RobertRo101 Just did another test. I let the machine cool off and then started it again, with the portafilter locked in I went a few minutes ago to check on it, after around 50-60 minutes of warmup, the thermometer read 92.3 in idle and as soon as I removed the portafilter, the temperature started increasing to 93.8. Manometer steam pressure was again around 0.5 bar.
Could it be that the portafilter actually catches the surplus of high temperature that I have been seeing in the past week? And if so, what could cause such an increase and what could fix it?
I am not a physicist, just an owner like you, so take what I say with a grain of salt: First, My portafilter is always in the machine (accept for when I knock out the puck and wash it, of course). I don’t lock it, just “store” it in the group in a way that it stays there and doesn’t push against the gasket. This way, when the group heats, the PF heats with it and is a part of the thermal mass of the group. When you make a shot everything is nice and hot, and you don’t confuse things with different temperatures of the group and the PF.
Common sense tells me the above is how the machine was designed to be used. If you think about it, the group and PF are like a large heat sink. They absorb heat from the water and lose heat to the surrounding air. The software uses the temperature of the water to calculate how to control the heating element to turn on and off so that the end result will be the set temperature in the group. So if you add/remove metal mass from the machine, or if part of the mass (the PF) is at a colder temperature because it wasn’t in the group, this can cause a change in the temperature of the water in the thermosyphon and confuse the software, resulting in the fluctuations you see in group temperature.
I don’t have a group thermometer so can’t confirm this, but you do, so it should be very easy to see if keeping the PF in the group stabilises the temperature or not. If it does, then your problem is solved.