Nuova Simonelli Oscar: Shot pull times vary to extreme

Equipment doesn't work? Troubleshooting? If you're handy, members can help.
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#1: Post by spetri »

I recently purchased a used Oscar I to replace a previous Oscar with broken boiler.

I have been having troubles with the shot times varying from 18 to 120+ seconds. Most often the first shot pulls at about 25-40 seconds, but the following shots are slow. I have noticed the pump noise is less on the slow shots.

Also, sometimes the cooling flush starts with a large amount of steam before it settles down.

Any suggestions on the cause(s) and how I can fix it?

I weigh each shot and my tamping pressure is fairly consistent. I did not have this problem with the old Oscar.
I have verified that the scales are working correctly.

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#2: Post by cafeIKE »

Do you purge the grinder for the first shot?
A Rocky is going to hold ??? stale coffee

What is the steam pressure?
Steam on the first shot is normal and the higher the steam pressure, the more steam from the group.

How much water flows direct from the group [no PF inserted] in 30s?

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spetri (original poster)

#3: Post by spetri (original poster) » replying to cafeIKE »

Yes, I purge the grinder

I don't know the steam pressure. How can I measure it?

I don't have a container, that will fit, big enough for 30 seconds. I did 5 x 15 second tests.
@15 Seconds by Grams with No PF. 1 Gram H20 = 1 ML
1) 207 G
2) 167 G
3) 144 G
4) 155 G
5) 201 G

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#4: Post by cafeIKE »

My error. I thought the Oscar had pressure gauge.
If you open the steam valve after the machine is hot, does the pressure always seem the same?

Water debit seems flaky.
How old is the pump?

If there is a strainer on the water inlet, is it clear?
If there is no strainer, is the inlet tube cut to prevent it 'attaching' to the reservoir:

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spetri (original poster)

#5: Post by spetri (original poster) » replying to cafeIKE »

It is a used machine. As far as I know, the pump is original.

The Oscar does not use hoses for water pickup. It has a removable water tank that drops into to a receptacle that doubles as the empty water safety switch.

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#6: Post by cafeIKE »

Buying a used machine that has not been fully rebuilt and demonstrably functions as new is often a risk.

You could try a descale.

If that doesn't help and you're handy and have the proper tools, disconnect the pump outlet and run a similar flow test. If the flow is consistent, then there is some stoppage in the brew circuit. If the flow is flaky replace the pump.


#7: Post by WWWired »

Outside Possibility: (1) The 3-way Solenoid Coil Failing (with your variable shot test data, a possibility nonetheless). The unobstructed flow rate for a 128 Series Parker 3-way Electrovalve (Solenoid) is about 250 milliliters or grams per 15 seconds. The flow rate for an E131F Series Parker 3-way Solenoid (Electrovalve) is about 400ml or grams per 15 second. Given this limiting governance of the 3-way Solenoid, your 200ish gram(ml)/15 second interval data isn't horrible (well in keeping with the 225 ml/minute capable by the standard Oscar 1 pump without any resistive force of a properly tamped coffee filled portafilter in place). The 2-way Solenoid on Oscars (typically a CEME 6610EN2, R08) is optimally capable of about 1000 milliliters or grams per 15 seconds when filling the Steam Boiler (separate from the HX Line/Group entirely). An erratic or weak/failing 3-way Solenoid Coil (the square block part of the Solenoid) may result in lower performance than when normally energized and moving easily to magnetize and move the central piston/plunger in the Solenoid to allow water passage; or

A Real Possibility: (2) The 3-way Solenoid's piston/plunger or orifices obstructed with minerals etc. (in which case response from "cafeIKE" above to descale to remove mineral buildups is solid advice). To try this solution, use a Citric Acid descale solution, about 2 tablespoons Citric Acid Powder (available in 1Kg bags from your local brewery or wine supply store), to 1 liter luke warm water. Several Treatments may be necessary if the mineral build ups are extensive. Citric Acid in this mixtures (2 Tbspns to 1 Liter water) is also a weak acid like Vinegar, but it is approximately 3 to 4x the stronger than vinegar (acetic acid) with no terrible aftertaste of continued fouling of remaining mineral deposits that can occur with vinegar. Here's a quick video about Citric Acid:
Credit to Whole Latte Love

