A week ago my engine temperature sometimes (not consistently) started creeping all the way up to the red whenever I was at a low RPM. There were no obvious signs my water pump was bad, and based on some reading I did and the gurgling coming from my heater core, I concluded it was probably just air in the cooling system. My overflow tank had run dry, and I've hardly added any coolant since I got the truck five years ago, so I figured it was neglect. So yesterday I put the front end on some ramps, slid my heat control all the way to the right (the hottest), and drained the system. When it stopped, I pulled the upper radiator hose and stuck a garden hose in the thermostat housing just to see if it flowed while cold. It did. So I let it flow until the fluid coming from the petcock ran clear, then shut off the water and pulled the thermostat. It wasn't corroded, but what appeared to be a rubber seat on the thermostat mechanism had worn away, and it was no longer seating (hence the cold water flow), so I put in a new one. So system flushed and thermostat replaced. With everything back together, I filled the radiator, started the engine, and used this DIY method to release the residual air bubbles while the engine warmed up. I raised the RPMs just a bit to speed things up. Within 10-15 minutes I had a hot upper radiator hose and hot air coming out of my vents, and after a while no more air bubbles. I thought I was golden, so I let the RPMs drop to normal idle... and the temp needle immediately started climbing all the way up. What gives? I followed all the steps to purge the air, and the hot radiator hose and vent air indicate the water pump is doing its job, right? Question: should I be able to *see* the coolant rushing by the radiator opening with the cap off once the thermostat opens? I seem to remember that being the case with my SlantSix engine (though it's been a while), but the coolant looks pretty still in this case. Observations? Suggestions?