Inaccurate Rally Gauges
Short answer: Erratic readings are largely the inaccuracy of the stock sending units, not necessarily a cooling-system fault. (1969 conversions are easier than 1967–68 — no full harness redo.) Factory gauges read poorly even with correct GM senders — a member who pressure-tested six GM oil senders found none read alike.
To know your real temperature, run an accurate mechanical gauge alongside the factory one and compare (remove it once you trust the factory reading, or note the offset). Test the thermostat by heating it in a pan of water with an accurate thermometer — it should fully open within a few degrees of its rating, and the engine should run within a few degrees of that rating when everything’s right.
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Content last modified: June 13, 2026 at 10:06 am
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