More Likely: (3) Scale/Mineral Buildups and/or the check-valve (one-way non-return water valve CO10), lines, and/or fittings between your Pump and HX-Line/3-way Solenoid/Group may be failing/obstructed (as mentioned in point (2) above with bits and pieces minerals for example), or worse, the pieces of the plastic "check-valve (Nuova Simonelli part number C010) may be having wee pieces break off the worn out plastic piece (you mentioned it's a second hand purchase I think, hence wear can be an issue). This assumes you have the plastic multi-piece rubik's-cube pea-sized check valve and not the metal check-valve Elektro's sells on it's website as an upgrade/replacement. In this instance, where the source of the lower flow rates are due to water obstruction in the location of 1-way check-valve (or associated fittings) between the Water-Reservoir/Pump and the HX-Line/Thermosiphon, then you may also notice some backflow from your Thermosiphon/HX-Line loop back to your Pump (and if you have an OPV kit installed, into your Water Reservoir as there is a Pump bypass possible with an aftermarket OPV kit modification). Check the temperature of your line just after your Pump to you 2-way Solenoid (if no OPV kit installed) and the temperature of your Water-Reservoir water to see if hot water from your Thermosiphon/HX-Line is back-flowing past the 1-way Check Valve as temperature (and therefore Pressure) increases in the HX-Line/Thermosiphon-loop. If no temperature increase, then the backflow should be eliminated but there could still be a forward flow issue through the 1-way check valve into the HX-Line . . . that said, however, the attention would likely shift to mineral/debris obstruction in the 3-Way Solenoid/Group location and connected fittings.

The issue of a disintegrating check-valve (see picture below, credit, is that little pieces will migrate through your HX Line and travel to your 3-way Solenoid where the tiny orifices of the Groups-3-way-Solenoid can become obstructed. This can cause problems like you are describing.

For that reason, the "Outside" possibilities (Failing 3-way Solenoid Coil etc) would be only be necessary next after first checking the HX Line pathway (fittings/tubes/Solenoid-orifices-and-components) for mineral obstructions or a failed check-valve disintegrating inside the HX Line Thermosiphon loop.

Fortunately, if you do have to resort to other causes than an obstruction/failing check/retaining valve, you can relatively easily check your 3-way Solenoid (the one attached to your Group): See point 12 on this page for how to test your 3-way Solenoid.

And for your Check-valve/Retaining-valve that keeps water from backflowing from the Boiler/HX Line, see point 8 on this page for what it looks like. It's in position 3 in this great Elektro's picture from the same webpage:

In this scenario, a disintigrating retaining/check-valve would occur at position 3 in the picture above, and small pieces might travel into the Boiler HX Line Tube (about 1 inch/2.5cm in diameter) at point 5 in the picture, then make the u-shaped path through the larger Steam Boiler, and exit at point 5a in the picture above, and follow the hot water line above the boiler to point 8 in the picture where it would meet the 3-Way Solenoid and following Thermosiphon pathway of the HX Line Loop after the 3-Way Solenoid. The orifices of the 3-way solenoid are very tiny, 1.3mm for the 128 Series Parker ZB09 3-way Solenoid, and 1.5mm for the E131F Series Parker 3-way Solenoid.

Credit to for great schematic.

An obstruction, either due scale/mineral buildup or due to a worn and disintegrating/failed retaining/check valve (designed to prevent back flow from HX Line into the Cold Water supply line from the pump), could be the culprits and that obstruction could be right at the check-valve, or if pieces of the plastic check valve have broken off and migrated in the HX Line, they could be somewhere near the 3-Way Solenoid orifices creating blockages and hence extra steam and slow shots (due to obstruction of flow).

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spetri (original poster)

#8: Post by spetri (original poster) »

Thank you for the explanation. There is a lot for me to review and figure out.

I have been confused about the descaling process for the Oscar. There are a few videos, and they go different ways, and the comments are contradictory and confusing. Any guidance? BTW I have Urnex Dezcal.

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#9: Post by cafeIKE »

Simplified HX Descale [PourOver] been working for me for over 15 years. Adapt as nec.

