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.

  • coolant tank cap or Radiator Cap?
    – Moab
    May 30, 2020 at 12:10
  • Welcome to Motor Vehicle Maintenance & Repair! May 30, 2020 at 13:02

1 Answer 1


If you didn't overheat the car, then you've done no damage. The radiator cap seals the system and allows pressure to build (to a point), which raises the boiling point of the coolant. This allows it to take more heat. If you didn't overheat your vehicle, there's nothing to worry about. The only thing you'd need to do is ensure you haven't lost any coolant in the process. If you did, you'll need to refill it.


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