Had a problem with sour espresso, recently installed Eric's thermometer and noticed that during my shots the temp was falling too low.

PID is set @ 200.

Pulling a shot through empty portafilter shows ~204-207 throughout the shot on Eric's thermometer, which would be about 200 at the puck as expected.

BUT, as soon as I put coffee in the basket and pull a real shot, it starts out fine, and then the temp at the grouphead thermometer quickly drops to 195, sometimes as low as just over 190, indicating the coffee is getting 185-190 degree water.

What could be causing this?

History:
Machine just came out of storage after a year of sitting. When it was put away I had a major leaking into the driptray problem (from the expansion valve tube), to fix it I just replaced the expansion valve seat.

Turned machine on after fixing the leak, and no water was coming out of the group-head. Pulled the mushroom (not a hideous amount of scale but a bit, cleaned it) and discovered that the middle gicleur center hole was completely clogged / blocked. Poked through it with a needle, soaked it overnight and all was well again.

Machine is plumbed in, ~34psi @ water regulator and ~8.5-9 bar during brew cycle

That is a puzzle. If you found scale, that is an indication there is more scale elsewhere, particularly in the steam boiler....maybe in the brew as well. Also if you found the mushroom, which I think has several holes, had a hole that was plugged you might look at that again. Also did you check the screen filter inside? Not sure that these relate to temp dropping, but they would affect flow...
It is also odd the grouphead thermometer would change so much since you are dealing with 5# of brass that the water moves through....Most of the temp studies are done in the portafilter and the water does pass thru heated brass after the temp probe. You could try the poor mans temp sensor with the styrofoam cup and poke thru digital temp sensor (maybe make you feel better?)..

If there were lots of scale, could it be possible that the preheating of the brew boiler water (HX tube in the steam tank if memory serve me correctly) is somehow insulated, assuming the steam is on..(try the experiment with steam on and off)....again perhaps descaling may help. DaveUK has talked in general terms about this. You could possibly do some research in that area....
Kind of a long shot, but that is the only way I could imagine some cooling during a shot - with non preheated water shooting into the brew tank....

Is the drop consistent with every shot? How about back-to-back shots?

Nettaiya wrote:PID is set @ 200.

You can try to change PID setting to higher/lower temperature to check if you have the same drop in temperature.

Nettaiya wrote:Pulling a shot through empty portafilter shows ~204-207 throughout the shot on Eric's thermometer, which would be about 200 at the puck as expected.

Are you running water through the group head for long enough?
Try pulling a shot with a blind filter.

Mine did the same thing for awhile. The gauge would drop as far low as 175*. It didn't seem to make any difference in the shot. I took it down to Great Infusions. They checked it out. It didn't 'perform' for them. They couldn't find anything wrong.I brought it back home and it hasn't done that since (two years).

One question though. Does it do the same thing when the steam boiler is on, or only when the brew boiler is working?