Desperate for help: Unbearable "battery acid" sourness event with dark toast [Gaggia Classic + Timemore S3 ESP]
Hi everyone,
I’m losing my mind here and desperately need some fresh eyes on my espresso setup. No matter what I do, every single shot tastes like straight-up battery acid. It’s a sharp, metallic, aggressive sourness that completely ruins the cup.
My Setup:
• Machine: NewGaggia Classic Pro E24 (upgraded with 9-bar spring)
• Grinder: Timemore S3 ESP (the s3 version with espresso-focused S2C burrs)
• Basket: IMS Competition non-pressurized basket (dosing ~20g)
The Problem:
Extreme, piercing sourness. This isn't just "bright, fruity acidity" – it's genuinely unpleasant. It happens even with freshly roasted dark roasts (~1 month old) and, bafflingly, even with 6-month-old stale medium-dark beans that shouldn't have any organic acids left in them.
Everything I’ve Tried & Ruled Out:
Grind Size & Ratios: At setting 28, a 1:2,5 ratio flows in about 30 seconds. Extremely sour. I tried going finer to increase extraction. At setting 25, the Gaggia completely chokes (zero flow). I tried tighter, traditional ratios: 1:2 in 30s, 25s, 20s, even 40-50s. I tried 1:3 in 40s, 30s, 20s. The sourness actually got worse at lower ratios, which strongly points to severe under-extraction.
I previously owned a HiBrew H10B and experienced the exact same issue. I assumed the harsh sourness was caused by the poor thermal stability of a cheap, budget machine, so I upgraded to the Gaggia Classic expecting a massive difference. To my horror, the exact same "battery acid" flavor carried over completely unchanged.
Water & Boiler: My baseline water is tap water filtered through a standard Brita-style pitcher.
◦ I tried a 50/50 mix of this filtered water and pure distilled water. No change.
◦ On my previous machine, I tried high-quality commercial bottled spring water to rule out the tap entirely — still pure acid.
◦ Today, I completely flushed the boiler and filled it entirely with fresh bottled spring water. Absolutely no change in the sourness.
If I use cheap, commercial pre-ground coffee in the Gaggia's pressurized basket with the same water, it comes out completely fine and not sour at all.
I suspected severe misalignment, so I did the paper test (spreading the grounds flat on a white sheet). IMO the grind looks uniform with no obvious large boulders.
Visually and physically, I haven't noticed any obvious issues with the machine's heating, though I don't have a thermometer to measure the exact temperatures. The steam wand produces strong, proper steam, the water coming straight out of the group head is piping hot, and the final espresso shot in the cup is hot as well. Furthermore, the water distribution from the shower screen looks completely even and uniform.
The machine and the portafilter (kept locked in the group head) are always preheated for at least 20 minutes before pulling any shots. Everything is thoroughly hot to the touch, so a cold group head or portafilter is definitely not the culprit.
I am attaching a photos of my workflow/puck prep and some photos of the grounds. Attaching video of example shot. I know this shot is too long (but its still sour even with dark roast), but this is just example how visually all my extractions looks clean from the bottomless portafilter—no obvious spraying or massive channeling.
Just to head off any "bitter-sour confusion" comments: Yes, I can absolutely tell the difference. When I pull a longer shot using the pressurized basket, the result becomes intensely bitter and astringent. I also know exactly what a proper, well-dialed espresso is supposed to taste like from visiting professional specialty cafes—this is definitely a textbook, aggressive under-extraction acid issue.
Am I missing something completely obvious? Any advice would be highly appreciated!