Looking for ideas on what the problem might be

Machine is a Rocket Cellini Plus V2. Tanked version with ulka pump.

If I put a back flush disc in and engage the pump while the heater is on, pressures ramps up to about 8 bar and then the pump goes very silent. Pressure stays at 8 or creeps up slowly.

If I do the same right at the top of the heating cycle when the heater turns off, everything works fine. I get 9 bar and hear the usual vibration pump sounds steadily.

Interestingly I found the following video which appears to show the exact problem I am having. Skip to about 1:30 where he is back flushing and you will see sometimes it's fine and sometimes the pump goes silent and it seems to correspond to when the heater is on.

https://youtu.be/IsKkV9wqyIY?t=1m30s

Any ideas?

It sounds like the pump is 'overheating', and the internal, and very small thermal high limit switch is stopping the power to it. Once cooled down, the pump works again. Likely time for a new pump.
On my Alexia, there is a relay that stops the heater when the pump engages. I've by-passed that relay and there is a definite change in voltage to the pump when the heater turns on and off. The pump changes pitch, but still runs however. It's the voltage drop that causes the problem.
The Alexia is the 120 volt North American model. In Europe they use 220 volts and the drop is less noticeable.

D.

Thanks for the response.

The problem is there even when the machine is cold and first turned on. If it was overheating related wouldn't it work under this condition?

All the major vendors sell Rocket machines. Best to call one or more of them to see if they know what could be causing the problem.
Unfortunately, the manual on Chris' Coffee does not contain any electrical information.
Hope this helps.

Just a quick update. .....

i was incorrect when I said the problem was there when the machine was cold. In fact I just tested it again and it worked fine until the machine was on for about 30 minutes or so and then the problem started reoccurring. So it seems like heat is a factor and the overheating theory may be correct.

http://www.myschiffman.org/cg/CIMG0381.jpg
Might be the overheat sensor, little white plastic part in the picture.

Complete rebuild: http://ulkapumprepair.blogspot.com/2008 ... -page.html

Part: http://www.chriscoffee.com/Klixon-Therm ... klixon.htm

The 'over heat' sensor' should be wired separately from the pump. To get really daring, with the espresso machine on a fire proof surface, an extinguisher near by. You could by-pass the said sensor, run the machine for a while and see if the pump keeps running.
I've found the sensor could be 'bad' and cutting the pump off when it's not really too hot, or the pump itself could be really hot, triggering the overheat cutout to occur.
If you follow this method, be prepared to have some smoke leak out of the pump, at which point you really need a new one.
Pumps come with built in sensors and you really can't replace one without the other.

My suggestion is to replace the pump and avoid the Saturday afternoon experiment.

D.

I have had a number of Ulka pumps wear out. While the problem for me was thermal related in the sense that it got worse as the machine got hotter, I never once caught it getting so hot that the thermal fuse actually did anything. It is possible however that the thermal fuse itself is failing and tripping too soon, its worth your while to (carefully) bypass the thermal fuse and see if things work better.

In all likelihood what you need is a new pump. You can maybe eke a bit more life out of the one you've got by turning the pressure down some, but eventually its going to need to be replaced. They are inexpensive and not hard to replace.

The Ulka pumps vary quite a bit in reliability, and you didn't necessarily do anything wrong. But here are some tips for maximizing their life.
- make sure it is well protected from the heat of your boiler. I personally replaced my dry crumbling factory insulation with melamine foam, and my most recent pump since doing that has lasted much longer than the others.

- minimize the amount of time it is on. They are rated for 1 minute on, but that is with 25C water, and your water in your reservoir will be hotter than 25C. I try not to run much more than 30 seconds. This is particularly important when refilling the boiler, the pump may want to run for a long time. Shut it off and give it a break.

- give it plenty of rest between uses. They are rated for 1 minute on 1 minute off, or 1 minute on 1.5 minute off, depending on the model. This isn't such a big deal in a normal espresso making workflow, but when backflush cleaning or refilling boiler remember to give it plenty of rest between uses.

Oh and one last tip, they don't like to pump hot liquid. If you mix a descale solution for example, let it cool before running it through your pump.