The old diaphragms I've come across ... more work than don't. Super easy to test with a hand vacuum pump. So either a bad thermal sensor or a crack in a vacuum line. This system uses vacuum ports with a very small orifice to delay open and close speed so they are not flapping open and closed or chattering with changes in vacuum ... easy for the little pin holes to get plugged.
A vacuum leak down stream of the sensor really doesn't show up in the way the engine runs because the orifices are so small. I've seen the lines crack at the sensor fittings a lot.
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