All MB engines (and most car engines in general) are designed to heat up to operating temp as quickly as possible. So the car that heats up faster is operating properly....if it takes a long time to heat up or if it never seems to quite get there, then your thermostat is probably in need of replacement.

My 3 liter M103 takes about 3-4 minutes of 35+ mph driving to get to 80C....maybe 5-6 minutes if its REALLY cold. At 30F it only takes it about 2-3 mins tops to get to temp on the highway.

The diesel's take much longer. My 617 with a working T-stat will take several miles or close to 10 minutes to reach full operating temp if its cold out and I'm not driving very fast. On the highway it takes about 4-5 minutes.

If its real cold (like 0-10F) my diesel will sometimes not make it all the way to operating temp....because the heater core is sucking away too much heat.

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