GMC SIERRA CK2 1999 Owner's Manual page 402

Long Trip/Highway Scheduled Maintenance -- Gasoline Engines
100,000 Miles (166 000 km)
j
Inspect spark plug wires. An Emission Control Service.
j
Replace spark plugs. An Emission Control Service.
j
Change automatic transmission fluid and filter if the vehicle's GVWR is over
8600 lbs. or if the vehicle is mainly driven under one or more of these conditions:
– In heavy city traffic where the outside temperature regularly reaches
90 _ F (32 _ C) or higher.
– In hilly or mountainous terrain.
– When doing frequent trailer towing.
– Uses such as found in taxi, police or delivery service.
Manual transmission fluid doesn't require change.
j If you haven't used your vehicle under severe service conditions listed previously
and, therefore, haven't changed your automatic transmission fluid, change both
the fluid and filter. Manual transmission fluid doesn't require change.
j Inspect Positive Crankcase Ventilation (PCV) valve.
An Emission Control Service.
150,000 Miles (240 000 km)
j 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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