Sorry for the delay in replying. Getting my kids back to college after the winter break.

I ran 2 tanks, over 1.5 hours, of dezcal thru the HX as per the elektros.it's "Unofficial" descaling instructions, followed by a tank of fresh water, and one with a little sodium bicarbonate. I tested the water after using the NaHCO3 test.

The Oscar is running better and is not so flaky. There was not much blue color to the water during the de-scaling. There were some white bits in the bottom of the glass measuring cup I was using to catch the discharge.

I did see the PO had done some mods. A new p-stat and a vacuum break. Pic included.

I will run the machine for a while to see how it goes. I'm hoping it was just a scale issue.

Thanks for the help!

Unfortunately, after a couple of weeks the shots have started pulling slowly again. I did another run using 4 liters of dezcal mixture over a couple of hours last night, but that did not fix the problem.

I have started taking the machine apart to remove tubing and test that it is not obstructed.

Any suggestions?

Any pictures you have of any pieces you do remove to inspect would assist greatly to determine the degree of scaling/mineral deposits.

spetri wrote:Unfortunately, after a couple of weeks the shots have started pulling slowly again. I did another run using 4 liters of dezcal mixture over a couple of hours last night, but that did not fix the problem.

I have started taking the machine apart to remove tubing and test that it is not obstructed.

Any suggestions?

Last night I opened up the Oscar removed the plastic tubes and the upper copper tube to the brew group and also did Elektros' "The water that comes out of the coffee group is insufficient or has little pressure" https://www.elektros.it/ca/en/tips_tric ... paragrafo7 operation.

The tubing showed no scale debris, with only a few very small pieces being washed out. This morning I tried making some coffees, but unfortunately it is going very slowly. 115 seconds for a 16 gram dose for 38 gram shot. (Not my usual 18 grams for 40 gram shot)

What regulates the brew group pressure? By sound and feel, I can't tell if I have a pressure or volume issue. With my luck probably both.

I guess I need a pressure gauge for portafilter. Any suggestions where to find and buy?

spetri wrote:. . . What regulates the brew group pressure? . . .

A perfect question that lasers right on in this issue

In the HX Line of a stock/factory model Nuova Simonelli Oscar 1, the Pump and the 3-Way Solenoid (Electrovalve or "EV") influence brew group pressure in a well functioning Oscar 1 hydraulic system's HX-Line/Group loop . . . and . . . of course the portafilter-gasket seal and grind-tamp-quantity of coffee powder (as this provides a semi-permeable-seal on the pump pressurized line terminating at the brew group when in operation). Increasing the pressure/temperature of the water in the HX-Line/Thermosiphon-Loop will also influence the pressure in the HX-Line/Thermosiphon-Loop during the first stages of extraction prior to the pump's effect of a nominal 9-11 bar develops (as temperature and pressure are directly related in a fixed-volume Pressure Vessel such as the HX-Line/Thermosiphon-Loop).

The Pump: Here is a picture of some specifications of a commonly used pump in the Nuova Simonelli Oscar 1 espresso machine:


A typical Ulka EX5, type E, model pump in a Nuova Simonelli Oscar 1 is capable of pushing approximately:

• 650 cc (ml) water per minute at 0 bar ......... (as without a portafilter inserted, no HX-Line/Pump-line/EV/check-valve obstructions)
• 225 cc (ml) water per minute at 10 bar ....... (good coffee power/tamping, components functioning well, no obstructions)
• 000 cc (ml) water per minute at 15 bar ....... (too fine/over tamped coffee, HX-Line/EV/fittings debris/obstructions, Thermosyphon-Stall)

The Group 3-way Solenoid (EV): A Nuova Simonelli Oscar 1 typical 3-way Solenoid can allow passage of a maximum of 1 liter per minute through a 1.3 mm orifice when energized in the right pressure-bar/pressure circumstances. Here's the Specifications for a common Oscar 1 3-way Solenoid:

spetri wrote:. . . I have noticed the pump noise is less on the slow shots . . .

This sounds like your pump is pushing against a higher than nominal pressure line (9-10bar). This will result in reduced flow.

Two possible front runners emerging are: (1) Thermosyphon Stall and (2) Faulty 1-way Check valve between pump and Thermosyphon Loop. Info provided by spetri suggests most components (the 3-way Solenoid, the Pump, descale) fully functional, with exception of the unknown status of the 1-way check valve.

Original Poster spetri has indicated a relatively clean of debris/mineral build up machine, with no solenoid issues (both by function and visual inspection), a fully functional pump operating within design parameters and specifications in post at #3 comment in this thread. The root cause of the fault is narrowed significantly with these excellent assessments and information from spetri.

