Just completed a 750 mile run in the 280. Car ran good and the cooling system worked good, except...

Running through the desert at 80+ MPH A/C on, the temp would climb a little, nothing to worry about maybe 200F. What bothered me was when climbing a grade, such as the Baker Grade on I-15N, the temp almost got into the red (happened on all grades, not just the Baker Grade). Slowing to 65-70 MPH allowed the temp to drop to normal. Also, on the down grades temp would drop back down to ~180F.

Now I know the system will heat up when working, such as climbing grades in the desert heat, but it seemed to get a bit too hot. Could 50/50 mix (with a bottle of Water Wetter) be a little too thick for desert/semi-desert operation? I'm thinking of siphoning off about 3/4 gallon of coolant and topping off with water. What do you think?

On the plus side, I pulled a steady 15.6 MPG on the highway and 13.8 in the city while in Las Vegas.

Water Wetter
 

I've tried the WW with about 70%/30% mixture of antifreeze/water with good results.

With A/C on and hot day (90F), temp stays below 100 C. Also have the "Aux Fan mod" which does turn the fans on earlier so that mitigates this response. The fans do help a lot.

But if you are going to do a lot of desert driving, looks like it couldn't hurt. Based on the WW site info, you should be able to remove about 30-45 deg F of heat with 70/30 mixture. Even better if the antifreeze is 'old'.

I bought some water wetter, but still have not put it in yet. Do you think I should do as Mike suggested and remove some of the mix and then add distilled water, before putting the water wetter in?
My area never gets colder thatn 40, and even that is very rare.
If so, can I just siphon the coolant out? Even it it does take some time (turkey baster)? I won't do it if I have to get under the car and unscrew the drain plug! Thanks guys.

That's what I did. Siphon some of the coolant off via the overflow bottle (OB) using a small electric drill driven pump, to clean out the contents of the OB.

Also remove the overflow line from the OB and use that to pour in the WW. In this manner, the WW is added into the engine block directly. Of course, the fluid is displaced and you see it rise up in the OB.

If you know you have 50/50, then this should be ok since there's about one gallon in my OB. To get more out, you could open the radiator drain plug to collect say 1-1/2 gallons of coolant. Then add the WW and water via the overflow line.

For my radiator, I used 1-1/2 bottles of WW.

Quick update
 

After doing my 3K oil change today, I siphoned off everything from the upper tank, a tad more than 1/2 gallon. Topped off with bottled water and a refresher of about 3 ounces of water wetter.

While the engine idled, circulating the new oil, I placed a thermometer in the upper tank and compared the thermometer to the temp gauge for accuracy. When the temp gauge showed a little over 175F the T-stat opened, and the thermometer rose. Finally stabilized ~190F ( with the gauge showing about 1/16 of an inch below the hash mark between 175 and max).

I'm going to watch it close for a few days, see what happens with the mix watered down a little. I'll get a highway run in, with a few grades, and see how it looks.

I did notice that the coolant I removed (with the water wetter in it) was very, very slick, most likely the slipperest water I've ever felt. I guess that's the extra lubricity of WW.