From the Haynes manual,
Auxiliary cooling fan
Models with front mounted radiator:
Switches on at 105°C
Switches off at 98°C
If you want to run pure water as the cooling agent in a mpi then good luck but you will have problems, it will boil and leak, new cap or not.
Furthermore we don’t know if the original poster has done their own work on the car but it strikes me that somebody with the insight and capability of changing the expansion cap, thermostat, water pump and head gasket would be beyond any knowledge that might be imparted on this site.
Dylan8660, when entering this forum it explicitly states not to give advice unless you know what you're talking about.
Running pure water, whilst not advisable, will not cause the engine to boil as the fan cuts in at 105c as you have stated.
You are remembering of course that the boiling point of water changes in a pressurized system?
For example, a modern engined car can have a triple stage fan that cuts in on different speeds at 107c, 115c and full speed at 117c. (citroen 2.2HDI engine)
Remember, boiling point occurs when the liquid's vapour pressure equals the surrounding enviromental pressure.
Furthermore, under no circumstance does increasing antifreeze concentration in anyway constitute a diagnostic test as it contributes no information on the problem.
Please attempt to help and not to mislead.
As regards the actual problem, i have nothing further to add beyond the other users posts.
