[tsk1979]

Peak operating temperature of an Automobile is not affected by Ambient temperature.
Now depending upon ambient temperature conditions, peak operating temperature may happen soon, or after some time(Summer/Winter).

Peak operating temperature varies from Engine to Engine. For example for Liva diesel its around 83 degree C. For Tata safari is 96 Degree C. That is why Liva recommended oil is 10W30, and Safari recommended is 10W40.

Now coming to hood vents, cooling of engine is primarily through the radiator. Not much heat will be lost unless you have a hoot scoop designed specifically to direct airflow around some parts. For example, you can have a scoop to direct air flow at the intercooler.

As far as your engine temp goes, not much of a change there unless its a high performance engine which requires special cooling paths to direct air. Mostly it pertains to rear engine mounts, where you have "vents" to direct air flow at radiator.

Last but not the least, I can't say about old school engines, but common rails with ECU control will do limp home if temp exceeds the parameters (eg 98 degree C for safari).
If cooling system is good, engine should never heat, even when climbing inclines in first gear. I have climbed passes at high altitude, and not faced any limp home indicator, except once. This was above 5500mts above sea level, and I was climbing continuously in 4L for quite some time. Air is very thin, so the cooling system could not cope up, and service light went on. Had to wait for a few minutes.

So if you are doing high altitude, if your cooling system is less than optimal, you will see issues. Older engines can exceed recommended temp, and this will cook engine oil. So make sure you use 20W50 or 15W50 in older engine, as this will be more resistant to some overheating as opposed to 10W40.