Hey guys.. been reading on here for a while and finally decided to make an account. I have a 2010 e550 coupe that has been really great to me aside from the usual annoyances. But I am worried now because the engine bay gets excessively hot as soon as I turn the car on and the radiator fan runs at full speed and sounds like a turbine if I start the car with the ac running. If I start the car with the ac off it does not do this. I have no codes so I am not really sure where to check. The ac still blows cold air so am not sure if that could be the issue. The car temp on the dash and when I hook up my scanner is normal 90 ish and doens't move ever. Any help would be appreciated as it sounds like a plane is going to take off whenever i'm at idle.

I had a very similar situation. It turned out to be the temperature sensor within the thermostat (the whole thermostat has to be changed). As it was explained to me, if any sensor related to the engine's temperature/control malfunctions the computer failsafes to full engine cooling (high fan speed etc....) to attempt to prevent damage to the engine from over heating.

Be certain to check for correct level of coolant in the expansion tank.
Low coolant and or coolant issues are often the problem

If my thermostat was bad wouldn’t I see the car overheating on the dash? It always shows a normal running temp on the needle in the car. Also this happens when the ac is on when I start the car if I start the car without the ac on it does not do this.

My coolant is good. When it is low I usually will get a warning on dash of low coolant level.

You did say "engine bay gets excessively hot" and that's concerning.. Lot's of possibilities with a 2010 car including Clogged Radiator If your 2010 Mercedes E550 Coupe has more than 50,000 miles on it.
Best of luck.

It does but I am not sure the cause as it doesnt seem to be the engine temp itself. The needle always seems to be at normal operating temp. Could it still overheat and give me these readings? I have no errors at all either.

I'd use an infrared thermometer or get a Temperature Indicator Stick to check the intake manifold temp.
And yes, the water temp could appear somewhat normal if the engine walls are scaled up preventing heat transfer to the coolant.
Obviously, these are WAG's based on keyboard banging and not evaluating the situation.
Again, I'm basing everything on your statement of "engine bay gets excessively hot" but you have a starting point.