3-second answer: After any coolant job: fill slowly, then run the engine with the radiator cap OFF and the heater on HIGH until the thermostat opens and the level stops dropping — then top off and cap it. On OBS trucks, the classic hidden air pocket is in the heater core and the intake crossover. Miss it and you'll chase a 'thermostat problem' that isn't one.
The procedure
- Cold engine. Drain the radiator (petcock) — and the block drains if you're doing a full flush (5.0/5.8 have block drains).
- Flush with water until it runs clear (a flush T or just repeated fills/drains).
- Fill with 50/50 green coolant slowly — pour, wait, pour. The system holds ~16-19 qt on V8s. PEAK conventional green concentrate mixed 50/50, or any green silicate 50/50 jug; skip orange Dex-Cool. See fluids.
- Leave the cap off. Start the engine, heater on HIGH (both blend doors open), and let it warm to the thermostat opening (watch the upper hose get hot).
- Keep topping off as air burps out — the level will drop several times.
- When the level stabilizes and the gauge holds normal, cap it, top the reservoir, done.
- If you still get a temp spike: park nose-up on an incline and re-bleed — that gets the last pocket out of the crossover.
When it's NOT air
Persistent overheating after a proper bleed = thermostat stuck (test in boiling water before replacing good parts), water pump impeller worn (the 5.0/5.8 composite impellers shed blades), or a blocked radiator. See cooling cluster.
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