generally i used mine if the temp overnight was going below +15
deg F. if it's not going to below 0 def F then if you have time you can plug in a couple of hours before startup. there is a remarkable difference in the warmup time when using a block heater.
you will notice it immediately.
let's not hope for winter yet though..:p

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