RX400 overheated in 101° weather / Low on coolant

[Pnuma]

After a long and HOT drive into the desert, we parked and idled with the AC on. Suddenly I got a dash warning that the car was overheating and immediately turned it off.

Checked. The coolant in the expansion tank was low. Not much, maybe a quart. 

After the car cooled I filled it with 7 Eleven pre-dilluted coolant and CAREFULLY drove home. I noticed on the drive that the car was fluctuating between normal engine temperature to about 20% hotter and then back down to normal. 

The car has never overheated before and it's now been a couple days / it's cooler and the car SEEMS fine. No more fluctuations on engine temperature either.

MY QUESTION: 

Do I need to take the car into the shop? Is it normal for the RX400 to become a little low on coolant? Is it normal for the engine to fluctuate 20% on very hot days?

(Also there doesn't appear to be any leaks where I park my car everyday.)

Thank you!

 

[RX400h]

How many miles are on the odometer?

Many years ago, on our way back from Arizona, there were signs on the highway just before an upgrade, warning drivers to turn off their vehicle's air conditioner to prevent overheating. We had rented a fairly new van at that time and it didn't have any overheating issues. However, we took our 1994 Nissan Quest van to Las Vegas during a hot Summer week and the engine lost power while the transmission was struggling to shift properly. After returning, the van never had a repeat of the drivability problem, probably because it never experienced such hot weather in San Diego.

The reason I brought up these stories is that driving long distances during high ambient temperatures can really be tough on an engine's cooling system. I think your RX will be just fine. Side note: I do occasionally have to top off the coolant tank - not very often, but the gallon of antifreeze is always handy when needed.

[Bykfixer]

The cooling system builds up pressure as temperatures climb, much like a crock pot on low.  

At times the pressure is relieved and when it "burps" sometimes a little coolant escapes. 

In extreme heat the cooling fans may not be enough to keep the coolant from overheating while the A/C is on at idle speed. While moving the air crossing the cooling fins helps the system work better. 

[Pnuma]

"how many miles from the odometer?"

 

I'm up to 178,000 miles now.

Definitely getting up there! I'm amazed the car has had as few problems as it has.

Thank you though guys! You've put my mind at ease.

[Bykfixer]

The Toyota is a well built automobile. The Lexus are a fine example.