Ford 2001 Mercury Cougar Owner's Manual page 220

Maintenance and care
Whenever coolant has been added,
the coolant level in the coolant
reservoir should be checked the
next few times you drive the
vehicle. If necessary, add enough
50/50 concentration of engine
coolant and distilled water to bring
the liquid level to the proper level.
If you have to add more than
1.0 liter (1.0 quart) of engine
coolant per month, have your
dealer check the engine cooling
system. Your cooling system may
have a leak. Operating an engine
with a low level of coolant can
result in engine overheating and
possible engine damage.
Changing engine coolant
Change your engine coolant
according to the appropriate
schedule listed in the Scheduled
Maintenance Guide". It is important
that the engine coolant be changed
at the specified intervals. The
corrosion protection of engine
coolant is depleted with time and
usage. Use of engine coolant with
depleted corrosion protection may
result in damage to the coolant
system. Color, specific gravity and
freeze point of the coolant are not
indicators of depletion.
To find out how much fluid your
vehicle's cooling system can hold,
refer to the Refill capacities in the
Capacities and Specifications
chapter. Fill your coolant reservoir
following the directions given in the
Adding engine coolant section.