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Since last post I tried many diagnostic avenues to understand why temp would reach normal and drop to cold within 30 seconds. I had in the past changed radiator to a larger 3 core. At that time never had this temp problem. The problem came about when the heads began having valve problems from cracks between intake and exhaust. Coolant loss began and I assumed that must have a direct correlation to the temp fluctuation. I replaced heads and the problem remained the same, good thing was got back engine performance. Following are procedures I tried. Three different thermostats with no change. Three different sending units no difference. One different gage, all resulted with same results. The temp still fluctuated. Here is where I began some off the wall stuff. I began to think that the oversized radiator was simply dumping so much cold coolant I would shunt off the flow via a clamp on upper radiator hose. It still did the same thing. I covered the radiator 100% with cardboard, still done it. I took off fan, he'll the dam thing wouldn't even overheat, it was 30 deg. Flushed heater core, PROBLEM DISAPPEARED?
