Cooling System; Checking Coolant - Honda 1990 Civic Sedan Owner's Manual

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Cooling System

Your Honda engine contains a number of aluminum parts. Therefore,
it requires an antifreeze/coolant specifically formulated to protect
the aluminum parts from corrosion. Failure to use a suitable
antifreeze/coolant may seriously shorten the life of the engine as
the result of rapid corrosion damage. Some antifreeze/coolants,
although labeled for use in engines containing aluminum, may not
provide adequate protection for your engine.
Therefore, use only a Honda RECOMMENDED antifreeze/coolant.
CHECK WITH YOUR AUTHORIZED HONDA DEALER.
For best corrosion protection, the mixture of coolant and water
must be maintained year-round at 50/50. Concentrations less than
50% coolant may not provide sufficient protection against
corrosion and freezing. Concentrations of greater than 60% coolant
will impair cooling efficiency and are not recommended. Low-
mineral drinking water or distilled water should be mixed with the
antifreeze/coolant. Coolant loss should be replenished by a mixture
containing the proper concentration of antifreeze and water.
Do not mix different antifreeze/coolants.
Do not use additional rust inhibitors or anti-rust products, as they
may not be compatible with the radiator coolant.
ENGINE DAMAGE CAUSED BY IMPROPER COOLANT USAGE IS
NOT COVERED BY THE NEW CAR WARRANTY.

Checking Coolant

Check the coolant level in the reserve tank when the engine is at
normal operating temperature.
Do not remove the radiator cap when the engine is hot; the
coolant is under pressure and could severely scald you.