GMC SIERRA CK2 1999 Owner's Manual page 391

Short Trip/City Scheduled Maintenance -- Gasoline Engines
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 cooling system and pressure cap.
An Emission Control Service.
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DATE
ACTUAL
SERVICED BY:
MILEAGE
7-29