Hi all!

I got myself a secondhand ECM Cellini. It is equipped with a rocket control box and a water reservoir with a baby reservoir underneath.

Some days the machine works flawless. Near perfect coffee and lots of steam. Some days are like:
with the blind filter in the portafilter

So the pump stutters. I've checked the connections to the pump. I've checked and cleaned the water reservoir connections. I've checked the pressostat (no direct connection to the pump) and the water level sensor in the boiler.

Any ideas in what direction I have to look for solving this problem?

What I've done so far:
- replaced micro switch lever
- replaced thermal protection pump

When I pull a shot -> no problem

If I open the steam valve during pulling a shot, the pump begins to stutter. So It has something to do with filling the boiler (low voltage), heating the boiler (high voltage) and running the pump (high voltage).

Do you have any idea how I can pinpoint this problem? Any help is appreciated!

I"m just guessing, but either electrical or mechanical. I would start by testing voltage and current with a meter at the pump while it is acting up. That will rule out power supply/ control box issue. Then move on to mechanical- bad vibe pump ? Is hot water back feeding into pump inlet ? Is air getting sucked into the pump ? ( vacuum leak). I would hold off tossing parts at it until you can locate your trouble.