I have a Bezzera BZ10 HX machine, with the OPV factory set at 12 bar. When I use a blind filter, the bar pressure steadily climbs to 12 bars and holds. However, when I attempt to extract coffee, the brew pressure normally reaches about 9 bars, and then drops between 4-6 during extraction. The result is under extracted coffee. I've adjusted both the grind and dose, but to no effect.

This happened about a week after purchase. I then returned the machine for inspection, and the vendor replaced the vibe pump and adjusted the OPV to 9. That worked for about a week, but then the problem returned. I just exchanged the machine for a direct replacement yesterday afternoon, only to discover the same problem this morning.

This is now my third pump, so the problem must be related to something else (I just don't know what!). Curiously, when I flush the machine, there's no steam/hissing like I would expect from an HX machine, but I have no idea if this is related in any way. I've read varying reports about this brew pressure issue online, but no definitive solution. In nearly every case, the person replaces the pump, which solves the problem for only a week or two. What is amiss here?

This thread Low Brewing Pressure Bezzera BZ07 most closely describes the problem I'm having, but I don't know if fiddling with the OPV would solve the problem. Besides, if I open the machine, then I void the warranty. Although, at this point, I'm ready to try just about anything

Any help would be greatly appreciated!

P.S. I'm using fresh beans from a local roaster, so that isn't the issue.

P.S.S. Added by moderator - see cross-post of same question on CoffeeGeek here.

jferreir wrote:...When I use a blind filter, the bar pressure steadily climbs to 12 bars and holds...

well, that would give some credit to the pump, to the OPV, and to the whole brewing circuit;
from this point, or with the newer 9 bar adjustment for the OPV, why not try to put the machine in a chocking status, by grinding much too fine than necessary for espresso, by filling enough the basket and tamping quite hardly; the machine should reach the same pressure limit like with the blind, then perhaps, due to some channeling, or after too much time, you could have a flow; from this point try to decrease gradually and one at a time, the dose and the grind, (same moderate tamping), until you have the proper flow, yet being at or close to the pressure limit;
BTW, what grinder do you have?

I have the Bezzera BB005 grinder. I have already tried what you suggested -- fine grind/choking, then working backwards - but that doesn't work. When I use ground coffee, the brew pressure never exceeds 8 or 9 bar. With a very fine grind, there will be no coffee flow, but the bar pressure still won't go above 8 or 9 bars.

it's really strange.
blind filter and fine ground coffee should behave in the same way.
could it be a grinder issue?
alberto

I haven't had an issue with the grinder, and the coffee grounds are quite uniform and consistent. I quite like the grinder, actually, but it's the espresso machine that's proven to be problematic. I'm at a loss to explain what might be causing the issue.

I would consult with the place of purchase once again as regards warranty issues. Does this happen during every brew event?

Here is a water schematic of the BZ10. I believe your boiler fill solenoid valve is opening at the inopportune time during the brew process due to loose fill probe wire or a faulty control board.

I agree that this could be the cause.
But why this doesn't happen with blind filter?

I should have said:

I believe your boiler fill solenoid valve is opening at the inopportune time during the brew process due to loose fill probe wire or a faulty control board OR your pump priming valve is not seating (closing) as it should. This valve is designed to close at a pressure of 4-6 bar (as per Ulka's spec). Since the priming valve and the OPV are joined with a "Y" piece, you can separate the two and run them back to the reservoir with some hardware store tubing.

During a simple open portafilter flush, the priming valve should flow a small amount back to the reservoir.

During the brew process, there should be no flow from the priming valve and some flow from the OPV tube.

During the blind basket process, there should be no flow from the priming valve and ~260 ml/minute from the OPV tube.

I think that this is a test that Jason can do without opening the machine.
He could separate the two plastic tubes at the inlet of the reservoir and looking at what happen
a) during a blind filter run
b) during extraction with choking (no output to the cup)
c) during a normal extraction

When I use the blind filter, the water flows back into the reservoir tank until it reaches 9 bars. After it hits 9 bars, there's no water returning to the tank. However, I just noticed that the machine is now having difficulty reaching 12 bars with the blind filter -- it slows to a crawl after hitting 9 bars, taking upwards of 25 seconds to reach 12 bars from start to finish.

When I attempt to extract coffee, there is a slow but consistent drip of water that goes back into the reservoir tank. This happens when the grind is too fine (no coffee flow), and when it's normal (adequate coffee flow).