when stopped and idling does opening the hood help the engine run cooler?
No. Get the cooling fan system checked. Since you did not post make, model, year, engine, I can only guess that the primary electric cooling fan is not kicking in like it should. This means, the radiator cannot cool the engine coolant while idling because of no air flow across it. Opening the hood will do nothing.
Sure about 1? C. If you are having problems with your car over heating when stopped, you car has a problem and that problems needs to be addressed. Like Busted I would start by making sure the fan(s) are running as designed.
Not enough to be worth the trouble.
thermostat stuck? It won’t open just by opening the hood; but perhaps if you opened the hood to replace it with one the that works? (Don’t do it hot or while the engine is running>
fan not running? Got to put air thruough that radiator somehow. Perhaps if you had the hood up and ran a big fan in front of your car? Or perhaps while you have the hood up, you can see if the fan is running, perhaps check the fan relay, and many be even check the temperture sensor?
No antifreese in the radiator? Don’t open when hot. but you can’t get any in if you don’t open the hood.
So, it might help to open the hood, but you probably need to do more than that.
If you have a separate fan for the air conditioner, you might try turning the air-conditioner on, as that may draw enough air through the radiator to resolve the heating while idling problem.