I have a 2000 Caravan 3.8L V6 with 117k miles. Always well cared for. All maintenance done on schedule, and recently had cooling system flushed.

Since the car was new, I make the same drive from San Diego to Las Vegas every year in the heat of the summer with A/C on and never a problem.

For the last 2 drives (last summer and now), my engine has overheated going up the 2 steep grades on the way. (There is a sign posted to turn off A/C on side of road.)

When I turn off A/C and turn on heater, temp drops to normal.

Never have any kind of overheating under other circumstances.

I've called several dealers service depts and received different answers. One that seemed the most confident said this is normal as engines age.

My question is this, is it normal for a high mileage engine to just start overheating (on hills in >100 degree weather) after 8 years of no overheating whatsoever under identical circumstances?

What would cause this, even if it is normal?