The best place would to be at the highest point of the coolant system on the motor. I would try cracking the sensor and see if coolant leaks out or air. If air, you may have to do this 4-5 times to purge the system. LS1 motor have this problem. It's possible to have 1 head gasket in wrong on the FE and run the motor for over 1 hour and just start to overheat. I know I mounted one wrong. Motor ran fine. but over an hour the temp was at 220 with a 180 thermostat. Plenty of flow of coolant but running hot. Try the holes, IMO I think you are going to push too much coolant through the thermostat housing without letting it heatup. It may fix the spike problem but might hurt the effectiveness of the coolant system. Dave I will get you the right number of STAT tomorrow for the thermostat with the larger opening. You must have alot of air in that motor that after 5-10 drives and till having this problem. I wonder if you have a head gasket not sealing, and it is bleeding compression gas into the coolant system. Motor heats up and the expansion is stopping the leak. Motor cools off and you have the air in the system again. What combo are you running, iron block aluminium heads, iron heads aluminium intake, all aluminium?? You have a recovery bottle so the hot coolant goes in there and when the motor cools off it is sucking coolant back into the motor but not all. There is a tester kit for checking combustion gases in the coolant system. Rick L. Ps used to drive on VA beach in the 70's, there was a patrol guy on the beach in the summer with a black jeep still there? I lost $50.00 to him in a drag race on the beach at 2:00a.m. Still a great beach for the sites.