Car overheating but not running hot?

Wow that is a lot of work done. Was this done by a shop or by you/friends/shade tree mechanic?

As for the temperature going lower after turning the car off, that is normal as you no longer have the engine generating heat.

If you are saying the vehicle is running in park, then I am guessing that is because less heat is being generated, but not sure on that point.

Was there any diagnosis done prior to the work? I mean sure replacing wear items on an older vehicle, like hoses and thermostat, are good starting points, and most likely need to be done anyway.

As for the thermostat. The opening temperature of the thermostat is irrelevant, as that only matters during warm up. Once the vehicle is at operating temperature 100% open is 100% open. Example in AZ you want a low temperature thermostat, since is hot outside, might as well open it up early, in a cold area like MN you want a higher temp thermostat so the vehicle will actually reach operating temperature when it is -10 F and the heating system will heat up more quickly.

You need some other way to test if is running hot. Like temperature gun on various hoses, then try to find the same vehicle, same engine etc, and test it in the same manner.

Have you burped the system? Are you using DexCool or spec fluid? Do you have too much coolant in there? What state are in? If a warm area you might want to go from a 50/50 mix to 60% water 40% coolant.

Does it have clean oil and the right weight, guessing 5w-30. I would use a synthetic blend like Valvoline in the red container.

How long have you owned this vehicle? Maybe oil is not flowing properly?

Put in a can of Motor Medic, follow the instructions. Then do an oil change and filter with the most basic oil.

After driving 1000 miles, put in another bottle of Motor Medic, follow instructions.

Then put in synthetic blend and new filter. This short cycle oil change should clean out any sludge.

Also turn the heat to high and cabin fan to medium and put food thermometer in the vent and record the temps as the vehicle warms up at idle, then when fully warm, slowly press the gas and see if the temp goes up. Just to set a baseline.

Remove the thermostat, then retest the temps on hoses and in the cabin. It should be lower due to more water flow.

Personally I would not use any aftermarket thermostat. Then put in an GM thermostat and see what happens.

Check the transmission fluid. Maybe that is overheating and contributing the overheating, if the transmission fluid goes through the radiator or right by it.

That is about all I have. If this does not work, find a GM specialist mechanic to diagnose it.

Multiple things are happening.

Driving slowly or not moving, less heat from engine operation, less air through the radiator, less coolant flow through the cooling system.

Driving fast, engine making more heat, more air through radiator, more coolant flow through the cooling system.

Maybe the thermostat is stuck partially open. It would allow adequate coolant flow through the radiator at idle or slow speed, but not enough flow for high speed.

Maybe the radiator has become thermally inefficient, so its ability to shed lots of heat is limited.

A 180° thermostat is not the proper temperature.

If everything is new, it's puzzling. Make sure the ignition timing is correct. If it's too retarded, the engine will run hotter.

Did the car always do this? How long have you had it?

The radiator cap is a 18 lb. cap, which means that the engine can see a lot higher temperatures then what you are experiencing and still be operating correctly. That means the coolant will not actually boil to what ever the temperature is for boiling water at 2 bars.

That said- a lean burn engine runs hotter! A lean burn out of range will produce an error code.

Hi no but the radiator cap needs replacing as the spring in the cap has become weak over time.

Your temp gauge may be out. If the car is running Ok and the fans are not on all the time, accept this is it's normal operating temperature.

It appears either the wrong thermostat was installed as the replacement, the water pump was installed backward or the head gasket wasn't installed using a torque wrench so it would be sealed equally at all bolts. Sell this car as soon as the problem is fixed aa this problem happens to almost all Chevy Impalas no matter what year they were produced. Why are you driving a car that has had problems since the first year it was introduced? Buy a new or used Toyota Prius with low mileage and you won't have problems with this or any problems if you keep servicing the car and drive close to the speed limit so you'll get at least 35 miles per gallon and if driving on a highway, you'll get even higher than 35 MPG. All Chevy Impalas are status symbols, and many have become low-riders. It's time to get rid of the car and get a car that has very few problems, and a Prius is one with very few problems.

Get an OBDII reader that reads sensor outputs to see what the actual temp is. I know a guy with a Ford diesel that has a gauge that doesn't read correctly, my code reader showed the temp was OK.

...too many possibilities to list....pressure test the cooling system, if its coolant isn't low. Check for oil in the radiator.

So how do you know the gage is correct in its reading. If it runs there all the time I'd not worry about the temperature at all as gages are not always correct. Sounds like you spent a lot of money for nothing at all.