my 2000 Silverado 4.8l is overheating as soon as it warms up.

I have replaced thermostat and water pump and flushed cooling system.

I've bled the cooling system multiple times.

I've pressure tested the cooling system and it holds pressure.

What could it be? Head gasket or cracked head? How do I test that? Compression test?

Thanks for any help. It's the wife's truck, need I say more?

 

Is it losing coolant? Is there oil in the coolant or coolant in the oil?

 

Are you relying strictly on the dash gauge or have you verified with another instrument?