[Gelbster]

With an air pocket(s) present, you can generate steam. This may cause localized, tremendous pressure, zero cooling and volcanic results.If the pocket is large enough it is quite possible the temperature sensor is not immersed in coolant so you have no clue what the real temperature is.
Airlift is a cheap tool.But the ramps+burp system works for some.If you temporarily extend the coolant overflow tube, you can easily catch all the ejected coolant rather than pollute.