Are you sure the thermostat is working properly? Sounds like you live in a really cold climate. I checked the Hayes Manual and the Chilton Manual and it does not give any info on the block heater..sorry!

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2000 Ford 7.3 Powerstroke 4x4
2006 Mazda Tribute
1983 Black 300 D (donated to charity)
1993 Teal 300 D (160K) Sold
"I love the smell of burnt diesel fuel in the morning, it smells like ....VICTORY"

Semper Fidelis
USMC 1973-1976