problem

[xzakx9]

i havent got my mk2 on the road yet but everytime i run it for a little while the water boils over......whats the problem with it??
zakx

[vw-bob]

its getting too hot 

sorry couldnt help myself, but i cant be of any assistance

[takethefifth]

Expansion tank cap knackered?

[xzakx9]

might be....how could i tell will it be obvious/???
zak

[Conker]

I think you would be able to see steam coming out of the cap.

Mine overheated for the first time a couple of days ago (albeit on a 150 mile trip) so I've been looking into this issue. The prime suspects are:

1  Expansion cap
2  Low level of coolant in the system
3  Poor ratio of coolant to water - should be circa 50:50
4  Air pockets in the system
5  System thermostat broken
6  Fan switch broken (is your fan coming on?)
7  Fan itself is broken
8  Dreaded headgasket


Perhaps you could tell us more about how it overheats, e.g. all the time, only at idle, is it drinking coolant, whats the oil temp like etc.

[Goops]

As Conker suggested its a process of elimination... follow the list (easiest/cheapest going forward)?

1  Expansion cap
2  Low level of coolant in the system
3  Poor ratio of coolant to water - should be circa 50:50
4  Air pockets in the system
5  System thermostat broken
6  Fan switch broken (is your fan coming on?)
7  Fan itself is broken
8  Dreaded headgasket

I have to say a flush is probably the most essential thing to do once you buy the motor! Worked a treat for me 

[rossko]

waterpump/thermostat would be my first guess... does the fan kick in?

[bobbymk11]

had same problem on my 1991 mk2 8v and tunred out needed new waterpump and thermostat...now runs fine...and also put old black expansion cap on..as new blue one was pi**....

[Agreeable Slick]

change the expansion cap, the seals on the caps are known for deteriorating and letting the water boil over. I think you need one of the blue ones, iirc, do a search of this section and it's mentioned in multiple topics.