Chrysler Repair: overheating (gauge goes to 3/4 scale when idling), coolant temperature sensor, water pump impeller

Question
I have a 2000 300M 3.5L. I had the radiator replaced, thermostat, the fan unit replaced, & the head gasket replaced. The temperature still goes up 3/4 almost to the red when idling. (I had a mechanic do it) What else can it be? I have taken it back to the mechanic twice already. Any suggestions? Thanks

Answer
Hi Missi,
It would be good to know whether the coolant temperature sensor is accurately reflecting the temperature of the coolant or not. It may be off-value. Have you verified the approximate temp of the system with a thermometer or infra-red temp sensor?
Have you experienced any loss of coolant from the overflow bottle or escape of steam when the gauge is that high?  Does the fan come on at that point, or earlier?
Have you been loosing coolant or having to add it?
Do you hear any bubbling sounds from the engine when you are warming it up?  Does the coolant level in the bottle go down while/after the engine has cooled down?
Answers to these questions would be helpful to me. It is possible that you simply have air trapped in the system which is fooling the thermostat and so it opens late. If there is a bleed nipple at the base of the pipe used to refill the system, then that could be opened (after attaching a hose to route any coolant away from the drive belts) and then the top radiator hose squeezed to force out any trapped air in the system.
The water pump impeller could be loose on its shaft.
Was the cooling system re-checked for the presence of exhaust gas in the system (which if present would indicate that there is still a head gasket leak)?
Let me know what you can about these questions and we can discuss it some more.
Roland