Chevrolet 1999 Monte Carlo Owner's Manual page 301

Scheduled Maintenance
150,000 Miles (240 000 km)
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Drain, flush and refill cooling system (or every 60 months since last service,
whichever occurs first). See "Engine Coolant" in the Index for what to use.
Inspect hoses. Clean radiator, condenser, pressure cap and neck. Pressure test
the cooling system and pressure cap.
An Emission Control Service.
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DATE
ACTUAL
SERVICED BY:
MILEAGE
7-13