My gauge can get a little twitchy, but only when it's slightly above 100 C. I figured it might have something to do with water above boiling point. The system is pretty well pressurized, though, the water in the reservoir will only start boiling if it's above 100 C and I open the reservoir cap (with a rag).

These fast subsecond twitches can't be thermodynamic. The fastest real changes I've seen are maybe from 100 to 90 over 30 seconds or so when coasting down a hill.

BTW, operating at 100 C is a new thing for me. It used to run more like 90 C (mild summer day, 75 mph). Haven't figured out the problem (if there is one) yet.

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1984 300D Turbo 350K 50K WVO (two tank) "Maeby"