Why does my espresso shot run too fast?

The normal case is simple. If 36 g comes out of an 18 g dose in 12 to 18 seconds, the grind is too coarse for that coffee. Go finer in small steps, three or four at a time on a stepless grinder, and re-pull. Fresh coffee needs finer settings than the same coffee will need in two weeks, and every new bag needs its own dial-in, so this is normal maintenance rather than a fault.

It becomes a real problem when finer grinding stops changing the time. That happens for a few specific reasons. The most common is underdosing: too little coffee in too large a basket leaves headspace, the puck floats and cracks when water hits it, and the shot blows straight through. Check the used puck. If it is wet, loose and cratered rather than a firm disc, add coffee or use a smaller basket.

The second reason is channeling. A shot with one big channel will run fast no matter how fine you grind, because the water is bypassing the bed rather than pushing through it. Grinding finer actually makes this worse: it raises pressure on the intact part of the puck and encourages the water to exploit the weak spot harder. Fix prep first, then revisit grind.

The third reason is hardware. Some grinders simply run out of adjustment at the fine end, particularly grinders sold as multi-purpose. If you are at zero or the burrs are chirping and the shot is still fast, that grinder cannot do espresso with that coffee. Similarly, worn or badly aligned burrs produce a wide spread of particle sizes, so the fines clog and the boulders leak, and no setting behaves predictably.

One thing that is not usually the answer: pressure. Home machines mostly deliver 9 bar or more at the pump, and a fast shot is a resistance problem, not a pump problem. Raising pressure to compensate for a coarse grind gives you a fast, thin, harsh shot rather than a good one.