FORD 1999 F-550 v.1 - Manual (Page 113)

Using the engine block heater (if equipped)
An engine block heater warms the
engine coolant, which improves
starting, warms up the engine faster
and allows the heater-defroster
system to respond quickly. Use of an
engine block heater is strongly
recommended if you live in a region
where temperatures reach -23°C
(-10°F) or below.
For best results, plug the heater in
at least three hours before starting
the vehicle. Using the heater for
longer than three hours will not harm the engine, so the heater can be
plugged in the night before starting the vehicle.
To prevent electrical shock, do not use your heater with
ungrounded electrical systems or two-pronged (cheater)
adapters.
Starting
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