I just bought a new car and out of curiosity, during one of my trips, logged the data using OBD scanner. I noted the max engine coolant temperature was around 100 deg C. What is the redline here? What temperature should I be alarmed at?

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Far enough below the coolant's boiling point that you don't need to worry about what happens when you shut the engine down (and you loose coolant flow, but still have a lot of residual heat).

The actual value will depend on the coolant that you're running, the concentration, and the setting of the pressure cap. But assuming a fairly standard coolant, diluted 50/50 with water, and a 15 PSIG cap you're looking at around 265° F or 130° C before things start to boil. The data sheet for the coolant should have a curve for different concentrations and pressures (although 50/50 and 15 PSIG is very standard).

Normal operating temperatures for a VW diesels are in the range of 170° F to 210° F with the thermostat set to open around 190° F. The actual gauge is controlled by the ECU and is filtered to read 190° F anywhere in that range (to avoid causing concern at the temperature varies in normal operation). The recommended coolant (G13), when diluted 50/50 has a boiling point of 228° F (109° C) at atmospheric pressure and 248° F (130° C) at 15 PSIG (roughly 2 bar or twice atmospheric pressure).

So it seems like what VW is saying is that with these engines somewhere around 40°F (22° C) below the boiling point is a reasonable margin.

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