Some caps don’t have a thread but a locking-type system so pressing and turning once is enough. Once you’ve finished, you can start the car and let the engine run for a couple of minutes so the coolant can circulate. After that, check the coolant level again, if it’s OK then the job is finished.
How long does engine coolant take to work?
Typically though it should be left to cool for 1-2 hours before it's cooled sufficiently to open the rad cap safely. If you're actually trying to check the coolant level then you probably need to wait more like 3-5 hours (as if the coolant is still warm it will show a falsely high reading due to expansion).
Should you add coolant while engine is running?
Make sure your engine is off and cool, the vehicle is in Park or Neutral, and the parking brake is set. If your engine is cold, the coolant level should be up to the cold fill line. Loosen the reservoir cap just a little, then step back while the pressure releases. Then, remove the cap completely.
What happens if you add coolant to a hot engine?
Adding cold coolant/antifreeze to a hot engine can cause cracks due to the sudden change in temperature, so even if you're in a rush, you should still take the time to wait for the engine to cool – or face a potentially huge repair bill.