I'd invest in a block heater. Its a waste of fuel and engine-life to run it so long for no reason. I haven't used my block heater yet this year....and it has been 5-10F several times. This morning it was about 16F....fired right up no problem. Just crossed 287k today!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I already have the block heater. Like I said, I was merely curious. She starts right up in the single digits without the block heater. I sucks to drive it cold at those temps, though. I swear, it must put out like 30hp for the first few minutes. I've started plugging it in just to shorten that period.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition