Instruments are often fed by a separate sensor than the ECU which can show up as a lag in the instrument indication. I have a scan gauge that I use to verify such anomalies.

I was suspicious of the thermostat in my work truck because gauge reading never left the lowest mark in the normal operating range. The scan gauge revealed that the engine was happily operating between 205 and 210 degrees.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250