Octavia slow water loss.

Put a match to it.

Dr. Bizarro

Look for the unapparent ones, I had a similar problem, it turned out to be an abraded pipe at the back of the engine that had the tiniest pin prick leak, but when the system was stinking hot and under pressurise leaked at the rate of a few drips per second.

Tom

Try to beg/borrow/hire a pressure tester. (If it helps, it's standard issue to AA patrols.) It really does show up the tiniest leak, and you can do it when the system is cold so you can look in every nook and cranny without getting burned!

Chris

Won't help - it's covered in water.

Could even be the expansion tank cap seal. As for natural wastage..it's certainly far more than my Capri uses and that has no expansion tank.

Unexplained coolant loss. It's old hat now but is still worth reading:

I would also be looking at a faulty cap on the reservoir tank, they do go home with age.

PDH

Water pump knackered. It will be changed when they do the cambelt, but make sure any rollers/tensioners are also changed.

How do you come to this conclusion? Is it a known fault with this engine? I'll be doing it myself, and was in 2 minds whether to change the water pump at the same time, I probably will now. Yes, I already have the tensioner. Ta Alan.

Plastic Impellers falling off is far from unknown, leaks also, but of course it could be leaking from somewhere else, get a pressure tester on it, cold first. Blow it up for 20-25psi (healthy system will cope with this fine), and if that doesnt show anything again with the engine near operating temp.

The w'pump, tensioner and rollers MUST be changed as a set on this engine as they are fairly notorious for throwing belts, and there's alot of valves to replace if it does! I would also suggest you reduce the interval to no more than 50k also.

Tim..