Under certain conditions it’s possible for coffee to bubble and sputter out of the portafilter making a bit of a mess.
This happens when the upper chamber has no water in it, preventing the internal cooling system from working correctly. This will happen if the machine is either under-filled with water, or when the machine takes too long to heat (over 6 minutes), allowing the upper chamber to boil dry.
When this happens you’ll notice steam stops emerging from the chimney. The reason this can cause bubbling is that the upper boiler contains water at boiling temperature, which cools the water from the lower boiler as it passes through. If there’s no water in the upper boiler, the water from the boiler isn’t cooled prior to coming into contact with the coffee. Because it’s still hot, some of that water is turned into steam as it passes through the ground coffee, which causes the coffee to bubble and sputter out of the top.
Firstly, make sure you’re putting the correct amount of water in the machine, which is 120g, just up to the fill-line in the boiler.
If you were already doing that, then you’ll need to use a higher heat setting on your cooker, so that the machine takes between 3 and 6 minutes to get up to temperature. This will ensure that there is still water in the upper boiler, which in turn means the water from the lower boiler will be cooled correctly before it reaches the coffee.
Note: If you’re having trouble getting your machine to brew in under 6 minutes and you’re using the heat exchanger plate, check that it’s not bowed or delaminating. If it is, it won’t transfer heat effectively to your machine. In this case, a new plate should resolve the problem; you can buy one here. This article explains how to look after your heat exchanger plate. If you’re using an induction stove, this article may help.