Since this is my first post, I'll start by saying thanks for all of the useful posts that have netted me some good shots as a pretty new barista:)

That said, I have a newer Oscar that I purchased used and is no longer functioning properly. The symptom is that the boiler over pressure valve is releasing pressure in bursts at a frequency of approximately once per second while pulling a shot only.

I took the maintenance steps of replacing the boiler over pressure valve (7 in the drawing) and thoroughly cleaning over pressure valve attached to the 3 way solenoid. I have additionally back flushed several times and this did not correct the problem.

Next, I connected a pressure gauge to the steam wand and watched this happen. Basically, as soon as I press the button to activate the pump, the boiler pressure increases to ~30psi and then the boiler over pressure valve kicks in. This happens with no portafilter attached!

Looking at the hydraulic diagram supplied by Nuova (attached to this post), I see that the pump is before the cold water supply for the boiler and suspect this is where the problem is. However, I don't know what part 3 in the diagram is called according to the Nuova parts list and I'd like to make sure my logic is sound before placing another parts order:)

The only other thing that I would suspect is that there is a leak in the hot water line (5 in the drawing) that runs through the boiler:(.

Thanks in advanced for any assistance!

Here is the diagram:)

If it helps, here is the translation from Italian to English:
0) WATER TANK
1) LACK OF WATER SENSOR
2) PUMP
3) SOLENOID LEVELLING
4) CHECK VALVE
5) HEAT EXCHANGER
6) LEVEL PROBE
7) SAFETY VALVE BOILER
8) PRESSURE
9) GROUP DISTRIBUTOR
10) SOLENOID VALVE REGULATOR
11) VALVE NEPLAS
12) LIQUID VALVE backflow
13) STEAM VALVE
14) HEATING BOILER

Eric, thanks for the detailed instructions:) I'll try to pull it off this evening and post pictures.

I purchased the Oscar in February and have been loving it ever since (until about a week ago). It was a trip out west early this year that made me realize that there is no good coffee in Tampa and that I have to make it myself:)

All I've done maintenance wise besides the OPVs is routine back flushes about once a week. Thanks again!

Eric, thank you sooo much! Indeed, the boiler solenoid valve was stuck in the open position:) There was some foreign material blocking the piston from sliding into the closed position. I used a small screw driver to remove the obviously visible material and then soaked the assembly in joe-glo since I didn't have white vinegar on hand.

Here is exactly what I did:
1. Unplugged machine
2. Removed the boiler solenoid pictured below

3. Removed the 3 plastic hoses attached to the solenoid assembly
4. Took a picture of and removed the wiring attached to the solenoid
5. Removed the coil portion of the solenoid
6. Removed the steel assemblies from the brass solenoid piston
7. Inspected the solenoid piston assembly and removed the culprit piece of crud blocking the solenoid from closing.
8. Soaked all pieces in cleaning solution for 45 minutes
9. Reassembled and reinstalled the solenoid
10. Drained the boiler

When I plugged the machine in, the boiler refilled, valve shut and I ran the group head while watching my pressure gauge hold steady:) Let it warm up fully and verified 19psi held steady on the pressure gauge at the steam wand while the group head was running.

Ran a couple of shots and I'm smiling again!