Gauges like anything else can fail. I have now come across three different failure modes with the blue sensor on different mk1 Fs, some more worrying than others.
The first one was occasional needle drop on sudden acceleration; needle previously reading normal would instantly drop to 0 for 15 seconds during overtaking then slowly climb back to normal at the higher revs. This sensor read a normal 80 ohms when measured at ambient, probably going open with a rush of hotter water on heavy acceleration - but I never got to measure that bit!
The second one was not ever responding to temperature - unnerving in an F! The sensor was binned before I could measure it but almost certainly stuck open.
The worst one I (hope to) have just fixed was characterised by occasional needle climbing into the red then making its way slowly (15 seconds) back to normal on a recent newly head gasketed car. Happily it gave the game away by doing it once when the ignition was switched on with the engine warm but not running. This one measured 3k9 ohms.
All were presumed, almost guaranteed, OEM sensors; not cheapo replacements.
So:
1. Needle drop before recovering.
2. No go.
3. False overheat.
Can you add any?
Temperature gauge failure modes
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