Ford 1997 Crown Victoria Manual page 159

A computer system controls the engine's idle
speed. When you start your vehicle, the engine's
idle speed normally runs higher than when it's
warmed up. These faster engine speeds will
make your vehicle move slightly faster than its
normal idle speed. It should, however, slow
down after a short time. If it does not, have the
idle speed checked.
If the engine idle speed does not slow down
automatically, do not allow your vehicle to idle
for more than 10 minutes. Have the vehicle
checked.
Extended idling at high engine speeds can
produce very high temperatures in the
engine and exhaust system, creating the
risk of fire or other damage.
Do not park, idle, or drive your vehicle in
dry grass or other dry ground cover. The
emission system heats up the engine
compartment and exhaust system, which
can start a fire.
If you consistently start your vehicle in subzero
temperatures, use an engine block heater (if
your vehicle has this option).
Engine Block Heater (If equipped)
Engine block heaters are strongly recommended
if you live in a region where temperatures reach
-10˚F (-23˚C) or below. An engine block heater
warms the engine coolant, which improves
starting, warms up the engine faster, and allows
the heater-defrost system to respond quickly.
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