So this morning I did something stupid. I had checked the coolant in my car and it was fractionally below the min point, but still fluid in the tank. I poured in some water so it filled to between the min and max and went to town with the intention of buying coolant. I heard something fall in the ground as I got into town. And when I stopped, realised I had not put the coolant cap back on. The car part shop was closed. Not knowing what to do, I got some tin foil and a rubber band and put it as a makeshift top. The coolant/water was still at the same level (that I can judge by eye). I saw no indication of splashing out.
In kind of lack of options, I drove home. My car is now parked in my garage and I have ordered a new cap and won't drive it until after I have put it on. When I got home, the coolant/water still seems to be at the original level where I filled it to and with the tin foil over it on the way home I know it didn't overflow.
My question is, how likely is it I have done major damage to my car? It's a Hyundai i30. Diesel. I had watched the temp guage in and out from town and it stayed just under 90 all the time. I was staying 2000rpm for the most part.