the way it's been out here i think that you need at least 3 solid hours plugged into the block heater to make a difference. i plugged in this a.m. (about +2 deg F ) for about 1.5 hours after sitting all night unplugged and it started 1st crack but took twice as many revolutions of the starter to fire up.
also the warmup time was greatly increased. took about 40 minutes on the highway (at 70-80 mph) to get the temp needle barely past 80.

the other day at a lower temperature i plugged for 4 hours and it was much easier for the initial start and quicker warmup.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP