La Marzocco GS3 AV - Coffee Pressure Question

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#1: Post by roughneck »

Hi folks,

I have a GS3 AV.

I haven't paid attention to it for a long time, but I just noticed that the pressure continues to rise above 9 bar (about 12 bar to be exact) during extraction (with coffee).

When empty, however, at free flow the pressure remains at 9 bar.

If I adjust the pressure while extracting (with coffee) to 9 bar, I have 7 bar when empty (free flow).
The amount of water when set at 9bar during free flow is around 500ml/min.

Can someone help me here?
Is that normal? Or how do I set it up correctly? Internet and operating instructions contradict each other here.

Thanks and Greetings

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#2: Post by Peppersass »

Sounds like your expansion valve isn't adjusted properly, or has gotten out of adjustment, or is clogged with scale.

When the pump bypass valve and expansion valve are adjusted correctly, the free-flow (no puck) pressure should be 9 BAR, the max pressure when re-heating after running the group should be 12 BAR and the pressure when pulling a shot should be about 10-10.5 BAR (there's about a 1-1.5 BAR pressure drop at the puck caused by the flow restrictor -- gicleur -- in the group head, so pressure there should be about 9 BAR during a pull.) Backflush pressure with a blind basket should be roughly 10.5-11 BAR.

First step is to adjust the expansion valve. The normal procedure is to run the group for about 15 seconds, then watch the pressure as the boiler reheats. You turn the part of the expansion valve that sticks down into the drain box so that the pressure reaches 12 BAR -- and no higher -- during the reheat cycle (see CAUTION note below.) Then check the free-flow pressure, which should be 9 BAR. If it isn't, the adjust the pump bypass valve so the free flow pressure is 9 BAR, then repeat the expansion valve adjustment. You might have to go back and forth between the two adjustments a couple of times.

CAUTION: Given that you haven't done the adjustment in a long time -- or ever -- the expansion valve threads may be jammed by scale. If you attempt to turn the expansion valve when the threads are jammed, you'll bend and twist the tube that connects the expansion valve to the coffee boiler. Most likely the tube will have to be replaced. To avoid this, you'll need two wrenches or a wrench and pair of pliers. Open the right side cover. You'll see the expansion valve screws into a male-male fitting that you can grab with a wrench to keep the tube from turning while you turn the lower part of the expansion valve with another wrench or pair of pliers. Which one depends on which version of the expansion valve you have. Newer versions have a couple of bevels at the bottom that allow you turn the tube with a wrench. Older versions have a smooth tube that requires a wrench. Best to cover the wrench with electrical tape and squeeze it very gently when turning. Otherwise, the tube will deform or get crushed.

If you can't turn the expansion valve, try wrapping the thread area with a rag soaked in distilled vinegar to dissolve the scale. If worse comes to worse, you may have to replace the expansion valve. The machine should be off and cold when you remove the valve, and use two wrenches to remove it so you don't damage the tube.

If you can turn the expansion valve, but the pressure still rises to 12 BAR during a shot, it's likely that the expansion valve is clogged with scale. You can try removing it and soaking it in distilled vinegar to remove the scale. If that doesn't do the trick, replace the valve.