Doug Winterburn wrote in
eb.com:

I had an overhearing problem with my 2004.5 Chev Duramax while towing
a 10,000 lb trailer up a long grade in the AZ summer. The problem was
exactly what you mention - the heat from the radiator blown by the fan
when it engaged right into the the air intake - a wicked heat loop!
The solution was to upgrade the air intake to the cold air intake from
the 2006 model as well as a much larger turbo intake to match the
larger size of the air intake plumbing. The next step would be a
vented hood, but the cold air intake solved the problem.

These trucks can generate some heat - the EGT can run about 1300F when
pulling a grade.

What's happening there is that hot air is less dense than cold
air, so with the hot radiator air going into the engine, there
was less mass of air for combustion and thus less power out.

Since you were trying to go up a hill you started pressing the
accellerator pedal harder, putting more fuel to the engine to
make more power to compensate. Too much fuel means the exhaust
gas temps go up. Odds are you were also making smoke at that
point.

1300 sounds very hot to me, but perhaps the little diesels
are different (if nothing else, the pyrometer is probably
closer to the exhaust port in the head).

John