Subaru 1999 Legacy Owner's Manual page 339

Maintenance and service
Rubber gaskets
HG0115
6. Put the radiator cap back on and tighten firmly. At this time, make
sure that the rubber gasket in the radiator cap is correctly in place.
7. Start and run the engine for more than five minutes at 2,000 to 3,000
rpm.
8. Stop the engine and wait until the coolant cools down (122 to 140 F
[50 to 60 C]). If there is any loss of coolant, add coolant to the radiator's
filler neck and to the reserve tank's "Full" level.
9. Put the radiator cap and reservoir cap back on and tighten firmly.
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