I know that there are many threads on this and believe me I have read them all in hopes I would not have to post...but here it goes.
2003 Z06, 25k miles air intake in the only mod.
This main issue is that the car overheats, 250-260 indicated at idle. The fans both come on but do nothing to stop the car from getting this hot.
I figured it was air in the system and went through that procedure with no luck. Read some more posts and figured in must be the water pump.
Installed a new water pump and stock thermostat today. Followed the refill procedure and the car seemed to be acting better. When both fans came on at 234* the temperature went down to 220 or so. I was happy and figured it was solved. I took it for a short drive and after a few miles it starting getting hot, even at cruising speeds. Pulled into a parking lot and it pegged out again at idle. Finally limped it home and here we are. There are no obstructions in the front of the car and all front air dam pieces are intact. The oil seems nice and clear and the coolant did not appear tainted either. WHAT ELSE IM I MISSING?
One more thing. The top radiator hose always seems to be under pressure while the lower hose that connects to the thermostat housing is not and appears to be flowing water.
Pressure is GOOD! Poor air flow and POOR coolant flow is BAD.... If it were me,,, I would reverse flush the radiator air path and see if that help.
If not, disassemble the cooling system... Remove the radiator, clean everything. Carefully examine the internal coolant passages in the radiator.. They should be SPOTLESS Clean. If there obstructed with CRUD,, that may be your issue..
Disconnected one side of the metal coolant crossover that runs from the corner of the heads in the front of the motor. No coolant came out, but as I filled the surge tank I finally got some seepage. I was advised that air can get trapped in that area of the heads and cause issues. I also cleaned the condenser and radiator the best I could with water and a vacuum. Appears to have solved some or most of the problem. Cruising speed temp w/o AC is 196-198, w/AC 201. Not much stop and go traffic but I did let it idle in the drive and it got to 225 or so, turned on the AC and the temps go down to 210 or so. Not flying the "Mission Accomplished" banner yet but looking better.