Shurflo extreme 4 pump acting oddly

[hshovic]

Greetings,

Last year I installed a Shurflo Extreme 4.0 pump. It worked fine for awhile, but I am having some problems now.

The major problem is surging. With any of the fixtures open, it surges from low to high to low to high.... But that is not all. Various pump people say it is because of an air leak on the inlet side. I found just the opposite.

I have a valve upstream for adding antifreeze. Just on a whim I opened this valve while the pump was surging, and it stopped surging. It sucked air for a while. Then I closed the valve, and the pump began picking up water again, and now it does not surge! It works fine.

Then I opened another fixture for awhile. Then it starts surging again.

So what gives?? Adding air actually fixes the problem..