My VW MK1 boils over coolant

I have a VW Golf Chico 1.8 1997 (South African) which i bought from a friend last year that keeps spilling the coolant off the resevoir. If i drive the car at low speeds (heavy traffic or rural roads) it heats up and spills the coolant over and when i open the bonnet(and feel the upper hose) i can hear the sound of it boiling.
I tried to change the thermostat but to my suprise there was no one installed(this suggests that there have been a problem with the cooling system and they decided to remove the thermostat as a short term solution). I fitted the thermostat and the temparature rised almost to the top and the radiator fan kept kicking in. I had no choice at the time but to remove it.
I am suspecting the water pump because when i flashed the system, water was running out fine from the bottom hose of the radiator.
Maybe the water pump is pumping less coolant than its suppose to, which in turn lets the same coolant in the engine for a longer time.
There is no coolant leaking around the water pump but i am highly considering replacing it, is this a good move?



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Do you know anybody with a mk1 that's working fine ? Could be worth swapping expansion bottle caps , or just buy a new cap , also with the cap off when you rev the engine you should see a good flow of water, also could be an air lock if it hasn't been refilled correctly.

I had a similar problem when i first bought mine but my fan wasn't coming in at all.

After cheching things like the thermo switch, thermostat etc. it turned out to be a hair line crack in the expansion bottle which was stopping the pressure from building up in the cooling system and making it boil over like you described.

I changed the expansion bottle and never had a problem again  

Ive also had a hairline crack in the bottle.