Citric Acid is W A Y less expensive than commerical products.


#10: Post by WWWired »

spetri wrote: . . . BTW I have Urnex Dezcal.
Urnex Dezcal's active ingredient is 90% Citric Acid. Definitely an excellent product - head and shoulders above vinegar.

cafeIKE's linked descale guide (in the comment just above this one) is brilliant and I 100% Agree with cafeIKE's comment about Citric Acid being cheaper than commercial products (even though it contains the same active ingredients) . . . and I might even speculate that Citric Acid is even cheaper than Vinegar now that cafeIKE has mentioned this important point on cost ;)

It sounds like you have settled on the possibility of mineral buildups (you would have the best feel for this as you're the closest to its operation).

First off, just a note that if you have a lot of mineral buildup - like sheets coating all interior surfaces of your boiler/pipes etc. - it is possible that descaling may cause bits and pieces of mineral deposits to break free and create other blockages in lines and orifices regulating water flow (such as your 3-way Solenoid), but this is no need to worry, just be aware if there are extensive mineral deposits inside your machine, you might experience sudden flow interference due to mineral scale breaking free and creating blockages or migrating in flow paths. Important Consdieration: If flow is interrupted/blocked suddenly, be aware not to run the machine to failure where a coil is burnt out or the pump is pushed beyond it's duty cycle (usually 1 min on/1 minute off). Just keep an eye on flow and watch for improving flow rates and let the machine have normal rests between running descale through and using the pump/solenoids so as not to overtax them. A good consideration for any espresso purchase is the provenance of the machine (was the seller asked how often they descaled or engaged in maintenance and if they used filtered water of the correct hardness?). I'm unsure of how much disassembly is being considered in this process of descaling and inspecting parts, so if descaling is the approach prefered for now, follow the instructions on the Urnex Dezcal and begin doing descale treatments . . . take not of the water that comes from your group when expelling the Urnex Dezcal solution for discolorations etc. If the mineral buildups are moderate, and the cause of the issue, descaling should show improvements with successive treatments until the desired flow efficiency is reached.

For a bigger job, like your Oscar, it might be necessary to do several descaling treatments if using those little packets (which is ok and might be preferable to just remove any mineral buildups in stages). Depending on the amount of mineral buildups in your Oscar, the amount of Dezcal/Citric Acid needed to remove these deposits could get expensive. Here's a picture of an extreme mineral buildup in a pipe (extreme case), in which case you can see that descaling will have minimal effects:

Photo Credit: By Александр Юрьевич Лебедев - Ранее нигде не публиковалась, Public Domain

And here's an example of a fitting from an Oscar of mine during full cleaning/renewal that is between the 2-way Solenoid and Steam Boiler cold water fill fitting . . . notice that before and after are the same fitting (straight connector) but after complete disassembly of the line and use of a solution of 2 Tablespoons of Citric Acid in 600ml (less water than 1 liter recommended) in an Ultrasonic cleaner bath several times:

Limescale buildup in a pipe, fittings, or in Electrovalves/Solenoids, can reduce thermal efficiency by lowering both liquid flow and thermal conduction of the pipe (HX Line/Steam Boiler interface), when used as a heat exchanger. Regular Descaling, BackFlushing, and other maintenance of an high end espresso machine such as the Nuova Simonelli Oscar (both I and II), Musica and other manufacturer Heat Exchange and Dual Boiler models is a good consideration. Here's a bit of info from Nuova Simonelli about maintenance considerations:

Here's a picture of a 1 Kilogram (2.2 pound) bag of 100% Citric Acid (including the price sticker) I purchased for $8.50 Canadian ($5 or $6 USA bucks) at a local Wine Brewers Supply Store (they're pretty common shops, you might have one nearby in your neighborhood possibly) . . . note that there are many brands, it doesn't have to be "Global" brand as long as it says Citric Acid (and is kinda like salt/sugar in consistency):

And a few more pictures of the descaling of the 1/4' Fitting above (no need for an expensive commercial Ultrasonic Cleaner) as pretty much any $20 unit will do, if one's available, from your local Facebook Marketplace or other local buy-and-sell site: