Chevrolet Repair: 350 OVERHEATING, cooling system pressure tester, head gasket problem

Question
I HAVE AN OLDER 350 WHICH HAS A NEW RADIATOR, NEW HOSES AND I AM ON MY SECOND THERMOSTAT. THE TEMP GAUAGE WILL PEG OUT THEN THE THERMOSTAT WILL OPEN UP AND IT WILL COOL DOWN A FRACTION FOR ABOUT 30 SECONDS, THEN HEAT RIGHT BACK UP. ANY IDEAS? BAD PUMP?

THANKS...............

Answer
Hi Steve...usually that means you have a head gasket problem....what happens is you have compression leaking into the cooling system that causes the water not to circulate.... then it heats up....install a cooling system pressure tester when engine is cold and watch gauge....if pressure builds really fast and goes past the pressure on the cap then you have a head gasket problem...because you installed a new stat, make sure you have all the air out of the system...leave rad cap off and allow engine to come to operating temp and top up rad as it goes down. once water is circulating top off again then you should have all the air out...allow engine to cool then install pressure tester....test your rad cap as  well...hope this helps