One question, I just called the engineers in Germany and we discussed this problem, does everyone who has posted here have a cap with this small hole in the center of it? This hole is not supposed to be here, apparently the mold is set up for this hole but it is for a future use and no parts should exist with this hole. Our going assumption is that this hole is letting air in reducing the suction so the pump cannot prime. I had assumed it was deliberate and was an air vent, but logically it reduces the suction and allows air intrusion as well.
So I am still having this problem. I am a little slow to respond because taking apart the whole unit is annoying and now that I know I can pull it out to fix I just make sure to never turn the power off. But I still need it fixed.
I do not have a hole on my cap. Here is a picture.
Since everything else checks out, I am nearly certain it is a weak impeller magnet. Can you message me a copy of the receipt and the serial number and we will get one out? The serial number is on the black tag on the pump cord.
Unfortunately I just left for vacation today (which is why I took apart the Tunze and cleaned it) so I wont be able to send until next week when I return.