P/S. I've just realized our offroad excursion was very dusty and lots of bugs. I am going to check the radiator fins and the A/C condenser fins tomorrow.

 

I run a 165k D2 with a new stock thermostat at altitude here in Denver.

took it off-road over the weekend at 8-11,000 feet and never got above 208F (was around 80F ambient at that altitude)

highway driving (uphill from 6,000 to 8,000 feet) with a/c on never got my temp above 204F

observed via ultra gauge

 

Well I take back what I said up thread. I guess I just haven't paid that much attention while driving in hot conditions.

Normally in Seattle, not hot... I don't notice a difference... As has been revealed in this thread most of the instances have been reported while driving under higher than normal load and temp conditions. Hills and heat.

So yesterday I drove I5 south from Seattle to Portland. Lots of hills, 70 to 75 most of the way.

Noticed an obvious 4 to 8 degree difference with the AC off when ambient air was above 95.

Not the ultragauge intake air, but the temp reflected on the HVAC unit.

The hottest I got was 203 on some hills with AC on. Using no AC I could get it down to 188, even 184 while coasting down hill. But generally 4 to 6 degrees cooler with HVAC in "econ" mode. Blowers on.

That was at freeway speeds. Now that I am in the urban concrete area with upper 90's for temps driving city traffic with AC on makes less difference.

The only factor with a major difference is that freeway driving at 70mph and above runs the (4.6) engine at over 2700rpm... something city traffic rarely does at consistency.

It is clear and obvious how higher RPM's and temps react to shutting off the AC. Certainly a notable horsepower hit... not sure if there is a corresponding mileage difference...

At the end of the day, I opted for AIR EFFIN CONDITIONING and being comfortable with the cooling system at mostly under 200 degrees.