Once possible Thermosyphon-Stall (see "Further Resources" below) is assessed and eliminated as a cause of the fault, possible quick measures could include if you still have your previous Oscar, mentioned in the original post #1, swapping out, one at a time:

• The HX-Line's 1-way Check/Retaining-Valve (between the 1/4" Double Fitting and Thermosyphon-Loop)
• The 3-way Group Solenoid (last resort)
• The Pump (last resort)

If your non-return 1-way check valve, that prevents backflow from your Thermosyphon-Loop to your Pump, is shot or malfunctioning (specially if you have the OPV kit installed) it might be difficult to maintain pressure and keep steam from developing in the Thermosyphon-Loop. Many of the experienced folks here will identify that Oscar 1 component (check-valve) as a common issue.

If the Pressure in your Steam Boiler is too high (above 1.3-ish bar) , it could be causing high pressure in your Thermosyphon-Loop as well, which when presented with the massive pressure drop of a "Cleaning Flush" will instantly cause the water in the Thermosyphon-Loop to flash-boil and produce a huge amount of steam. If the machine is running a pressure above 1.3 bar, a longer Cooling Flush than usual may be needed as the flash boil caused by the pressure drop in the Thermosyphon-Loop by the Cooling Flush may be greater as well. Likely shouldn't see more pressure than 1.3 bar influence on the Thermosyphon-Loop until the pump is energized and begins pushing its 9-11 bar nominal pressure. At 1.3 bar water boils at 107.1℃ (224.8℉) in a fixed volume vessel (such as a Steam Boiler or Thermosyphon-Loop). But drop the pressure to 1 bar, and water suddenly boils at 99.6℃ and presto, steam and boiling water instantly. That's what a Cooling Flush does . . . it opens the closed Thermosyphon-Loop to the Atmosphere of Earth, which as Astronauts will boast is 1 bar. So if you have water at a pressure of 1.3 bar (or higher due to previous pump action) and temperature of 107.1℃ (224.8℉) in the Thermosyphon-Loop, as long as the pressure is stable at 1.3 bar and holding, (no pressure leaks from the 1-way check valve or 3-way Group Solenoid valve), then the water should not produce steam or boil at all. A properly sealed, Thermosyphon-Loop with functional flow control valves at all points is crucial to avoiding steam buildup in the Loop and the resultant Thermosyphon Stalling.

Further Resources:

• Check out Dan Kehn's discussion of the dynamics of Cooling Flushes and ideal pressure in a Steam Boiler indicating 1 bar (boiling point of 99.6℃ or 211.4℉) to 1.3 bar (boiling point of 107.1℃ or 224.8℉). At 2 bar, 2mm thick copper, Stainless Steel or Brass Steam Boiler is becoming dangerous and at 3 bar risk of explosion of such a pressure vessel begins to become an issue.
• And homeburrero discusses a bit about Thermosyphon Stall in a Home-Barista discussion here, where he references Randy Glass' article Understanding and Preventing Thermosyphon Stall, where

  Randy Glass wrote:To prevent this, whenever doing a cooling or cleaning flush, avoid doing short flushes. A longer flush pushes out any steam or air, and it will replace enough of the hot water in the thermosyphon system with cold to lower the temperature to below the boiling point so that the flash boil cannot take place. Whenever opening the group to the atmosphere when the machine is at or near operating temperature, flush for at least a few seconds. I do not know what that amount of time would be, but generally, the longer the machine has been idle, the longer the flush. I would guess at least three or four seconds. I have eliminated stalls since conscientiously avoiding short flushes.
  If a stall takes place the only solution is a very long flush to rid the system of air. In severe cases this might have to be repeated three or four times with a bit of a rest between flushes to reinstate the thermosyphon flow.

If you do want to try to adjust your Pressurestat, here's a good YouTube video discussing how to (please UNPLUG machine when putting a screwdriver anywhere near a Pressurestate, despite the video showing an experienced professional demonstrating a machine with power to the pressurestat): Credit: YouTube and Whole Latte Love

If none of these bring your Oscar 1 back to full function, then the trouble-shooting search will continue beyond these involved components

Possibly Thermistor, if installed

Definitely excellent point by cafeIKE to verify the Thermistor on any higher end espresso machine. Some have a Thermistor and some don't, as cafeIKE mentioned, and these are a real sore spot often to check first Assessing such fault sources can really make a difference in getting to the root of the cause of a fault

Here's a picture of the location of an Oscar 1 Pump Thermistor (if installed, which it isn't in this particular model version of the Oscar 1):