While checking the throttle body sync on my Mk 2 I noticed that when I blipped the throttle, in closing suddenly it caused a back surge of coolant which sprayed up to the top of the reservoir, some of which went into the overflow pipe. Trickle trickle, she pea'd green.
The Mk 1 used to do it too, and others on the forum have noticed the same thing. Now I know why

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Rob.