I did a fairly stupid thing and drove my car with half of the coolant removed from it.
I finished changing the timing belt on my 1998 Toyota Corolla (1.3 4E-FE engine), and I started it a month ago and completed it the other day. The reason being I hurt my knee so couldn't physically get under the car. I did intend to replace the water pump before I hurt my knee, and I drained the coolant from the radiator in preparation for it. I didn't turn the car on or flush the system in order to drain it completely, so I know some coolant was still in the engine and reserve tank. A month later, I finished the job. I was preoccupied with a squeal from the engine, so once I diagnosed that issue I decided to take it for a spin around the block.
The drive consisted of a 30mph stretch of road, followed by a 50mph stretch, and then 30mph. I traveled for no more than 5 or 10 minutes. The moment I saw the needle hit H on the dashboard I turned on the heater and threw it to maximum so it blew hot air in to the cabin away from the engine until I got home. I did notice that the air wasn't exactly hot.
I parked up and turned the engine off. With the engine off I could hear bubbling, like a kettle was boiling. I lifted the bonnet up and left it for a few hours after it. I didn't open the radiator in fear of whatever liquid was there would scald me.
Yesterday I filled the radiator with 50/50 mix of red coolant (which is what should work on my car). Once the radiator was full I turned the engine on, let it get to temperature and the liquid went down and I topped it up until it wouldn't take any more liquid. The coolant I had came in a 5L bottle, and about half of it went in to the car.
Without driving the car which I can't do at the moment until I get it booked in for an MOT as it expired the day before Yesterday, with what I've described, what sort of damage could I have done to my engine, and what can I do as a pre-emptive measure to fix any damage before it damages anything else?
EDIT: Thought it would be worth updating this question after driving the car for some time after this issue.
Since I drove it without coolant, there has been no noticeable problems with the engine. There is no water in the oil and no mayonaise gunk under the oil cap. I have dodged a bullet, and the car is going from strength to strength!