I tried to add this €0.02 of mine before, but the 'Cat had a nap, so apologies for firing off my mouth from the hip ....

"Pumping out" might originally have referred directly to removing polluted bilge water from an oil tanker; this would imply making the pollution in question Somebody Else's problem. Take that basic idea, apply it to the tailpipe of a rust bucket or a people carrier, and there y'go: instant cliché.