Easy check for a "slight" head gasket leak at the water jacket - any up-to-date shop should have a sniffer that detects hydrocarbons in the antifreeze. Any positive reaction and you need a head gasket. I had a slight leak on another car, could not locate it, was losing coolant and had no tell tale sign of H2O vapor in the exhaust. A friend with a shop stuck the sniffer in the upper tank which showed a gasket leak. Test took just a few seconds.

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Mike Tangas
73 280 SEL 4.5